diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/crypto/BlockSignatureBuffer.java b/biscuit-core/src/main/java/org/eclipse/biscuit/crypto/BlockSignatureBuffer.java
index 579e4334..860c5c65 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/crypto/BlockSignatureBuffer.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/crypto/BlockSignatureBuffer.java
@@ -84,9 +84,7 @@ public static byte[] generateBlockSignaturePayloadV0(
     }
     var toVerify = ByteBuffer.allocate(capacity).order(ByteOrder.LITTLE_ENDIAN);
     toVerify.put(payload);
-    if (externalSignature.isPresent()) {
-      toVerify.put(externalSignature.get().getSignature());
-    }
+    externalSignature.ifPresent(signature -> toVerify.put(signature.getSignature()));
     toVerify.putInt(nextKey.getAlgorithm().getNumber());
     toVerify.put(nextKeyBytes);
     toVerify.flip();
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Check.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Check.java
index 9094c1da..438a718f 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Check.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Check.java
@@ -55,8 +55,8 @@ public Schema.CheckV2 serialize() {
       default:
     }
 
-    for (int i = 0; i < this.queries.size(); i++) {
-      b.addQueries(this.queries.get(i).serialize());
+    for (Rule query : this.queries) {
+      b.addQueries(query.serialize());
     }
 
     return b.build();
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Combinator.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Combinator.java
index 87ccd00a..3cc65915 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Combinator.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Combinator.java
@@ -39,10 +39,10 @@ public boolean hasNext() {
 
   @Override
   public Pair<Origin, Map<Long, Term>> next() {
-    if (this.nextElement == null || !this.nextElement.isPresent()) {
+    if (this.nextElement == null || this.nextElement.isEmpty()) {
       this.nextElement = getNext();
     }
-    if (this.nextElement == null || !this.nextElement.isPresent()) {
+    if (this.nextElement == null || this.nextElement.isEmpty()) {
       throw new NoSuchElementException();
     } else {
       Pair<Origin, Map<Long, Term>> t = this.nextElement.get();
@@ -136,10 +136,6 @@ public Optional<Pair<Origin, Map<Long, Term>>> getNext() {
         }
       }
 
-      if (this.currentIt == null) {
-        return Optional.empty();
-      }
-
       Optional<Pair<Origin, Map<Long, Term>>> opt = this.currentIt.getNext();
 
       if (opt.isPresent()) {
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/FactSet.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/FactSet.java
index 19a6b34c..a28801be 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/FactSet.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/FactSet.java
@@ -63,7 +63,7 @@ public void merge(FactSet other) {
     }
   }
 
-  public Stream stream(TrustedOrigins blockIds) {
+  public Stream<Pair<Origin, Fact>> stream(TrustedOrigins blockIds) {
     return facts.entrySet().stream()
         .filter(
             entry -> {
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/MatchedVariables.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/MatchedVariables.java
index f29b18db..7cbc0187 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/MatchedVariables.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/MatchedVariables.java
@@ -36,7 +36,7 @@ public Optional<Term> get(final long key) {
   }
 
   public boolean isComplete() {
-    return this.variables.values().stream().allMatch((v) -> v.isPresent());
+    return this.variables.values().stream().allMatch(Optional::isPresent);
   }
 
   public Optional<Map<Long, Term>> complete() {
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Predicate.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Predicate.java
index c3a1a6f3..a7f0ee98 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Predicate.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Predicate.java
@@ -47,8 +47,7 @@ public boolean match(final Predicate rulePredicate) {
   }
 
   public Predicate clone() {
-    final List<Term> terms = new ArrayList<>();
-    terms.addAll(this.terms);
+    final List<Term> terms = new ArrayList<>(this.terms);
     return new Predicate(this.name, terms);
   }
 
@@ -78,19 +77,17 @@ public int hashCode() {
   public String toString() {
     return this.name
         + "("
-        + String.join(
-            ", ",
-            this.terms.stream()
-                .map((i) -> (i == null) ? "(null)" : i.toString())
-                .collect(Collectors.toList()))
+        + this.terms.stream()
+            .map((i) -> (i == null) ? "(null)" : i.toString())
+            .collect(Collectors.joining(", "))
         + ")";
   }
 
   public Schema.PredicateV2 serialize() {
     Schema.PredicateV2.Builder builder = Schema.PredicateV2.newBuilder().setName(this.name);
 
-    for (int i = 0; i < this.terms.size(); i++) {
-      builder.addTerms(this.terms.get(i).serializeTerm());
+    for (Term term : this.terms) {
+      builder.addTerms(term.serializeTerm());
     }
 
     return builder.build();
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Rule.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Rule.java
index e7457720..93e96aba 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Rule.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Rule.java
@@ -86,14 +86,14 @@ public Stream<Result<Pair<Origin, Fact>, Error>> apply(
               if (!generatedVariables.containsKey(var.value())) {
                 // throw new Error("variables that appear in the head should appear in the body
                 // as well");
-                return Stream.of(Result.<Pair<Origin, Fact>, Error>err(new Error.InternalError()));
+                return Stream.of(Result.err(new Error.InternalError()));
               }
               p.terms().set(index, generatedVariables.get(var.value()));
             }
           }
 
           origin.add(ruleOrigin);
-          return Stream.of(Result.<Pair<Origin, Fact>, Error>ok(new Pair<>(origin, new Fact(p))));
+          return Stream.of(Result.ok(new Pair<>(origin, new Fact(p))));
         });
   }
 
@@ -194,12 +194,12 @@ public Rule(
   public Schema.RuleV2 serialize() {
     Schema.RuleV2.Builder b = Schema.RuleV2.newBuilder().setHead(this.head.serialize());
 
-    for (int i = 0; i < this.body.size(); i++) {
-      b.addBody(this.body.get(i).serialize());
+    for (Predicate predicate : this.body) {
+      b.addBody(predicate.serialize());
     }
 
-    for (int i = 0; i < this.expressions.size(); i++) {
-      b.addExpressions(this.expressions.get(i).serialize());
+    for (Expression expression : this.expressions) {
+      b.addExpressions(expression.serialize());
     }
 
     for (Scope scope : this.scopes) {
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/SchemaVersion.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/SchemaVersion.java
index 28f3f5c5..6542a243 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/SchemaVersion.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/SchemaVersion.java
@@ -6,6 +6,7 @@
 package org.eclipse.biscuit.datalog;
 
 import java.util.List;
+import java.util.Objects;
 import java.util.Optional;
 import org.eclipse.biscuit.crypto.PublicKey;
 import org.eclipse.biscuit.datalog.expressions.Expression;
@@ -130,10 +131,8 @@ private static boolean containsV33Op(List<Expression> expressions) {
       for (Op op : e.getOps()) {
         if (op instanceof Op.Unary) {
           Op.Unary b = (Op.Unary) op;
-          switch (b.getOp()) {
-            case TypeOf:
-              return true;
-            default:
+          if (Objects.requireNonNull(b.getOp()) == Op.UnaryOp.TypeOf) {
+            return true;
           }
         } else if (op instanceof Op.Binary) {
           Op.Binary b = (Op.Binary) op;
@@ -174,9 +173,6 @@ private static boolean containsV33Term(Term term) {
     if (term instanceof Term.Array) {
       return true;
     }
-    if (term instanceof Term.Map) {
-      return true;
-    }
-    return false;
+    return term instanceof Term.Map;
   }
 }
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Scope.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Scope.java
index 1659dcd5..0d573352 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Scope.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Scope.java
@@ -16,8 +16,8 @@ public enum Kind {
     PublicKey
   }
 
-  private Kind kind;
-  private long publicKey;
+  private final Kind kind;
+  private final long publicKey;
 
   private Scope(Kind kind, long publicKey) {
     this.kind = kind;
@@ -106,7 +106,7 @@ public boolean equals(Object o) {
   @Override
   public int hashCode() {
     int result = kind != null ? kind.hashCode() : 0;
-    result = 31 * result + (int) (publicKey ^ (publicKey >>> 32));
+    result = 31 * result + Long.hashCode(publicKey);
     return result;
   }
 }
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/SymbolTable.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/SymbolTable.java
index d95b49b3..43d0c76c 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/SymbolTable.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/SymbolTable.java
@@ -72,7 +72,7 @@ public String fromEpochIsoDate(long epochSec) {
   private final List<PublicKey> publicKeys;
 
   public long insert(final String symbol) {
-    int index = this.DEFAULT_SYMBOLS.indexOf(symbol);
+    int index = DEFAULT_SYMBOLS.indexOf(symbol);
     if (index == -1) {
       index = this.symbols.indexOf(symbol);
       if (index == -1) {
@@ -114,7 +114,7 @@ public Term add(final String symbol) {
 
   public Optional<Long> get(final String symbol) {
     // looking for symbol in default symbols
-    long index = this.DEFAULT_SYMBOLS.indexOf(symbol);
+    long index = DEFAULT_SYMBOLS.indexOf(symbol);
     if (index == -1) {
       // looking for symbol in usages defined symbols
       index = this.symbols.indexOf(symbol);
@@ -129,8 +129,8 @@ public Optional<Long> get(final String symbol) {
   }
 
   public Optional<String> getSymbol(int i) {
-    if (i >= 0 && i < this.DEFAULT_SYMBOLS.size() && i < DEFAULT_SYMBOLS_OFFSET) {
-      return Optional.of(this.DEFAULT_SYMBOLS.get(i));
+    if (i >= 0 && i < DEFAULT_SYMBOLS.size() && i < DEFAULT_SYMBOLS_OFFSET) {
+      return Optional.of(DEFAULT_SYMBOLS.get(i));
     } else if (i >= DEFAULT_SYMBOLS_OFFSET && i < this.symbols.size() + DEFAULT_SYMBOLS_OFFSET) {
       return Optional.of(this.symbols.get(i - DEFAULT_SYMBOLS_OFFSET));
     } else {
@@ -155,9 +155,9 @@ public String formatRule(final Rule r) {
 
   public String formatRuleBody(final Rule r) {
     final List<String> preds =
-        r.body().stream().map((p) -> this.formatPredicate(p)).collect(Collectors.toList());
+        r.body().stream().map(this::formatPredicate).collect(Collectors.toList());
     final List<String> expressions =
-        r.expressions().stream().map((c) -> this.formatExpression(c)).collect(Collectors.toList());
+        r.expressions().stream().map(this::formatExpression).collect(Collectors.toList());
 
