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:

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

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

6. Stream<Double> randoms = Stream.generate(Math::randonm); it creates an infinite stream of random numbers.
7. 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