Starter Set of Functional Interfaces

Java Development Kit 8 has a number of functional interfaces. Here we review the starter set—the interfaces we frequently encounter and need to first get familiar with. All the interfaces we see here are part of the java.util.function package.

Consumer<T>

Description

Represents an operation that will accept an input and returns nothing. For this to be useful, it will have to cause side effects.

Abstract method

accept()

Default method(s)

andThen()

Popular usage

As a parameter to the forEach() method

Primitive specializations

IntConsumer, LongConsumer, DoubleConsumer, …

Supplier<T>

Description

A factory that’s expected to return either a new instance or a precreated instance

Abstract method

get()

Default method(s)

—

Popular usage

To create lazy infinite Streams and as the parameter to the Optional class’s orElseGet() method

Primitive specializations

IntSupplier, LongSupplier, DoubleSupplier, …

Predicate<T>

Description

Useful for checking if an input argument satisfies some condition

Abstract method

test()

Default method(s)

and(), negate(), and or()

Popular usage

As a parameter to Stream’s methods, like filter() and anyMatch()

Primitive specializations

IntPredicate, LongPredicate, DoublePredicate, …

Function<T, R>

Description

A transformational interface that represents an operation intended to take in an argument and return an appropriate result

Abstract method

apply()

Default method(s)

andThen(), compose()

Popular usage

As a parameter to Stream’s map() method

Primitive specializations

IntFunction, LongFunction, DoubleFunction, IntToDoubleFunction, DoubleToIntFunction, …