     String res = String.join(", ", preds);
     if (!expressions.isEmpty()) {
@@ -170,7 +170,7 @@ public String formatRuleBody(final Rule r) {
     if (!r.scopes().isEmpty()) {
       res += " trusting ";
       final List<String> scopes =
-          r.scopes().stream().map((s) -> this.formatScope(s)).collect(Collectors.toList());
+          r.scopes().stream().map(this::formatScope).collect(Collectors.toList());
       res += String.join(", ", scopes);
     }
     return res;
@@ -199,13 +199,7 @@ public String formatScope(final Scope scope) {
   }
 
   public String formatPredicate(final Predicate p) {
-    List<String> ids =
-        p.terms().stream()
-            .map(
-                (t) -> {
-                  return this.formatTerm(t);
-                })
-            .collect(Collectors.toList());
+    List<String> ids = p.terms().stream().map(this::formatTerm).collect(Collectors.toList());
     return Optional.ofNullable(this.formatSymbol((int) p.name())).orElse("<?>")
         + "("
         + String.join(", ", ids)
@@ -220,15 +214,14 @@ public String formatTerm(final Term i) {
     } else if (i instanceof Term.Date) {
       return fromEpochIsoDate(((Term.Date) i).value());
     } else if (i instanceof Term.Integer) {
-      return "" + ((Term.Integer) i).value();
+      return String.valueOf(((Term.Integer) i).value());
     } else if (i instanceof Term.Str) {
       return "\"" + this.formatSymbol((int) ((Term.Str) i).value()) + "\"";
     } else if (i instanceof Term.Bytes) {
       return "hex:" + Utils.byteArrayToHexString(((Term.Bytes) i).value()).toLowerCase();
     } else if (i instanceof Term.Set) {
       final List<String> values =
-          ((Term.Set) i)
-              .value().stream().map((v) -> this.formatTerm(v)).collect(Collectors.toList());
+          ((Term.Set) i).value().stream().map(this::formatTerm).collect(Collectors.toList());
       if (values.isEmpty()) {
         return "{,}";
       } else {
@@ -239,11 +232,10 @@ public String formatTerm(final Term i) {
       return "null";
     } else if (i instanceof Term.Array) {
       final List<String> values =
-          ((Term.Array) i)
-              .value().stream().map((v) -> this.formatTerm(v)).collect(Collectors.toList());
+          ((Term.Array) i).value().stream().map(this::formatTerm).collect(Collectors.toList());
       return "[" + String.join(", ", values) + "]";
     } else if (i instanceof Term.Map) {
-      ArrayList<String> entries = new ArrayList();
+      ArrayList<String> entries = new ArrayList<>();
       for (Map.Entry<MapKey, Term> entry : ((Term.Map) i).value().entrySet()) {
         entries.add(formatTerm(entry.getKey()) + ": " + formatTerm(entry.getValue()));
       }
@@ -275,15 +267,15 @@ public String formatCheck(final Check c) {
         break;
     }
     final List<String> queries =
-        c.queries().stream().map((q) -> this.formatRuleBody(q)).collect(Collectors.toList());
+        c.queries().stream().map(this::formatRuleBody).collect(Collectors.toList());
     return prefix + String.join(" or ", queries);
   }
 
   public String formatWorld(final World w) {
     final List<String> facts =
-        w.getFacts().stream().map((f) -> this.formatFact(f)).collect(Collectors.toList());
+        w.getFacts().stream().map(this::formatFact).collect(Collectors.toList());
     final List<String> rules =
-        w.getRules().stream().map((r) -> this.formatRule(r)).collect(Collectors.toList());
+        w.getRules().stream().map(this::formatRule).collect(Collectors.toList());
 
     StringBuilder b = new StringBuilder();
     b.append("World {\n\tfacts: [\n\t\t");
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/TemporarySymbolTable.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/TemporarySymbolTable.java
index d2b91b90..9dca6279 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/TemporarySymbolTable.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/TemporarySymbolTable.java
@@ -12,9 +12,9 @@
 import java.util.Optional;
 
 public final class TemporarySymbolTable {
-  private SymbolTable base;
-  private int offset;
-  private List<String> symbols;
+  private final SymbolTable base;
+  private final int offset;
+  private final List<String> symbols;
 
   public TemporarySymbolTable(SymbolTable base) {
     this.offset = DEFAULT_SYMBOLS_OFFSET + base.currentOffset();
@@ -42,7 +42,7 @@ public long insert(final String symbol) {
 
     int index = this.symbols.indexOf(symbol);
     if (index != -1) {
-      return (long) (this.offset + index);
+      return this.offset + index;
     }
     this.symbols.add(symbol);
     return this.symbols.size() - 1 + this.offset;
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Term.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Term.java
index 2d7dfe80..9ca22a69 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Term.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/Term.java
@@ -103,11 +103,7 @@ public boolean equals(Object o) {
       if (this == o) {
         return true;
       }
-      if (o == null || getClass() != o.getClass()) {
-        return false;
-      }
-
-      return true;
+      return o != null && getClass() == o.getClass();
     }
 
     @Override
@@ -177,7 +173,7 @@ public boolean equals(Object o) {
 
     @Override
     public int hashCode() {
-      return (int) (value ^ (value >>> 32));
+      return Long.hashCode(value);
     }
 
     @Override
@@ -244,12 +240,12 @@ public boolean equals(Object o) {
 
     @Override
     public int hashCode() {
-      return (int) (value ^ (value >>> 32));
+      return Long.hashCode(value);
     }
 
     @Override
     public String toString() {
-      return "" + this.value;
+      return String.valueOf(this.value);
     }
 
     public Schema.TermV2 serializeTerm() {
@@ -395,7 +391,7 @@ public boolean equals(Object o) {
 
     @Override
     public int hashCode() {
-      return (int) (value ^ (value >>> 32));
+      return Long.hashCode(value);
     }
 
     public Schema.TermV2 serializeTerm() {
@@ -470,7 +466,7 @@ public boolean equals(Object o) {
 
     @Override
     public int hashCode() {
-      return (int) (value ^ (value >>> 32));
+      return Long.hashCode(value);
     }
 
     @Override
@@ -543,7 +539,7 @@ public int hashCode() {
 
     @Override
     public String toString() {
-      return "" + this.value;
+      return String.valueOf(this.value);
     }
 
     public Schema.TermV2 serializeTerm() {
@@ -610,7 +606,7 @@ public int hashCode() {
 
     @Override
     public String toString() {
-      return "" + value;
+      return String.valueOf(value);
     }
 
     public Schema.TermV2 serializeTerm() {
@@ -712,7 +708,7 @@ public int hashCode() {
 
     @Override
     public String toString() {
-      return "" + value;
+      return String.valueOf(value);
     }
 
     public Schema.TermV2 serializeTerm() {
@@ -737,7 +733,7 @@ public static Result<Term, Error.FormatError> deserializeV2(Schema.TermV2 term)
         return Result.err(
             new Error.FormatError.DeserializationError("invalid Term kind, expected map"));
       }
-      java.util.HashMap<MapKey, Term> values = new HashMap();
+      java.util.HashMap<MapKey, Term> values = new HashMap<>();
       Schema.Map s = term.getMap();
 
       for (Schema.MapEntry l : s.getEntriesList()) {
@@ -819,16 +815,16 @@ public int hashCode() {
 
     @Override
     public String toString() {
-      if (value.size() == 0) {
+      if (value.isEmpty()) {
         return "{,}";
       }
-      String s = "{";
+      StringBuilder s = new StringBuilder("{");
       int count = 0;
       for (Term elem : value) {
-        s += elem;
+        s.append(elem);
         count += 1;
         if (count < value.size()) {
-          s += ", ";
+          s.append(", ");
         }
       }
       return s + "}";
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/World.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/World.java
index e7ff9d71..5c42083d 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/World.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/World.java
@@ -147,7 +147,7 @@ public String print(SymbolTable symbolTable) {
 
     s.append("World {\n\t\tfacts: [");
     for (Map.Entry<Origin, HashSet<Fact>> entry : this.facts.facts().entrySet()) {
-      s.append("\n\t\t\t" + entry.getKey() + ":");
+      s.append("\n\t\t\t").append(entry.getKey()).append(":");
       for (Fact f : entry.getValue()) {
         s.append("\n\t\t\t\t");
         s.append(symbolTable.formatFact(f));
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/expressions/Expression.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/expressions/Expression.java
index e6917f0e..b568fecc 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/expressions/Expression.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/expressions/Expression.java
@@ -32,7 +32,7 @@ public ArrayList<Op> getOps() {
   // FIXME: should return a Result<Term, error::Expression>
   public Term evaluate(Map<Long, Term> variables, TemporarySymbolTable temporarySymbolTable)
       throws Error.Execution {
-    Deque<Op> stack = new ArrayDeque<Op>(16); // Default value
+    Deque<Op> stack = new ArrayDeque<>(16); // Default value
     for (Op op : ops) {
       op.evaluate(stack, variables, temporarySymbolTable);
     }
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/expressions/Op.java b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/expressions/Op.java
index e1aae371..bc26c1e5 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/expressions/Op.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/datalog/expressions/Op.java
@@ -6,7 +6,7 @@
 package org.eclipse.biscuit.datalog.expressions;
 
 import biscuit.format.schema.Schema;
-import java.io.UnsupportedEncodingException;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayDeque;
 import java.util.ArrayList;
 import java.util.Deque;
@@ -14,6 +14,7 @@
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
 import org.eclipse.biscuit.datalog.MapKey;
@@ -90,11 +91,7 @@ public void evaluate(
             if (s.isEmpty()) {
               throw new Error.Execution("string not found in symbols for id" + value);
             } else {
-              try {
-                stack.push(new Term.Integer(s.get().getBytes("UTF-8").length));
-              } catch (UnsupportedEncodingException e) {
-                throw new Error.Execution("cannot calculate string length: " + e.toString());
-              }
+              stack.push(new Term.Integer(s.get().getBytes(StandardCharsets.UTF_8).length));
             }
           } else if (value instanceof Term.Bytes) {
             stack.push(new Term.Integer(((Term.Bytes) value).value().length));
@@ -536,7 +533,7 @@ public void evaluate(
               Closure closure = (Closure) right;
               Term result = closure.call(variables, temporarySymbolTable);
               if (result instanceof Term.Bool) {
-                stack.push((Term.Bool) result);
+                stack.push(result);
               }
             } else {
               stack.push(new Term.Bool(false));
@@ -551,14 +548,14 @@ public void evaluate(
               Closure closure = (Closure) right;
               Term result = closure.call(variables, temporarySymbolTable);
               if (result instanceof Term.Bool) {
-                stack.push((Term.Bool) result);
+                stack.push(result);
               }
             }
           }
           break;
         case Intersection:
           if (right instanceof Term.Set && left instanceof Term.Set) {
-            HashSet<Term> intersec = new HashSet<Term>();
+            HashSet<Term> intersec = new HashSet<>();
             HashSet<Term> setRight = ((Term.Set) right).value();
             HashSet<Term> setLeft = ((Term.Set) left).value();
             for (Term locId : setRight) {
@@ -571,7 +568,7 @@ public void evaluate(
           break;
         case Union:
           if (right instanceof Term.Set && left instanceof Term.Set) {
-            HashSet<Term> union = new HashSet<Term>();
+            HashSet<Term> union = new HashSet<>();
             HashSet<Term> setRight = ((Term.Set) right).value();
             HashSet<Term> setLeft = ((Term.Set) left).value();
             union.addAll(setRight);
@@ -608,22 +605,14 @@ public void evaluate(
               stack.push(new Term.Null());
             } else {
               Term element = array.get(index);
-              if (element != null) {
-                stack.push(element);
-              } else {
-                stack.push(new Term.Null());
-              }
+              stack.push(Objects.requireNonNullElseGet(element, Term.Null::new));
             }
           }
           if (right instanceof MapKey && left instanceof Term.Map) {
             MapKey key = (MapKey) right;
             HashMap<MapKey, Term> map = ((Term.Map) left).value();
             Term value = map.get(key);
-            if (value != null) {
-              stack.push(value);
-            } else {
-              stack.push(new Term.Null());
-            }
+            stack.push(Objects.requireNonNullElseGet(value, Term.Null::new));
           }
           break;
         case Any:
@@ -1088,7 +1077,7 @@ Term call(Map<Long, Term> variables, TemporarySymbolTable temporarySymbolTable)
       if (arity() != 0) {
         throw new Error.Execution("called closure with arity " + arity() + "with no arguments");
       }
-      Deque<Op> stack = new ArrayDeque<Op>(16); // Default value
+      Deque<Op> stack = new ArrayDeque<>(16); // Default value
       for (Op op : ops) {
         op.evaluate(stack, variables, temporarySymbolTable);
       }
@@ -1107,7 +1096,7 @@ Term call(Term arg, Map<Long, Term> variables, TemporarySymbolTable temporarySym
         throw new Error.Execution(
             Error.Execution.Kind.ShadowedVariable, "closure parameter shadows variable");
       }
-      Deque<Op> stack = new ArrayDeque<Op>(16); // Default value
+      Deque<Op> stack = new ArrayDeque<>(16); // Default value
       for (Op op : ops) {
         op.evaluate(stack, variables, temporarySymbolTable);
       }
@@ -1165,7 +1154,7 @@ public static Result<Op, Error.FormatError> deserializeV1(Schema.OpClosure closu
       ArrayList<Op> ops = new ArrayList<>();
 
