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Functional Programming

Introducing Functional Programming

Java defines a functional interface as a interface that contains a single abstract method. Functional interfaces are used as the basis for lambda expressions in functional programming. A lambda expression is a block of code that gets passed around, like an anonymous method.

The example code uses the lambda expression a -> a.canHoop(); where the arrow separates the parameter (a) from the body (a.canHoop()). The same expression could be also definded as (Animal a) -> {return a.canHoop()};

Built-In Functional Interfaces

The functional interfaces below were introduced in the java.util.function package with Java 8.

Functional interface #Parameters Return Type Single Abstract Method Example code
Supplier<T> 0 T get supplier code
Consumer<T> 1 (T) void accept consumer code
BiConsumer<T,U> 2 (T,U) void accept biConsumer code
Predicate<T> 1 (T) boolean test predicate code
BiPredicate<T,U> 2 (T,U) boolean test biPredicate code
Function<T,R> 1 (T) R apply function code
BiFunction<T,U,R> 2 (T,U) R apply biFunction code
UnaryOperator<T> 1 (T) T apply unary operator code
BinaryOperator<T> 2 (T,T) T apply binary operator code

Streams

A stream in Java is a sequence of data. A stream pipeline is the operations that run on a stream to produce a result. We can think of a stream pipeline as an assembly line in a factory. There are three parts to a stream pipeline:

  • Source: Where the stream comes from.
  • Intermediate operations: Transforms the stream into another one. There can be as few or as a many intermediate operations as you'd like.
  • Terminal operation: Actually produces a result.

Creating Stream Sources

In Java, the Stream interface is the java.util.stream package. There are a few ways to create a finite stream:

  1. Stream<String> empty = Stream.empty(); it creates an empty stream.
  2. Stream<Integer> singleElement = Stream.of(1); it creates a stream with a single element.
  3. Stream<Integer> fromArray = Stream.of(1,2,3); it creates a stream from an array.

Java also provides a convenient way to convert from a list to a stream:

  1. List<String> list = Arrays.asList("a","b","c");
  2. Stream<String> fromList = list.stream();

We can't create an infinite list, though, which makes streams more powerful:

  1. Stream<Double> randoms = Stream.generate(Math::randonm); it creates an infinite stream of random numbers.
  2. Stream<Integer> oddNumbers = Stream.iterate(1,n -> n+2); it generates odd numbers as long as you need them.

Using Common Terminal Operations

It's possible to perform a terminal operation without any intermediate operations but not the other way around.

Method What happends for infinite streams Return value Reduction Example code
count() Does not terminate long Yes count code
min()/max() Does not terminate Optional<T> Yes min code
findAny()/findFirst() Terminates Optional<T> No findAny code
allMatch()/anyMatch()/noneMatch() Sometimes terminates boolean No anyMatch code
forEach() Does not terminate void No anyMatch code
reduce() Does not terminate Varies yes reduce code
collect() Does not terminate Varies yes collect code