Java: Functional Interfaces Overview

Systematic overview of the most important functional interfaces in Java.

Quick links:

• Runnable
• Suppliers
• Consumers
• Predicates
• Functions
• Operators

Runnable

The simplest functional interface, which has been around since the beginning of Java. Takes no input, and has no output.

Class	Return	Method	Params
Runnable	void	run	-

Suppliers

Supply output without input.

Class	Return	Method	Params
Supplier	T	get	-
Primitive Return
BooleanSupplier	boolean	getAsBoolean	-
DoubleSupplier	double	getAsDouble	-
IntSupplier	int	getAsInt	-
LongSupplier	long	getAsLong	-

Consumers

Consume input without output.

Class	Return	Method	Params
Consumer	void	accept	T
Primitive Parameter
DoubleConsumer	void	accept	double
IntConsumer	void	accept	int
LongConsumer	void	accept	long
Two Parameters
BiConsumer	void	accept	T, U
Two Parameters (One Primitive)
ObjDoubleConsumer	void	accept	T, double
ObjIntConsumer	void	accept	T, int
ObjLongConsumer	void	accept	T, long

Predicates

Return a boolean depending on some input.

Class	Return	Method	Params
Predicate	boolean	test	T
Primitive Parameter
DoublePredicate	boolean	test	double
IntPredicate	boolean	test	int
LongPredicate	boolean	test	long
Two Parameters
BiPredicate	boolean	test	T, U

Functions

Transform input to output.

Class	Return	Method	Params
Function	R	apply	T
Primitive Parameter
DoubleFunction	R	apply	double
IntFunction	R	apply	int
LongFunction	R	apply	long
Primitive Return
ToDoubleFunction	double	applyAsDouble	T
ToIntFunction	int	applyAsInt	T
ToLongFunction	long	applyAsLong	T
Primitive Parameter and Return
DoubleToIntFunction	int	applyAsInt	double
DoubleToLongFunction	long	applyAsLong	double
IntToDoubleFunction	double	applyAsDouble	int
IntToLongFunction	long	applyAsLong	int
LongToDoubleFunction	double	applyAsDouble	long
LongToIntFunction	int	applyAsInt	long
Two Parameters
BiFunction	R	apply	T, U
Two Primitive Parameters
ToDoubleBiFunction	double	applyAsDouble	T, U
ToIntBiFunction	int	applyAsInt	T, U
ToLongBiFunction	long	applyAsLong	T, U

Operators

Specialized functions where input and output have the same type.

Class	Return	Method	Params
UnaryOperator	T	apply	T
Primitive
DoubleUnaryOperator	double	applyAsDouble	double
IntUnaryOperator	int	applyAsInt	int
LongUnaryOperator	long	applyAsLong	long
Two Parameters
BinaryOperator	T	apply	T, T
Two Primitive Parameters
DoubleBinaryOperator	double	applyAsDouble	double, double
IntBinaryOperator	int	applyAsInt	int, int
LongBinaryOperator	long	applyAsLong	long, long