       for (long param : closure.getParamsList()) {
-        params.add(new Long(param));
+        params.add(param);
       }
 
       for (Schema.Op op : closure.getOpsList()) {
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/Authorizer.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/Authorizer.java
index 024afcd9..8394c996 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/Authorizer.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/Authorizer.java
@@ -187,7 +187,7 @@ public void updateOnToken() throws Error.FailedLogic {
           TrustedOrigins ruleTrustedOrigins =
               TrustedOrigins.fromScopes(
                   convertedRule.scopes(), blockTrustedOrigins, i + 1, this.publicKeyToBlockId);
-          world.addRule((long) i + 1, ruleTrustedOrigins, convertedRule);
+          world.addRule(i + 1, ruleTrustedOrigins, convertedRule);
         }
       }
     }
@@ -415,8 +415,7 @@ public Long authorize(RunLimits limits) throws Error {
       org.eclipse.biscuit.datalog.Check c = this.checks.get(i).convert(symbolTable);
       boolean successful = false;
 
-      for (int j = 0; j < c.queries().size(); j++) {
-        org.eclipse.biscuit.datalog.Rule query = c.queries().get(j);
+      for (org.eclipse.biscuit.datalog.Rule query : c.queries()) {
         TrustedOrigins ruleTrustedOrigins =
             TrustedOrigins.fromScopes(
                 query.scopes(), authorizerTrustedOrigins, Long.MAX_VALUE, this.publicKeyToBlockId);
@@ -501,8 +500,8 @@ public Long authorize(RunLimits limits) throws Error {
     for (int i = 0; i < this.policies.size(); i++) {
       Policy policy = this.policies.get(i);
 
-      for (int j = 0; j < policy.queries().size(); j++) {
-        org.eclipse.biscuit.datalog.Rule query = policy.queries().get(j).convert(symbolTable);
+      for (Rule policyQuery : policy.queries()) {
+        org.eclipse.biscuit.datalog.Rule query = policyQuery.convert(symbolTable);
         TrustedOrigins policyTrustedOrigins =
             TrustedOrigins.fromScopes(
                 query.scopes(), authorizerTrustedOrigins, Long.MAX_VALUE, this.publicKeyToBlockId);
@@ -513,7 +512,7 @@ public Long authorize(RunLimits limits) throws Error {
         }
 
         if (res) {
-          if (this.policies.get(i).kind() == Policy.Kind.ALLOW) {
+          if (policy.kind() == Policy.Kind.ALLOW) {
             policyResult = Optional.of(Result.ok(i));
           } else {
             policyResult = Optional.of(Result.err(i));
@@ -597,7 +596,7 @@ public String formatWorld() {
     StringBuilder facts = new StringBuilder();
     for (Map.Entry<Origin, HashSet<org.eclipse.biscuit.datalog.Fact>> entry :
         this.world.getFacts().facts().entrySet()) {
-      facts.append("\n\t\t" + entry.getKey() + ":");
+      facts.append("\n\t\t").append(entry.getKey()).append(":");
       for (org.eclipse.biscuit.datalog.Fact f : entry.getValue()) {
         facts.append("\n\t\t\t");
         facts.append(this.symbolTable.formatFact(f));
@@ -605,13 +604,13 @@ public String formatWorld() {
     }
     final List<String> rules =
         this.world.getRules().stream()
-            .map((r) -> this.symbolTable.formatRule(r))
+            .map(this.symbolTable::formatRule)
             .collect(Collectors.toList());
 
     List<String> checks = new ArrayList<>();
 
     for (int j = 0; j < this.checks.size(); j++) {
-      checks.add("Authorizer[" + j + "]: " + this.checks.get(j).toString());
+      checks.add("Authorizer[" + j + "]: " + this.checks.get(j));
     }
 
     if (this.token != null) {
@@ -649,7 +648,7 @@ public String formatWorld() {
     }
 
     return "World {\n\tfacts: ["
-        + facts.toString()
+        + facts
         // String.join(",\n\t\t", facts) +
         + "\n\t],\n\trules: [\n\t\t"
         + String.join(",\n\t\t", rules)
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/Biscuit.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/Biscuit.java
index 65f49bf6..34ab15b8 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/Biscuit.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/Biscuit.java
@@ -120,7 +120,6 @@ private static Biscuit make(
     } else {
       SerializedBiscuit s = container.getOk();
 
-      Optional<SerializedBiscuit> c = Optional.of(s);
       return new Biscuit(authority, blocks, authority.getSymbolTable(), s);
     }
   }
@@ -339,10 +338,7 @@ public Biscuit attenuate(final SecureRandom rng, final KeyPair keypair, Block bl
       symbolTable.insert(pk);
     }
 
-    ArrayList<Block> blocks = new ArrayList<>();
-    for (Block b : copiedBiscuit.blocks) {
-      blocks.add(b);
-    }
+    ArrayList<Block> blocks = new ArrayList<>(copiedBiscuit.blocks);
     blocks.add(block);
 
     SerializedBiscuit container = containerRes.getOk();
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/Block.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/Block.java
index 45f9882c..ab037885 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/Block.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/Block.java
@@ -118,7 +118,7 @@ public String print(SymbolTable symbolTable) {
     s.append(this.context);
     if (this.externalKey.isPresent()) {
       s.append("\n\t\texternal key: ");
-      s.append(this.externalKey.get().toString());
+      s.append(this.externalKey.get());
     }
     s.append("\n\t\tscopes: [");
     for (Scope scope : this.scopes) {
@@ -171,16 +171,16 @@ public String printCode(SymbolTable symbolTable) {
         s.append(this.externalKey.get().toString());
     }*/
     for (Scope scope : this.scopes) {
-      s.append("trusting " + localSymbols.formatScope(scope) + "\n");
+      s.append("trusting ").append(localSymbols.formatScope(scope)).append("\n");
     }
     for (Fact f : this.facts) {
-      s.append(localSymbols.formatFact(f) + ";\n");
+      s.append(localSymbols.formatFact(f)).append(";\n");
     }
     for (Rule r : this.rules) {
-      s.append(localSymbols.formatRule(r) + ";\n");
+      s.append(localSymbols.formatRule(r)).append(";\n");
     }
     for (Check c : this.checks) {
-      s.append(localSymbols.formatCheck(c) + ";\n");
+      s.append(localSymbols.formatCheck(c)).append(";\n");
     }
 
     return s.toString();
@@ -202,16 +202,16 @@ public Schema.Block serialize() {
       b.setContext(this.context);
     }
 
-    for (int i = 0; i < this.facts.size(); i++) {
-      b.addFactsV2(this.facts.get(i).serialize());
+    for (Fact fact : this.facts) {
+      b.addFactsV2(fact.serialize());
     }
 
-    for (int i = 0; i < this.rules.size(); i++) {
-      b.addRulesV2(this.rules.get(i).serialize());
+    for (Rule rule : this.rules) {
+      b.addRulesV2(rule.serialize());
     }
 
-    for (int i = 0; i < this.checks.size(); i++) {
-      b.addChecksV2(this.checks.get(i).serialize());
+    for (Check check : this.checks) {
+      b.addChecksV2(check.serialize());
     }
 
     for (Scope scope : this.scopes) {
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/Policy.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/Policy.java
index 0eaff09c..960aea40 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/Policy.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/Policy.java
@@ -25,7 +25,7 @@ public enum Kind {
   }
 
   private final List<Rule> queries;
-  private Kind kind;
+  private final Kind kind;
 
   public Policy(List<Rule> queries, Kind kind) {
     this.queries = queries;
@@ -42,8 +42,7 @@ public Policy(Rule query, Kind kind) {
 
   @Override
   public String toString() {
-    final List<String> qs =
-        queries.stream().map((q) -> q.bodyToString()).collect(Collectors.toList());
+    final List<String> qs = queries.stream().map(Rule::bodyToString).collect(Collectors.toList());
 
     switch (this.kind) {
       case ALLOW:
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/RevocationIdentifier.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/RevocationIdentifier.java
index 3324e277..5eb35b78 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/RevocationIdentifier.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/RevocationIdentifier.java
@@ -9,7 +9,7 @@
 import org.eclipse.biscuit.token.builder.Utils;
 
 public final class RevocationIdentifier {
-  private byte[] bytes;
+  private final byte[] bytes;
 
   public RevocationIdentifier(byte[] bytes) {
     this.bytes = bytes;
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/ThirdPartyBlockContents.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/ThirdPartyBlockContents.java
index cc76e3c4..db84c2d7 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/ThirdPartyBlockContents.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/ThirdPartyBlockContents.java
@@ -16,9 +16,9 @@
 import org.eclipse.biscuit.error.Error;
 
 public final class ThirdPartyBlockContents {
-  private byte[] payload;
-  private byte[] signature;
-  private PublicKey publicKey;
+  private final byte[] payload;
+  private final byte[] signature;
+  private final PublicKey publicKey;
 
   ThirdPartyBlockContents(byte[] payload, byte[] signature, PublicKey publicKey) {
     this.payload = payload;
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/UnverifiedBiscuit.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/UnverifiedBiscuit.java
index 48168bd2..e7e5cbea 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/UnverifiedBiscuit.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/UnverifiedBiscuit.java
@@ -171,10 +171,7 @@ private UnverifiedBiscuit attenuate(final SecureRandom rng, final KeyPair keypai
       symbols.add(s);
     }
 
-    ArrayList<Block> blocks = new ArrayList<>();
-    for (Block b : copiedBiscuit.blocks) {
-      blocks.add(b);
-    }
+    ArrayList<Block> blocks = new ArrayList<>(copiedBiscuit.blocks);
     blocks.add(block);
     SerializedBiscuit container = containerRes.getOk();
 
@@ -318,10 +315,7 @@ public UnverifiedBiscuit appendThirdPartyBlock(
 
     SymbolTable symbols = new SymbolTable(copiedBiscuit.symbolTable);
 
