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, …
