Functional Interfaces

Introduction#

In Java 8+, a functional interface is an interface that has just one abstract method (aside from the methods of Object). See JLS §9.8. Functional Interfaces.

List of standard Java Runtime Library functional interfaces by signature

Parameter Types	Return Type	Interface
()	void	Runnable
()	T	Supplier
()	boolean	BooleanSupplier
()	int	IntSupplier
()	long	LongSupplier
()	double	DoubleSupplier
(T)	void	Consumer<T>
(T)	T	UnaryOperator<T>
(T)	R	Function<T,R>
(T)	boolean	Predicate<T>
(T)	int	ToIntFunction<T>
(T)	long	ToLongFunction<T>
(T)	double	ToDoubleFunction<T>
(T, T)	T	BinaryOperator<T>
(T, U)	void	BiConsumer<T,U>
(T, U)	R	BiFunction<T,U,R>
(T, U)	boolean	BiPredicate<T,U>
(T, U)	int	ToIntBiFunction<T,U>
(T, U)	long	ToLongBiFunction<T,U>
(T, U)	double	ToDoubleBiFunction<T,U>
(T, int)	void	ObjIntConsumer<T>
(T, long)	void	ObjLongConsumer<T>
(T, double)	void	ObjDoubleConsumer<T>
(int)	void	IntConsumer
(int)	R	IntFunction<R>
(int)	boolean	IntPredicate
(int)	int	IntUnaryOperator
(int)	long	IntToLongFunction
(int)	double	IntToDoubleFunction
(int, int)	int	IntBinaryOperator
(long)	void	LongConsumer
(long)	R	LongFunction<R>
(long)	boolean	LongPredicate
(long)	int	LongToIntFunction
(long)	long	LongUnaryOperator
(long)	double	LongToDoubleFunction
(long, long)	long	LongBinaryOperator
(double)	void	DoubleConsumer
(double)	R	DoubleFunction<R>
(double)	boolean	DoublePredicate
(double)	int	DoubleToIntFunction
(double)	long	DoubleToLongFunction
(double)	double	DoubleUnaryOperator
(double, double)	double	DoubleBinaryOperator