-    ArrayList<Block> blocks = new ArrayList<>();
-    for (Block b : copiedBiscuit.blocks) {
-      blocks.add(b);
-    }
+    ArrayList<Block> blocks = new ArrayList<>(copiedBiscuit.blocks);
     blocks.add(block);
 
     return new UnverifiedBiscuit(copiedBiscuit.authority, blocks, symbols, container);
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Biscuit.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Biscuit.java
index 99eb173a..759c9f00 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Biscuit.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Biscuit.java
@@ -21,13 +21,13 @@
 import org.eclipse.biscuit.token.builder.parser.Parser;
 
 public final class Biscuit {
-  private SecureRandom rng;
-  private Signer root;
+  private final SecureRandom rng;
+  private final Signer root;
   private String context;
-  private List<Fact> facts;
-  private List<Rule> rules;
-  private List<Check> checks;
-  private List<Scope> scopes;
+  private final List<Fact> facts;
+  private final List<Rule> rules;
+  private final List<Check> checks;
+  private final List<Scope> scopes;
   private Optional<Integer> rootKeyId;
 
   public Biscuit(final SecureRandom rng, final Signer root) {
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Block.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Block.java
index c070e0c3..ef666402 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Block.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Block.java
@@ -32,10 +32,10 @@
 
 public final class Block {
   private String context;
-  private List<Fact> facts;
-  private List<Rule> rules;
-  private List<Check> checks;
-  private List<Scope> scopes;
+  private final List<Fact> facts;
+  private final List<Rule> rules;
+  private final List<Check> checks;
+  private final List<Scope> scopes;
 
   public Block() {
     this.context = "";
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Check.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Check.java
index a08b7b3d..d4d8c923 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Check.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Check.java
@@ -7,6 +7,7 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Objects;
 import java.util.stream.Collectors;
 import org.eclipse.biscuit.datalog.Check.Kind;
 import org.eclipse.biscuit.datalog.SymbolTable;
@@ -59,8 +60,7 @@ public Kind kind() {
 
   @Override
   public String toString() {
-    final List<String> qs =
-        queries.stream().map((q) -> q.bodyToString()).collect(Collectors.toList());
+    final List<String> qs = queries.stream().map(Rule::bodyToString).collect(Collectors.toList());
 
     switch (kind) {
       case ONE:
@@ -85,7 +85,7 @@ public boolean equals(Object o) {
 
     Check check = (Check) o;
 
-    return queries != null ? queries.equals(check.queries) : check.queries == null;
+    return Objects.equals(queries, check.queries);
   }
 
   @Override
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Expression.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Expression.java
index 7ac58d27..c5a59390 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Expression.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Expression.java
@@ -25,7 +25,7 @@ public final org.eclipse.biscuit.datalog.expressions.Expression convertExpr(
   }
 
   public static Expression convertFrom(ArrayList<Op> ops, SymbolTable symbolTable) {
-    Deque<Expression> stack = new ArrayDeque<Expression>(16);
+    Deque<Expression> stack = new ArrayDeque<>(16);
     for (Op op : ops) {
       if (op instanceof org.eclipse.biscuit.datalog.Term) {
         org.eclipse.biscuit.datalog.Term v = (org.eclipse.biscuit.datalog.Term) op;
@@ -479,8 +479,8 @@ public String toString() {
   }
 
   public static final class Closure extends Expression {
-    private ArrayList<String> params;
-    private Expression body;
+    private final ArrayList<String> params;
+    private final Expression body;
 
     public Closure(Expression body) {
       this.params = new ArrayList<>();
@@ -493,10 +493,10 @@ public Closure(ArrayList<String> params, Expression body) {
     }
 
     public void toOpcodes(SymbolTable symbolTable, List<Op> ops) {
-      ArrayList<Long> paramIndexes = new ArrayList<Long>();
+      ArrayList<Long> paramIndexes = new ArrayList<>();
       for (String param : params) {
         long index = symbolTable.insert(param);
-        paramIndexes.add(new Long(index));
+        paramIndexes.add(index);
       }
       ArrayList<Op> bodyOps = new ArrayList<>();
       body.toOpcodes(symbolTable, bodyOps);
@@ -541,19 +541,19 @@ public boolean equals(Object o) {
 
     @Override
     public String toString() {
-      if (params.size() == 0) {
+      if (params.isEmpty()) {
         return body.toString();
       }
 
-      String s = null;
+      StringBuilder s = null;
       for (String param : params) {
         if (s == null) {
-          s = "$" + param;
+          s = new StringBuilder("$" + param);
         } else {
-          s = s + ", $" + param;
+          s.append(", $").append(param);
         }
       }
-      return s + " -> " + body.toString();
+      return s + " -> " + body;
     }
   }
 }
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Fact.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Fact.java
index c7148294..90e97ca8 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Fact.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Fact.java
@@ -8,6 +8,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
@@ -20,7 +21,7 @@ public final class Fact implements Cloneable {
   Optional<Map<String, Optional<Term>>> variables;
 
   public Fact(String name, List<Term> terms) {
-    Map<String, Optional<Term>> variables = new HashMap<String, Optional<Term>>();
+    Map<String, Optional<Term>> variables = new HashMap<>();
     for (Term term : terms) {
       if (term instanceof Term.Variable) {
         variables.put(((Term.Variable) term).value, Optional.empty());
@@ -41,7 +42,7 @@ private Fact(Predicate predicate, Optional<Map<String, Optional<Term>>> variable
   }
 
   public void validate() throws Error.Language {
-    if (!this.variables.isEmpty()) {
+    if (this.variables.isPresent()) {
       List<String> invalidVariables =
           variables.get().entrySet().stream()
               .flatMap(
@@ -75,21 +76,21 @@ public Fact set(String name, Term term) throws Error.Language {
 
   public Fact applyVariables() {
     this.variables.ifPresent(
-        laVariables -> {
-          this.predicate.terms =
-              this.predicate.terms.stream()
-                  .flatMap(
-                      t -> {
-                        if (t instanceof Term.Variable) {
-                          Optional<Term> term =
-                              laVariables.getOrDefault(((Term.Variable) t).value, Optional.empty());
-                          return term.map(t2 -> Stream.of(t2)).orElse(Stream.empty());
-                        } else {
-                          return Stream.of(t);
-                        }
-                      })
-                  .collect(Collectors.toList());
-        });
+        laVariables ->
+            this.predicate.terms =
+                this.predicate.terms.stream()
+                    .flatMap(
+                        t -> {
+                          if (t instanceof Term.Variable) {
+                            Optional<Term> term =
+                                laVariables.getOrDefault(
+                                    ((Term.Variable) t).value, Optional.empty());
+                            return term.stream();
+                          } else {
+                            return Stream.of(t);
+                          }
+                        })
+                    .collect(Collectors.toList()));
     return this;
   }
 
@@ -129,7 +130,7 @@ public boolean equals(Object o) {
 
     Fact fact = (Fact) o;
 
-    return predicate != null ? predicate.equals(fact.predicate) : fact.predicate == null;
+    return Objects.equals(predicate, fact.predicate);
   }
 
   @Override
@@ -140,13 +141,7 @@ public int hashCode() {
   @Override
   public Fact clone() {
     Predicate p = this.predicate.clone();
-    Optional<Map<String, Optional<Term>>> variables =
-        this.variables.map(
-            v -> {
-              Map<String, Optional<Term>> m = new HashMap<>();
-              m.putAll(v);
-              return m;
-            });
+    Optional<Map<String, Optional<Term>>> variables = this.variables.map(HashMap::new);
     return new Fact(p, variables);
   }
 }
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Predicate.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Predicate.java
index c6843d46..10e20dbf 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Predicate.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Predicate.java
@@ -7,6 +7,7 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Objects;
 import java.util.stream.Collectors;
 import org.eclipse.biscuit.datalog.SymbolTable;
 
@@ -51,9 +52,8 @@ public static Predicate convertFrom(
 
   @Override
   public String toString() {
-    final List<String> i =
-        terms.stream().map((term) -> term.toString()).collect(Collectors.toList());
-    return "" + name + "(" + String.join(", ", i) + ")";
+    final List<String> i = terms.stream().map(Object::toString).collect(Collectors.toList());
+    return name + "(" + String.join(", ", i) + ")";
   }
 
   @Override
@@ -67,10 +67,10 @@ public boolean equals(Object o) {
 
     Predicate predicate = (Predicate) o;
 
-    if (name != null ? !name.equals(predicate.name) : predicate.name != null) {
+    if (!Objects.equals(name, predicate.name)) {
       return false;
     }
-    return terms != null ? terms.equals(predicate.terms) : predicate.terms == null;
+    return Objects.equals(terms, predicate.terms);
   }
 
   @Override
@@ -83,7 +83,7 @@ public int hashCode() {
   @Override
   public Predicate clone() {
     String name = this.name;
-    List<Term> terms = new ArrayList<Term>(this.terms.size());
+    List<Term> terms = new ArrayList<>(this.terms.size());
     terms.addAll(this.terms);
     return new Predicate(name, terms);
   }
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Rule.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Rule.java
index 73ce4501..19dab756 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Rule.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Rule.java
@@ -9,6 +9,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
 import java.util.stream.Collectors;
@@ -57,12 +58,9 @@ public Rule(
 
   @Override
   public Rule clone() {
-    List<Predicate> body = new ArrayList<>();
-    body.addAll(this.body);
-    List<Expression> expressions = new ArrayList<>();
-    expressions.addAll(this.expressions);
-    List<Scope> scopes = new ArrayList<>();
-    scopes.addAll(this.scopes);
+    List<Predicate> body = new ArrayList<>(this.body);
+    List<Expression> expressions = new ArrayList<>(this.expressions);
+    List<Scope> scopes = new ArrayList<>(this.scopes);
     Predicate head = this.head.clone();
     return new Rule(head, body, expressions, scopes);
   }
@@ -90,7 +88,7 @@ public void applyVariables() {
                         if (t instanceof Term.Variable) {
                           Optional<Term> term =
                               laVariables.getOrDefault(((Term.Variable) t).value, Optional.empty());
-                          return term.map(t2 -> Stream.of(t2)).orElse(Stream.of(t));
+                          return term.map(Stream::of).orElse(Stream.of(t));
                         } else {
                           return Stream.of(t);
                         }
@@ -105,7 +103,7 @@ public void applyVariables() {
                             Optional<Term> term =
                                 laVariables.getOrDefault(
                                     ((Term.Variable) t).value, Optional.empty());
-                            return term.map(t2 -> Stream.of(t2)).orElse(Stream.of(t));
+                            return term.map(Stream::of).orElse(Stream.of(t));
                           } else {
                             return Stream.of(t);
                           }
@@ -171,7 +169,7 @@ public Result<Rule, String> validateVariables() {
     return Result.err(
         "rule head or expressions contains variables that are not "
             + "used in predicates of the rule's body: "
-            + freeVariables.toString());
+            + freeVariables);
   }
 
   public org.eclipse.biscuit.datalog.Rule convert(SymbolTable symbolTable) {
@@ -228,16 +226,16 @@ public boolean equals(Object o) {
 
     Rule rule = (Rule) o;
 
