Java Streams Examples

Article

The code refers to the following articles:

• English: Java Streams (with Examples)
• German: Java Streams (mit Beispielen)

and, in the package eu.happycoders.lambdas, to the articles

• English: Java Lambda Expressions (with Examples)
• German: Java Lambda-Ausdrücke (mit Beispielen)

and, in the package eu.happycoders.functionalinterfaces, to the articles

• English: Java Functional Interfaces (with Examples)
• German: Funktionale Interfaces in Java (mit Beispielen)

and, in the package eu.happycoders.reduce, to the articles

• English: Java Stream reduce() (with Examples)
• German: Java Stream reduce() (mit Beispielen)

and, in the package eu.happycoders.tomap, to the article

• German: Java Collectors.toMap() (mit Beispielen)

Contents

All examples work on the same data model: the records Book and Author, the enum Genre, and the Library class with eleven public-domain classics.

Every class with a main() method is one chapter of the article and prints the outputs shown there. Run one with java -cp target/classes eu.happycoders.streams.<ClassName> after mvn compile:

• Ch1Intro – the same task as a loop and as a stream; the anatomy of a stream pipeline
• Ch2Lambdas – lambda expressions, method references, functional interfaces
• Ch3Sources – creating streams from collections, arrays, ranges, files, and generators
• Ch4Intermediate – filter(), map(), flatMap(), mapMulti(), distinct(), sorted(), limit(), skip(), takeWhile(), dropWhile(), peek(), gather()
• Ch5Terminal – forEach(), collect() and collectors, toList(), toArray(), reduce(), count(), min(), max(), findFirst(), anyMatch(), and the others
• Ch6Lazy – lazy evaluation: element-by-element processing, short-circuiting, stateful operations, single use
• Ch7Parallel – parallel streams
• Ch8Mistakes – common mistakes and how to avoid them

The lambda article has its own chapters in eu.happycoders.lambdas, on the same data model (run them with java -cp target/classes eu.happycoders.lambdas.<ClassName>):

• Ch1AnonymousVsLambda – the same comparator as an anonymous class and as a lambda
• Ch2Syntax – parameters, expression and block body, explicit types, var, the unnamed parameter _
• Ch3TargetType – the four contexts of a target type, functional interfaces, overloaded methods
• Ch4VariableAccess – effectively final, the one-element-array workaround, this (with Scope and BookFilter)
• Ch5MethodReferences – the four kinds of method references, this:: and super::, an ambiguous reference
• Ch6WhereUsed – lambdas in Comparator, collections, maps, Optional, and threads
• Ch7UnderTheHood – the generated class, identity of capturing and non-capturing lambdas (with LambdaDemo for javap)
• Ch8Mistakes – a lambda that is too long, recursion, a lambda in a stack trace (with LambdaTrace)

The functional-interfaces article has its chapters in eu.happycoders.functionalinterfaces (run them with java -cp target/classes eu.happycoders.functionalinterfaces.<ClassName> from the repository root - two chapters read files relative to it):

• Ch1Definition – Runnable as the simplest case; Comparator implemented by a named class (YearComparator) and by a lambda; the abstract method called directly; default methods
• Ch2Annotation – @FunctionalInterface on TitleFormatter, the error for a second abstract method, an interface without the annotation
• Ch3CoreFour – Function, Predicate, Consumer, Supplier with andThen(), compose(), identity(), and(), or(), negate(), not(), isEqual(), orElseGet(), toCollection()
• Ch4Operators – UnaryOperator in List.replaceAll(), BinaryOperator in reduce(), maxBy(), Map.merge()
• Ch5TwoParameters – BiFunction, BiConsumer, BiPredicate in Map.forEach(), computeIfPresent(), replaceAll(), Files.find()
• Ch6Primitives – ToIntFunction, IntPredicate, IntUnaryOperator, IntFunction, IntBinaryOperator, IntSupplier, ObjIntConsumer on an IntStream; boxing
• Ch7Wildcards – Predicate<? super T> and Function<? super T, ? extends R>, and a method of our own with and without the wildcard
• Ch8OutsidePackage – Runnable, Callable, Comparator, FileFilter, DirectoryStream.Filter, TemporalAdjuster
• Ch9Custom – TitleFormatter vs. Function (nominal typing), ThrowingFunction for checked exceptions
• Ch10Mistakes – Function<T, Boolean> instead of Predicate, Function<T, Void> instead of Consumer, UnaryOperator<Integer> instead of IntUnaryOperator

The reduce() article has its chapters in eu.happycoders.reduce (run them with java -cp target/classes eu.happycoders.reduce.<ClassName>):

• Ch1HowItWorks – the sum of 1 to 5 with reduce() and as a loop; every call of the accumulator
• Ch2Variants – the three variants of reduce() on full and empty streams, and the two of IntStream
• Ch3Applications – product, minimum with BinaryOperator.minBy(), a BigDecimal sum (with Prices) and a sum of the record Money, DoubleStream.sum() against reduce(), predicates and functions chained with reduce(), and and(), andThen() and allMatch() as the more readable alternatives
• Ch4Parallel – accumulator and combiner calls in a parallel stream; a non-neutral identity, a non-associative accumulator, and the accumulator used as combiner, each with its fix
• Ch5ReduceVsCollect – an ArrayList as identity, collect() and toList(), string concatenation against joining(), Collectors.reducing() as downstream collector
• Ch6Fold – Gatherers.fold() in a parallel stream
• Ch7Mistakes – get() on an empty Optional, mapToInt() instead of boxing, overflow with Math::multiplyExact, BigInteger

The toMap() article has its chapters in eu.happycoders.tomap (run them with java -cp target/classes eu.happycoders.tomap.<ClassName>):

• Ch1HowItWorks – title to year with toMap() and as a loop; Function.identity() for a lookup map
• Ch2Variants – the three variants of toMap(): a duplicate key, the merge function, TreeMap::new; a throwing merge function against collectingAndThen()
• Ch3Merging – counting with Integer::sum, concatenating titles (and groupingBy() with joining()), the oldest book with BinaryOperator.minBy()
• Ch4MapType – the order of the HashMap, LinkedHashMap in stream order, EnumMap per genre
• Ch5Null – the NullPointerException for a null value with two and three arguments, filter() and collect() with three arguments as ways out, a null key in a TreeMap
• Ch6Unmodifiable – toUnmodifiableMap(): UnsupportedOperationException, merge function, null key
• Ch7Applications – filtering, transforming and inverting a map; two lists to one map
• Ch8VsGroupingBy – lists and counts with toMap() and groupingBy(), nested maps with toMap() as downstream collector
• Ch9Parallel – the later number wins: toMap() against toConcurrentMap() in a parallel stream
• Ch10Mistakes – a merge function that returns null, and counting first as the fix; swallowed duplicates

The code requires Java 25 or newer.

Additional Resources

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