-    if (head != null ? !head.equals(rule.head) : rule.head != null) {
+    if (!Objects.equals(head, rule.head)) {
       return false;
     }
-    if (body != null ? !body.equals(rule.body) : rule.body != null) {
+    if (!Objects.equals(body, rule.body)) {
       return false;
     }
-    if (scopes != null ? !scopes.equals(rule.scopes) : rule.scopes != null) {
+    if (!Objects.equals(scopes, rule.scopes)) {
       return false;
     }
-    return expressions != null ? expressions.equals(rule.expressions) : rule.expressions == null;
+    return Objects.equals(expressions, rule.expressions);
   }
 
   @Override
@@ -255,8 +253,7 @@ public String bodyToString() {
     String res = "";
 
     if (!r.body.isEmpty()) {
-      final List<String> b =
-          r.body.stream().map((pred) -> pred.toString()).collect(Collectors.toList());
+      final List<String> b = r.body.stream().map(Predicate::toString).collect(Collectors.toList());
       res += String.join(", ", b);
     }
 
@@ -265,9 +262,7 @@ public String bodyToString() {
         res += ", ";
       }
       final List<String> e =
-          r.expressions.stream()
-              .map((expression) -> expression.toString())
-              .collect(Collectors.toList());
+          r.expressions.stream().map(Object::toString).collect(Collectors.toList());
       res += String.join(", ", e);
     }
 
@@ -275,8 +270,7 @@ public String bodyToString() {
       if (!r.body.isEmpty() || !r.expressions.isEmpty()) {
         res += " ";
       }
-      final List<String> e =
-          r.scopes.stream().map((scope) -> scope.toString()).collect(Collectors.toList());
+      final List<String> e = r.scopes.stream().map(Scope::toString).collect(Collectors.toList());
       res += "trusting " + String.join(", ", e);
     }
 
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Scope.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Scope.java
index de97578c..1c9726eb 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Scope.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Scope.java
@@ -17,9 +17,9 @@ enum Kind {
     Parameter,
   }
 
-  private Kind kind;
-  private PublicKey publicKey;
-  private String parameter;
+  private final Kind kind;
+  private final PublicKey publicKey;
+  private final String parameter;
 
   private Scope(Kind kind) {
     this.kind = kind;
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Term.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Term.java
index c5778e6b..ea8a23f7 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Term.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/Term.java
@@ -221,11 +221,7 @@ public boolean equals(Object o) {
       if (this == o) {
         return true;
       }
-      if (o == null || getClass() != o.getClass()) {
-        return false;
-      }
-
-      return true;
+      return o != null && getClass() == o.getClass();
     }
 
     @Override
@@ -440,16 +436,16 @@ public java.util.Set<Term> getValue() {
 
     @Override
     public String toString() {
-      if (value.size() == 0) {
+      if (value.isEmpty()) {
         return "{,}";
       }
-      String s = "{";
+      StringBuilder s = new StringBuilder("{");
       int count = 0;
       for (Term elem : value) {
-        s += elem;
+        s.append(elem);
         count += 1;
         if (count < value.size()) {
-          s += ", ";
+          s.append(", ");
         }
       }
       return s + "}";
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/parser/Error.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/parser/Error.java
index 8bd3f98d..0e58d787 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/parser/Error.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/parser/Error.java
@@ -5,6 +5,8 @@
 
 package org.eclipse.biscuit.token.builder.parser;
 
+import java.util.Objects;
+
 public final class Error extends Exception {
   String input;
   String message;
@@ -31,10 +33,10 @@ public boolean equals(Object o) {
 
     Error error = (Error) o;
 
-    if (input != null ? !input.equals(error.input) : error.input != null) {
+    if (!Objects.equals(input, error.input)) {
       return false;
     }
-    return message != null ? message.equals(error.message) : error.message == null;
+    return Objects.equals(message, error.message);
   }
 
   @Override
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/parser/Parser.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/parser/Parser.java
index f1b8992f..e835f872 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/parser/Parser.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/builder/parser/Parser.java
@@ -219,7 +219,6 @@ public static Result<Pair<String, Rule>, Error> rule(String s) {
       return Result.err(new Error(s, "rule arrow not found"));
     }
 
-    List<Predicate> predicates = new ArrayList<Predicate>();
     s = s.substring(2);
 
     var bodyRes = ruleBody(s);
@@ -288,7 +287,6 @@ public static Result<Pair<String, Policy>, Error> policy(String s) {
       return Result.err(new Error(s, "missing policy prefix"));
     }
 
-    List<Rule> queries = new ArrayList<>();
     var bodyRes = checkBody(s);
     if (bodyRes.isErr()) {
       return Result.err(bodyRes.getErr());
@@ -326,7 +324,6 @@ public static Result<Pair<String, List<Rule>>, Error> checkBody(String s) {
     }
     queries.add(valid.getOk());
 
-    int i = 0;
     while (true) {
       if (s.isEmpty()) {
         break;
@@ -359,7 +356,7 @@ public static Result<Pair<String, List<Rule>>, Error> checkBody(String s) {
   }
 
   public static Result<RuleBody, Error> ruleBody(String s) {
-    List<Predicate> predicates = new ArrayList<Predicate>();
+    List<Predicate> predicates = new ArrayList<>();
     List<Expression> expressions = new ArrayList<>();
 
     while (true) {
@@ -406,17 +403,17 @@ public static Result<Pair<String, Predicate>, Error> predicate(String s) {
     String name = tn._1;
     s = tn._2;
 
-    if (name.length() == 0) {
+    if (name.isEmpty()) {
       return Result.err(new Error(s, "no predicate name"));
     }
 
     s = space(s);
-    if (s.length() == 0 || s.charAt(0) != '(') {
+    if (s.isEmpty() || s.charAt(0) != '(') {
       return Result.err(new Error(s, "opening parens not found for predicate " + name));
     }
     s = s.substring(1);
 
-    List<Term> terms = new ArrayList<Term>();
+    List<Term> terms = new ArrayList<>();
     while (true) {
 
       s = space(s);
@@ -445,7 +442,7 @@ public static Result<Pair<String, Predicate>, Error> predicate(String s) {
     }
     String remaining = s.substring(1);
 
-    return Result.ok(new Pair<String, Predicate>(remaining, new Predicate(name, terms)));
+    return Result.ok(new Pair<>(remaining, new Predicate(name, terms)));
   }
 
   public static Result<Pair<String, List<Scope>>, Error> scopes(String s) {
@@ -455,7 +452,7 @@ public static Result<Pair<String, List<Scope>>, Error> scopes(String s) {
     s = s.substring("trusting".length());
     s = space(s);
 
-    List<Scope> scopes = new ArrayList<Scope>();
+    List<Scope> scopes = new ArrayList<>();
 
     while (true) {
       s = space(s);
@@ -499,7 +496,7 @@ public static Result<Pair<String, Scope>, Error> scope(String s) {
         return Result.err(new Error(s, "unrecognized parameter"));
       }
       Pair<String, String> t = res.getOk();
-      if (!s.isEmpty() && s.charAt(0) == '}') {
+      if (s.charAt(0) == '}') {
         return Result.ok(new Pair<>(t._1, Scope.parameter(t._2)));
       } else {
         return Result.err(new Error(s, "unrecognized parameter end"));
@@ -544,12 +541,12 @@ public static Result<Pair<String, Predicate>, Error> factPredicate(String s) {
     s = tn._2;
 
     s = space(s);
-    if (s.length() == 0 || s.charAt(0) != '(') {
+    if (s.isEmpty() || s.charAt(0) != '(') {
       return Result.err(new Error(s, "opening parens not found for fact " + name));
     }
     s = s.substring(1);
 
-    List<Term> terms = new ArrayList<Term>();
+    List<Term> terms = new ArrayList<>();
     while (true) {
 
       s = space(s);
@@ -578,7 +575,7 @@ public static Result<Pair<String, Predicate>, Error> factPredicate(String s) {
     }
     String remaining = s.substring(1);
 
-    return Result.ok(new Pair<String, Predicate>(remaining, new Predicate(name, terms)));
+    return Result.ok(new Pair<>(remaining, new Predicate(name, terms)));
   }
 
   public static Result<Pair<String, String>, Error> name(String s) {
@@ -586,7 +583,7 @@ public static Result<Pair<String, String>, Error> name(String s) {
     String name = t._1;
     String remaining = t._2;
 
-    return Result.ok(new Pair<String, String>(remaining, name));
+    return Result.ok(new Pair<>(remaining, name));
   }
 
   public static Result<Pair<String, Term>, Error> term(String s) {
@@ -762,7 +759,7 @@ public static Result<Pair<String, Term.Str>, Error> string(String s) {
     String string = s.substring(1, index + 1);
     String remaining = s.substring(index + 2);
 
-    return Result.ok(new Pair<String, Term.Str>(remaining, (Term.Str) Utils.string(string)));
+    return Result.ok(new Pair<>(remaining, (Term.Str) Utils.string(string)));
   }
 
   public static Result<Pair<String, Term.Integer>, Error> integer(String s) {
@@ -788,7 +785,7 @@ public static Result<Pair<String, Term.Integer>, Error> integer(String s) {
     long i = Long.parseLong(s.substring(0, index2));
     String remaining = s.substring(index2);
 
-    return Result.ok(new Pair<String, Term.Integer>(remaining, (Term.Integer) Utils.integer(i)));
+    return Result.ok(new Pair<>(remaining, (Term.Integer) Utils.integer(i)));
   }
 
   public static Result<Pair<String, Term.Date>, Error> date(String s) {
@@ -798,7 +795,7 @@ public static Result<Pair<String, Term.Date>, Error> date(String s) {
     try {
       OffsetDateTime d = OffsetDateTime.parse(t._1);
       String remaining = t._2;
-      return Result.ok(new Pair<String, Term.Date>(remaining, new Term.Date(d.toEpochSecond())));
+      return Result.ok(new Pair<>(remaining, new Term.Date(d.toEpochSecond())));
     } catch (DateTimeParseException e) {
       return Result.err(new Error(s, "not a date"));
     }
@@ -813,7 +810,7 @@ public static Result<Pair<String, Term.Variable>, Error> variable(String s) {
         takewhile(
             s.substring(1), (c) -> Character.isAlphabetic(c) || Character.isDigit(c) || c == '_');
 
-    return Result.ok(new Pair<String, Term.Variable>(t._2, (Term.Variable) Utils.var(t._1)));
+    return Result.ok(new Pair<>(t._2, (Term.Variable) Utils.var(t._1)));
   }
 
   public static Result<Pair<String, Term.Null>, Error> nullTerm(String s) {
@@ -846,7 +843,7 @@ public static Result<Pair<String, Term.Array>, Error> array(String s) {
     }
     s = s.substring(1);
 
-    ArrayList<Term> terms = new ArrayList<Term>();
+    ArrayList<Term> terms = new ArrayList<>();
     while (true) {
 
       s = space(s);
@@ -890,7 +887,7 @@ public static Result<Pair<String, Term.Map>, Error> map(String s) {
     }
     s = s.substring(1);
 
-    HashMap<MapKey, Term> v = new HashMap<MapKey, Term>();
+    HashMap<MapKey, Term> v = new HashMap<>();
     while (true) {
       s = space(s);
 
@@ -953,13 +950,13 @@ public static Result<Pair<String, Term.Set>, Error> set(String s) {
       s = space(s.substring(1));
       if (s.charAt(0) == '}') {
         s = s.substring(1);
-        return Result.ok(new Pair<>(s, new Term.Set(new HashSet<Term>())));
+        return Result.ok(new Pair<>(s, new Term.Set(new HashSet<>())));
       } else {
         return Result.err(new Error(s, "closing brace not found"));
       }
     }
 
-    HashSet<Term> terms = new HashSet<Term>();
+    HashSet<Term> terms = new HashSet<>();
     while (true) {
 
       s = space(s);
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/format/SerializedBiscuit.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/format/SerializedBiscuit.java
index 80eddd4f..62a2f8a0 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/format/SerializedBiscuit.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/format/SerializedBiscuit.java
@@ -242,9 +242,7 @@ public byte[] serialize() throws Error.FormatError.SerializationError {
     }
 
     biscuitBuilder.setProof(proofBuilder.build());
-    if (!this.rootKeyId.isEmpty()) {
-      biscuitBuilder.setRootKeyId(this.rootKeyId.get());
-    }
+    this.rootKeyId.ifPresent(biscuitBuilder::setRootKeyId);
 
     Schema.Biscuit biscuit = biscuitBuilder.build();
 
@@ -342,10 +340,7 @@ public Result<SerializedBiscuit, Error.FormatError> append(
       SignedBlock signedBlock =
           new SignedBlock(block, nextKey, signature, externalSignature, blockSignatureVersion);
 
-      ArrayList<SignedBlock> blocks = new ArrayList<>();
-      for (SignedBlock bl : this.blocks) {
-        blocks.add(bl);
-      }
+      ArrayList<SignedBlock> blocks = new ArrayList<>(this.blocks);
       blocks.add(signedBlock);
 
       Proof proof = new Proof.NextSecret(next);
@@ -420,11 +415,9 @@ static Result<org.eclipse.biscuit.crypto.PublicKey, Error> verifyAuthorityBlockS
     }
 
     var verificationResult = publicKey.verify(payload.getOk(), signedBlock.getSignature());
-    if (verificationResult.isPresent()) {
-      return Result.err(verificationResult.get());
-    }
-
-    return Result.ok(signedBlock.getKey());
+    return verificationResult
+        .<Result<PublicKey, Error>>map(Result::err)
+        .orElseGet(() -> Result.ok(signedBlock.getKey()));
   }
 
   static Result<org.eclipse.biscuit.crypto.PublicKey, Error> verifyBlockSignature(
@@ -465,7 +458,6 @@ static Result<org.eclipse.biscuit.crypto.PublicKey, Error> verifyBlockSignature(
   }
 
   public Pair<Block, List<Block>> extractBlocks(SymbolTable symbolTable) throws Error {
-    ArrayList<Optional<org.eclipse.biscuit.crypto.PublicKey>> blockExternalKeys = new ArrayList<>();
     var authRes = Block.fromBytes(this.authority.getBlock(), Optional.empty());
     if (authRes.isErr()) {
       throw authRes.getErr();
@@ -474,7 +466,6 @@ public Pair<Block, List<Block>> extractBlocks(SymbolTable symbolTable) throws Er
     for (org.eclipse.biscuit.crypto.PublicKey pk : authority.getPublicKeys()) {
       symbolTable.insert(pk);
     }
-    blockExternalKeys.add(Optional.empty());
 
     for (String s : authority.getSymbolTable().symbols()) {
       symbolTable.add(s);
@@ -495,9 +486,7 @@ public Pair<Block, List<Block>> extractBlocks(SymbolTable symbolTable) throws Er
       // blocks with external signatures keep their own symbol table
       if (bdata.getExternalSignature().isPresent()) {
         // symbolTable.insert(bdata.externalSignature.get().key);
-        blockExternalKeys.add(Optional.of(bdata.getExternalSignature().get().getKey()));
       } else {
-        blockExternalKeys.add(Optional.empty());
         for (String s : block.getSymbolTable().symbols()) {
           symbolTable.add(s);
         }
diff --git a/biscuit-core/src/main/java/org/eclipse/biscuit/token/format/SignedBlock.java b/biscuit-core/src/main/java/org/eclipse/biscuit/token/format/SignedBlock.java
index 21610dde..0629f0c4 100644
--- a/biscuit-core/src/main/java/org/eclipse/biscuit/token/format/SignedBlock.java
+++ b/biscuit-core/src/main/java/org/eclipse/biscuit/token/format/SignedBlock.java
@@ -9,11 +9,11 @@
 import org.eclipse.biscuit.crypto.PublicKey;
 
 public class SignedBlock {
-  private byte[] block;
-  private PublicKey key;
-  private byte[] signature;
-  private Optional<ExternalSignature> externalSignature;
-  private int version;
+  private final byte[] block;
+  private final PublicKey key;
+  private final byte[] signature;
+  private final Optional<ExternalSignature> externalSignature;
+  private final int version;
 
   public SignedBlock(
       byte[] block,
diff --git a/biscuit/src/test/java/org/eclipse/biscuit/datalog/ExpressionTest.java b/biscuit/src/test/java/org/eclipse/biscuit/datalog/ExpressionTest.java
index 93b607f5..5fd67380 100644
--- a/biscuit/src/test/java/org/eclipse/biscuit/datalog/ExpressionTest.java
+++ b/biscuit/src/test/java/org/eclipse/biscuit/datalog/ExpressionTest.java
@@ -27,7 +27,7 @@ public void testNegate() throws Error.Execution {
 
     Expression e =
         new Expression(
-            new ArrayList<Op>(
+            new ArrayList<>(
                 Arrays.asList(
                     new Term.Integer(1),
                     new Term.Variable(SymbolTable.DEFAULT_SYMBOLS_OFFSET + 2),
@@ -51,7 +51,7 @@ public void testAddsStr() throws Error.Execution {
 
     Expression e =
         new Expression(
-            new ArrayList<Op>(
+            new ArrayList<>(
                 Arrays.asList(
                     new Term.Str(SymbolTable.DEFAULT_SYMBOLS_OFFSET),
                     new Term.Str(SymbolTable.DEFAULT_SYMBOLS_OFFSET + 1),
@@ -72,7 +72,7 @@ public void testContainsStr() throws Error.Execution {
 
     Expression e =
         new Expression(
-            new ArrayList<Op>(
+            new ArrayList<>(
                 Arrays.asList(
                     new Term.Str(SymbolTable.DEFAULT_SYMBOLS_OFFSET),
                     new Term.Str(SymbolTable.DEFAULT_SYMBOLS_OFFSET + 1),
@@ -92,7 +92,7 @@ public void testNegativeContainsStr() throws Error.Execution {
 
     Expression e =
         new Expression(
-            new ArrayList<Op>(
+            new ArrayList<>(
                 Arrays.asList(
                     new Term.Str(SymbolTable.DEFAULT_SYMBOLS_OFFSET),
                     new Term.Str(SymbolTable.DEFAULT_SYMBOLS_OFFSET + 1),
@@ -111,7 +111,7 @@ public void testIntersectionAndContains() throws Error.Execution {
 
     Expression e =
         new Expression(
-            new ArrayList<Op>(
+            new ArrayList<>(
                 Arrays.asList(
                     new Term.Set(
                         new HashSet<>(
diff --git a/biscuit/src/test/java/org/eclipse/biscuit/datalog/WorldTest.java b/biscuit/src/test/java/org/eclipse/biscuit/datalog/WorldTest.java
index 844a0bef..a944cf0d 100644
--- a/biscuit/src/test/java/org/eclipse/biscuit/datalog/WorldTest.java
+++ b/biscuit/src/test/java/org/eclipse/biscuit/datalog/WorldTest.java
@@ -10,9 +10,9 @@
 
 import java.time.Instant;
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.HashSet;
 import java.util.Iterator;
+import java.util.List;
 import java.util.stream.Collectors;
 import org.eclipse.biscuit.datalog.expressions.Expression;
 import org.eclipse.biscuit.datalog.expressions.Op;
@@ -34,26 +34,26 @@ public void testFamily() throws Error {
     final long grandparent = syms.insert("grandparent");
     final long sibling = syms.insert("siblings");
 
-    w.addFact(new Origin(0), new Fact(new Predicate(parent, Arrays.asList(a, b))));
-    w.addFact(new Origin(0), new Fact(new Predicate(parent, Arrays.asList(b, c))));
-    w.addFact(new Origin(0), new Fact(new Predicate(parent, Arrays.asList(c, d))));
+    w.addFact(new Origin(0), new Fact(new Predicate(parent, List.of(a, b))));
+    w.addFact(new Origin(0), new Fact(new Predicate(parent, List.of(b, c))));
+    w.addFact(new Origin(0), new Fact(new Predicate(parent, List.of(c, d))));
 
     final Rule r1 =
         new Rule(
             new Predicate(
                 grandparent,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("grandparent")),
                     new Term.Variable(syms.insert("grandchild")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     parent,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("grandparent")),
                         new Term.Variable(syms.insert("parent")))),
                 new Predicate(
                     parent,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("parent")),
                         new Term.Variable(syms.insert("grandchild"))))),
             new ArrayList<>());
@@ -62,35 +62,29 @@ public void testFamily() throws Error {
     FactSet queryRuleResult = w.queryRule(r1, (long) 0, new TrustedOrigins(0), syms);
     System.out.println(
         "grandparents query_rules: ["
-            + String.join(
-                ", ",
-                queryRuleResult.stream()
-                    .map((f) -> syms.formatFact(f))
-                    .collect(Collectors.toList()))
+            + queryRuleResult.stream().map(syms::formatFact).collect(Collectors.joining(", "))
             + "]");
     System.out.println(
         "current facts: ["
-            + String.join(
-                ", ",
-                w.getFacts().stream().map((f) -> syms.formatFact(f)).collect(Collectors.toList()))
+            + w.getFacts().stream().map(syms::formatFact).collect(Collectors.joining(", "))
             + "]");
 
     final Rule r2 =
         new Rule(
             new Predicate(
                 grandparent,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("grandparent")),
                     new Term.Variable(syms.insert("grandchild")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     parent,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("grandparent")),
                         new Term.Variable(syms.insert("parent")))),
                 new Predicate(
                     parent,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("parent")),
                         new Term.Variable(syms.insert("grandchild"))))),
             new ArrayList<>());
@@ -104,13 +98,13 @@ public void testFamily() throws Error {
         new Rule(
             new Predicate(
                 parent,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("parent")),
                     new Term.Variable(syms.insert("child")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     parent,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("parent")),
                         new Term.Variable(syms.insert("child"))))),
             new ArrayList<>());
@@ -123,29 +117,28 @@ public void testFamily() throws Error {
     }
     final Rule query2 =
         new Rule(
-            new Predicate(parent, Arrays.asList(new Term.Variable(syms.insert("parent")), b)),
-            Arrays.asList(
-                new Predicate(parent, Arrays.asList(new Term.Variable(syms.insert("parent")), b))),
+            new Predicate(parent, List.of(new Term.Variable(syms.insert("parent")), b)),
+            List.of(new Predicate(parent, List.of(new Term.Variable(syms.insert("parent")), b))),
             new ArrayList<>());
     System.out.println(
         "parents of B: ["
             + String.join(
                 ", ",
                 w.queryRule(query2, (long) 0, new TrustedOrigins(0), syms).stream()
-                    .map((f) -> syms.formatFact(f))
+                    .map(syms::formatFact)
                     .collect(Collectors.toSet()))
             + "]");
     final Rule query3 =
         new Rule(
             new Predicate(
                 grandparent,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("grandparent")),
                     new Term.Variable(syms.insert("grandchild")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     grandparent,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("grandparent")),
                         new Term.Variable(syms.insert("grandchild"))))),
             new ArrayList<>());
@@ -154,42 +147,41 @@ public void testFamily() throws Error {
             + String.join(
                 ", ",
                 w.queryRule(query3, (long) 0, new TrustedOrigins(0), syms).stream()
-                    .map((f) -> syms.formatFact(f))
+                    .map(syms::formatFact)
                     .collect(Collectors.toSet()))
             + "]");
 
-    w.addFact(new Origin(0), new Fact(new Predicate(parent, Arrays.asList(c, e))));
+    w.addFact(new Origin(0), new Fact(new Predicate(parent, List.of(c, e))));
     w.run(syms);
 
     final Rule query4 =
         new Rule(
             new Predicate(
                 grandparent,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("grandparent")),
                     new Term.Variable(syms.insert("grandchild")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     grandparent,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("grandparent")),
                         new Term.Variable(syms.insert("grandchild"))))),
             new ArrayList<>());
     final FactSet res = w.queryRule(query4, (long) 0, new TrustedOrigins(0), syms);
     System.out.println(
         "grandparents after inserting parent(C, E): ["
-            + String.join(
-                ", ", res.stream().map((f) -> syms.formatFact(f)).collect(Collectors.toSet()))
+            + String.join(", ", res.stream().map(syms::formatFact).collect(Collectors.toSet()))
             + "]");
 
     final FactSet expected =
         new FactSet(
             new Origin(0),
             new HashSet<>(
-                Arrays.asList(
-                    new Fact(new Predicate(grandparent, Arrays.asList(a, c))),
-                    new Fact(new Predicate(grandparent, Arrays.asList(b, d))),
-                    new Fact(new Predicate(grandparent, Arrays.asList(b, e))))));
+                List.of(
+                    new Fact(new Predicate(grandparent, List.of(a, c))),
+                    new Fact(new Predicate(grandparent, List.of(b, d))),
+                    new Fact(new Predicate(grandparent, List.of(b, e))))));
     assertEquals(expected, res);
 
     w.addRule(
@@ -198,18 +190,18 @@ public void testFamily() throws Error {
         new Rule(
             new Predicate(
                 sibling,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("sibling1")),
                     new Term.Variable(syms.insert("sibling2")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     parent,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("parent")),
                         new Term.Variable(syms.insert("sibling1")))),
                 new Predicate(
                     parent,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("parent")),
                         new Term.Variable(syms.insert("sibling2"))))),
             new ArrayList<>()));
@@ -219,13 +211,13 @@ public void testFamily() throws Error {
         new Rule(
             new Predicate(
                 sibling,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("sibling1")),
                     new Term.Variable(syms.insert("sibling2")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     sibling,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("sibling1")),
                         new Term.Variable(syms.insert("sibling2"))))),
             new ArrayList<>());
@@ -234,7 +226,7 @@ public void testFamily() throws Error {
             + String.join(
                 ", ",
                 w.queryRule(query5, (long) 0, new TrustedOrigins(0), syms).stream()
-                    .map((f) -> syms.formatFact(f))
+                    .map(syms::formatFact)
                     .collect(Collectors.toSet()))
             + "]");
   }
@@ -256,39 +248,39 @@ public void testNumbers() throws Error {
     final long t2 = syms.insert("t2");
     final long join = syms.insert("join");
 
-    w.addFact(new Origin(0), new Fact(new Predicate(t1, Arrays.asList(new Term.Integer(0), abc))));
-    w.addFact(new Origin(0), new Fact(new Predicate(t1, Arrays.asList(new Term.Integer(1), def))));
-    w.addFact(new Origin(0), new Fact(new Predicate(t1, Arrays.asList(new Term.Integer(2), ghi))));
-    w.addFact(new Origin(0), new Fact(new Predicate(t1, Arrays.asList(new Term.Integer(3), jkl))));
-    w.addFact(new Origin(0), new Fact(new Predicate(t1, Arrays.asList(new Term.Integer(4), mno))));
+    w.addFact(new Origin(0), new Fact(new Predicate(t1, List.of(new Term.Integer(0), abc))));
+    w.addFact(new Origin(0), new Fact(new Predicate(t1, List.of(new Term.Integer(1), def))));
+    w.addFact(new Origin(0), new Fact(new Predicate(t1, List.of(new Term.Integer(2), ghi))));
+    w.addFact(new Origin(0), new Fact(new Predicate(t1, List.of(new Term.Integer(3), jkl))));
+    w.addFact(new Origin(0), new Fact(new Predicate(t1, List.of(new Term.Integer(4), mno))));
 
     w.addFact(
         new Origin(0),
-        new Fact(new Predicate(t2, Arrays.asList(new Term.Integer(0), aaa, new Term.Integer(0)))));
+        new Fact(new Predicate(t2, List.of(new Term.Integer(0), aaa, new Term.Integer(0)))));
     w.addFact(
         new Origin(0),
-        new Fact(new Predicate(t2, Arrays.asList(new Term.Integer(1), bbb, new Term.Integer(0)))));
+        new Fact(new Predicate(t2, List.of(new Term.Integer(1), bbb, new Term.Integer(0)))));
     w.addFact(
         new Origin(0),
-        new Fact(new Predicate(t2, Arrays.asList(new Term.Integer(2), ccc, new Term.Integer(1)))));
+        new Fact(new Predicate(t2, List.of(new Term.Integer(2), ccc, new Term.Integer(1)))));
 
     FactSet res =
         w.queryRule(
             new Rule(
                 new Predicate(
                     join,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("left")),
                         new Term.Variable(syms.insert("right")))),
-                Arrays.asList(
+                List.of(
                     new Predicate(
                         t1,
-                        Arrays.asList(
+                        List.of(
                             new Term.Variable(syms.insert("id")),
                             new Term.Variable(syms.insert("left")))),
                     new Predicate(
                         t2,
-                        Arrays.asList(
+                        List.of(
                             new Term.Variable(syms.insert("t2_id")),
                             new Term.Variable(syms.insert("right")),
                             new Term.Variable(syms.insert("id"))))),
@@ -304,10 +296,10 @@ public void testNumbers() throws Error {
         new FactSet(
             new Origin(0),
             new HashSet<>(
-                Arrays.asList(
-                    new Fact(new Predicate(join, Arrays.asList(abc, aaa))),
-                    new Fact(new Predicate(join, Arrays.asList(abc, bbb))),
-                    new Fact(new Predicate(join, Arrays.asList(def, ccc))))));
+                List.of(
+                    new Fact(new Predicate(join, List.of(abc, aaa))),
+                    new Fact(new Predicate(join, List.of(abc, bbb))),
+                    new Fact(new Predicate(join, List.of(def, ccc))))));
     assertEquals(expected, res);
 
     res =
@@ -315,25 +307,25 @@ public void testNumbers() throws Error {
             new Rule(
                 new Predicate(
                     join,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("left")),
                         new Term.Variable(syms.insert("right")))),
-                Arrays.asList(
+                List.of(
                     new Predicate(
                         t1,
-                        Arrays.asList(
+                        List.of(
                             new Term.Variable(syms.insert("id")),
                             new Term.Variable(syms.insert("left")))),
                     new Predicate(
                         t2,
-                        Arrays.asList(
+                        List.of(
                             new Term.Variable(syms.insert("t2_id")),
                             new Term.Variable(syms.insert("right")),
                             new Term.Variable(syms.insert("id"))))),
-                Arrays.asList(
+                List.of(
                     new Expression(
-                        new ArrayList<Op>(
-                            Arrays.asList(
+                        new ArrayList<>(
+                            List.of(
                                 new Term.Variable(syms.insert("id")),
                                 new Term.Integer(1),
                                 new Op.Binary(Op.BinaryOp.LessThan)))))),
@@ -348,33 +340,33 @@ public void testNumbers() throws Error {
         new FactSet(
             new Origin(0),
             new HashSet<>(
-                Arrays.asList(
-                    new Fact(new Predicate(join, Arrays.asList(abc, aaa))),
-                    new Fact(new Predicate(join, Arrays.asList(abc, bbb))))));
+                List.of(
+                    new Fact(new Predicate(join, List.of(abc, aaa))),
+                    new Fact(new Predicate(join, List.of(abc, bbb))))));
     assertEquals(expected, res);
   }
 
-  private final FactSet testSuffix(
+  private FactSet testSuffix(
       final World w, SymbolTable syms, final long suff, final long route, final String suffix)
       throws Error {
     return w.queryRule(
         new Rule(
             new Predicate(
                 suff,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("app_id")),
                     new Term.Variable(syms.insert("domain")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     route,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("route_id")),
                         new Term.Variable(syms.insert("app_id")),
                         new Term.Variable(syms.insert("domain"))))),
-            Arrays.asList(
+            List.of(
                 new Expression(
-                    new ArrayList<Op>(
-                        Arrays.asList(
+                    new ArrayList<>(
+                        List.of(
                             new Term.Variable(syms.insert("domain")),
                             syms.add(suffix),
                             new Op.Binary(Op.BinaryOp.Suffix)))))),
@@ -397,26 +389,22 @@ public void testStr() throws Error {
     w.addFact(
         new Origin(0),
         new Fact(
-            new Predicate(
-                route, Arrays.asList(new Term.Integer(0), app_0, syms.add("example.com")))));
+            new Predicate(route, List.of(new Term.Integer(0), app_0, syms.add("example.com")))));
     w.addFact(
         new Origin(0),
-        new Fact(
-            new Predicate(route, Arrays.asList(new Term.Integer(1), app_1, syms.add("test.com")))));
+        new Fact(new Predicate(route, List.of(new Term.Integer(1), app_1, syms.add("test.com")))));
     w.addFact(
         new Origin(0),
-        new Fact(
-            new Predicate(route, Arrays.asList(new Term.Integer(2), app_2, syms.add("test.fr")))));
+        new Fact(new Predicate(route, List.of(new Term.Integer(2), app_2, syms.add("test.fr")))));
     w.addFact(
         new Origin(0),
         new Fact(
             new Predicate(
-                route, Arrays.asList(new Term.Integer(3), app_0, syms.add("www.example.com")))));
+                route, List.of(new Term.Integer(3), app_0, syms.add("www.example.com")))));
     w.addFact(
         new Origin(0),
         new Fact(
-            new Predicate(
-                route, Arrays.asList(new Term.Integer(4), app_1, syms.add("mx.example.com")))));
+            new Predicate(route, List.of(new Term.Integer(4), app_1, syms.add("mx.example.com")))));
 
     FactSet res = testSuffix(w, syms, suff, route, ".fr");
     for (Iterator<Fact> it = res.stream().iterator(); it.hasNext(); ) {
@@ -427,8 +415,7 @@ public void testStr() throws Error {
         new FactSet(
             new Origin(0),
             new HashSet<>(
-                Arrays.asList(
-                    new Fact(new Predicate(suff, Arrays.asList(app_2, syms.add("test.fr")))))));
+                List.of(new Fact(new Predicate(suff, List.of(app_2, syms.add("test.fr")))))));
     assertEquals(expected, res);
 
     res = testSuffix(w, syms, suff, route, "example.com");
@@ -440,12 +427,10 @@ public void testStr() throws Error {
         new FactSet(
             new Origin(0),
             new HashSet<>(
-                Arrays.asList(
-                    new Fact(new Predicate(suff, Arrays.asList(app_0, syms.add("example.com")))),
-                    new Fact(
-                        new Predicate(suff, Arrays.asList(app_0, syms.add("www.example.com")))),
-                    new Fact(
-                        new Predicate(suff, Arrays.asList(app_1, syms.add("mx.example.com")))))));
+                List.of(
+                    new Fact(new Predicate(suff, List.of(app_0, syms.add("example.com")))),
+                    new Fact(new Predicate(suff, List.of(app_0, syms.add("www.example.com")))),
+                    new Fact(new Predicate(suff, List.of(app_1, syms.add("mx.example.com")))))));
     assertEquals(expected, res);
   }
 
@@ -471,33 +456,33 @@ public void testDate() throws Error {
 
     w.addFact(
         new Origin(0),
-        new Fact(new Predicate(x, Arrays.asList(new Term.Date(t1.getEpochSecond()), abc))));
+        new Fact(new Predicate(x, List.of(new Term.Date(t1.getEpochSecond()), abc))));
     w.addFact(
         new Origin(0),
-        new Fact(new Predicate(x, Arrays.asList(new Term.Date(t3.getEpochSecond()), def))));
+        new Fact(new Predicate(x, List.of(new Term.Date(t3.getEpochSecond()), def))));
 
     final Rule r1 =
         new Rule(
             new Predicate(
                 before,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("date")), new Term.Variable(syms.insert("val")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     x,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("date")),
                         new Term.Variable(syms.insert("val"))))),
-            Arrays.asList(
+            List.of(
                 new Expression(
-                    new ArrayList<Op>(
-                        Arrays.asList(
+                    new ArrayList<>(
+                        List.of(
                             new Term.Variable(syms.insert("date")),
                             new Term.Date(t2_timestamp),
                             new Op.Binary(Op.BinaryOp.LessOrEqual)))),
                 new Expression(
-                    new ArrayList<Op>(
-                        Arrays.asList(
+                    new ArrayList<>(
+                        List.of(
                             new Term.Variable(syms.insert("date")),
                             new Term.Date(0),
                             new Op.Binary(Op.BinaryOp.GreaterOrEqual))))));
@@ -512,34 +497,33 @@ public void testDate() throws Error {
         new FactSet(
             new Origin(0),
             new HashSet<>(
-                Arrays.asList(
+                List.of(
                     new Fact(
-                        new Predicate(
-                            before, Arrays.asList(new Term.Date(t1.getEpochSecond()), abc))))));
+                        new Predicate(before, List.of(new Term.Date(t1.getEpochSecond()), abc))))));
     assertEquals(expected, res);
 
     final Rule r2 =
         new Rule(
             new Predicate(
                 after,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("date")), new Term.Variable(syms.insert("val")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     x,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("date")),
                         new Term.Variable(syms.insert("val"))))),
-            Arrays.asList(
+            List.of(
                 new Expression(
-                    new ArrayList<Op>(
-                        Arrays.asList(
+                    new ArrayList<>(
+                        List.of(
                             new Term.Variable(syms.insert("date")),
                             new Term.Date(t2_timestamp),
                             new Op.Binary(Op.BinaryOp.GreaterOrEqual)))),
                 new Expression(
-                    new ArrayList<Op>(
-                        Arrays.asList(
+                    new ArrayList<>(
+                        List.of(
                             new Term.Variable(syms.insert("date")),
                             new Term.Date(0),
                             new Op.Binary(Op.BinaryOp.GreaterOrEqual))))));
@@ -554,10 +538,9 @@ public void testDate() throws Error {
         new FactSet(
             new Origin(0),
             new HashSet<>(
-                Arrays.asList(
+                List.of(
                     new Fact(
-                        new Predicate(
-                            after, Arrays.asList(new Term.Date(t3.getEpochSecond()), def))))));
+                        new Predicate(after, List.of(new Term.Date(t3.getEpochSecond()), def))))));
     assertEquals(expected, res);
   }
 
@@ -575,31 +558,30 @@ public void testSet() throws Error {
 
     w.addFact(
         new Origin(0),
-        new Fact(new Predicate(x, Arrays.asList(abc, new Term.Integer(0), syms.add("test")))));
+        new Fact(new Predicate(x, List.of(abc, new Term.Integer(0), syms.add("test")))));
     w.addFact(
         new Origin(0),
-        new Fact(new Predicate(x, Arrays.asList(def, new Term.Integer(2), syms.add("hello")))));
+        new Fact(new Predicate(x, List.of(def, new Term.Integer(2), syms.add("hello")))));
 
     final Rule r1 =
         new Rule(
             new Predicate(
                 int_set,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("sym")), new Term.Variable(syms.insert("str")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     x,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("sym")),
                         new Term.Variable(syms.insert("int")),
                         new Term.Variable(syms.insert("str"))))),
-            Arrays.asList(
+            List.of(
                 new Expression(
-                    new ArrayList<Op>(
-                        Arrays.asList(
+                    new ArrayList<>(
+                        List.of(
                             new Term.Set(
-                                new HashSet<>(
-                                    Arrays.asList(new Term.Integer(0L), new Term.Integer(1L)))),
+                                new HashSet<>(List.of(new Term.Integer(0L), new Term.Integer(1L)))),
                             new Term.Variable(syms.insert("int")),
                             new Op.Binary(Op.BinaryOp.Contains))))));
     System.out.println("testing r1: " + syms.formatRule(r1));
@@ -612,8 +594,7 @@ public void testSet() throws Error {
         new FactSet(
             new Origin(0),
             new HashSet<>(
-                Arrays.asList(
-                    new Fact(new Predicate(int_set, Arrays.asList(abc, syms.add("test")))))));
+                List.of(new Fact(new Predicate(int_set, List.of(abc, syms.add("test")))))));
     assertEquals(expected, res);
 
     final long abcSymId = syms.insert("abc");
@@ -623,24 +604,24 @@ public void testSet() throws Error {
         new Rule(
             new Predicate(
                 symbol_set,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("sym")),
                     new Term.Variable(syms.insert("int")),
                     new Term.Variable(syms.insert("str")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     x,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("sym")),
                         new Term.Variable(syms.insert("int")),
                         new Term.Variable(syms.insert("str"))))),
-            Arrays.asList(
+            List.of(
                 new Expression(
-                    new ArrayList<Op>(
-                        Arrays.asList(
+                    new ArrayList<>(
+                        List.of(
                             new Term.Set(
                                 new HashSet<>(
-                                    Arrays.asList(new Term.Str(abcSymId), new Term.Str(ghiSymId)))),
+                                    List.of(new Term.Str(abcSymId), new Term.Str(ghiSymId)))),
                             new Term.Variable(syms.insert("sym")),
                             new Op.Binary(Op.BinaryOp.Contains),
                             new Op.Unary(Op.UnaryOp.Negate))))));
@@ -655,34 +636,32 @@ public void testSet() throws Error {
         new FactSet(
             new Origin(0),
             new HashSet<>(
-                Arrays.asList(
+                List.of(
                     new Fact(
                         new Predicate(
-                            symbol_set,
-                            Arrays.asList(def, new Term.Integer(2), syms.add("hello")))))));
+                            symbol_set, List.of(def, new Term.Integer(2), syms.add("hello")))))));
     assertEquals(expected, res);
 
     final Rule r3 =
         new Rule(
             new Predicate(
                 string_set,
-                Arrays.asList(
+                List.of(
                     new Term.Variable(syms.insert("sym")),
                     new Term.Variable(syms.insert("int")),
                     new Term.Variable(syms.insert("str")))),
-            Arrays.asList(
+            List.of(
                 new Predicate(
                     x,
-                    Arrays.asList(
+                    List.of(
                         new Term.Variable(syms.insert("sym")),
                         new Term.Variable(syms.insert("int")),
                         new Term.Variable(syms.insert("str"))))),
-            Arrays.asList(
+            List.of(
                 new Expression(
-                    new ArrayList<Op>(
-                        Arrays.asList(
-                            new Term.Set(
-                                new HashSet<>(Arrays.asList(syms.add("test"), syms.add("aaa")))),
+                    new ArrayList<>(
+                        List.of(
+                            new Term.Set(new HashSet<>(List.of(syms.add("test"), syms.add("aaa")))),
                             new Term.Variable(syms.insert("str")),
                             new Op.Binary(Op.BinaryOp.Contains))))));
     System.out.println("testing r3: " + syms.formatRule(r3));
@@ -695,11 +674,10 @@ public void testSet() throws Error {
         new FactSet(
             new Origin(0),
             new HashSet<>(
-                Arrays.asList(
+                List.of(
                     new Fact(
                         new Predicate(
-                            string_set,
-                            Arrays.asList(abc, new Term.Integer(0), syms.add("test")))))));
+                            string_set, List.of(abc, new Term.Integer(0), syms.add("test")))))));
     assertEquals(expected, res);
   }
 
@@ -718,16 +696,16 @@ public void testResource() throws Error {
     final Term read = syms.add("read");
     final Term write = syms.add("write");
 
-    w.addFact(new Origin(0), new Fact(new Predicate(right, Arrays.asList(file1, read))));
-    w.addFact(new Origin(0), new Fact(new Predicate(right, Arrays.asList(file2, read))));
-    w.addFact(new Origin(0), new Fact(new Predicate(right, Arrays.asList(file1, write))));
+    w.addFact(new Origin(0), new Fact(new Predicate(right, List.of(file1, read))));
+    w.addFact(new Origin(0), new Fact(new Predicate(right, List.of(file2, read))));
+    w.addFact(new Origin(0), new Fact(new Predicate(right, List.of(file1, write))));
 
     final long caveat1 = syms.insert("caveat1");
     // r1: caveat2(#file1) <- resource(#ambient, #file1)
     final Rule r1 =
         new Rule(
-            new Predicate(caveat1, Arrays.asList(file1)),
-            Arrays.asList(new Predicate(resource, Arrays.asList(file1))),
+            new Predicate(caveat1, List.of(file1)),
+            List.of(new Predicate(resource, List.of(file1))),
             new ArrayList<>());
 
     System.out.println("testing caveat 1(should return nothing): " + syms.formatRule(r1));
@@ -746,11 +724,11 @@ public void testResource() throws Error {
     // 0?, #read)
     final Rule r2 =
         new Rule(
-            new Predicate(caveat2, Arrays.asList(var0)),
-            Arrays.asList(
-                new Predicate(resource, Arrays.asList(var0)),
-                new Predicate(operation, Arrays.asList(read)),
-                new Predicate(right, Arrays.asList(var0, read))),
+            new Predicate(caveat2, List.of(var0)),
+            List.of(
+                new Predicate(resource, List.of(var0)),
+                new Predicate(operation, List.of(read)),
+                new Predicate(right, List.of(var0, read))),
             new ArrayList<>());
 
     System.out.println("testing caveat 2: " + syms.formatRule(r2));