Java 8 introduced basic support for first-class functions. The functions, unlike in other languages, aren’t represented by only handful of types. Instead, Java 8 uses dozens of various types depending on arity, parameter types and the return type. The standard documentation is pretty unwieldy, so for my and your convenience, I prepared a list of all functional interfaces in a more useful order.

Few things to have in mind:

• All interfaces are in java.util.function package unless otherwise noted.

• Interfaces are specialized only for boolean, int, long and double, and only sometimes. Other primitive types, and all primitive types in certain situations, will have to be boxed.

Nullary functions

function type	Java type
() → void	java.lang.Runnable
() → boolean	BooleanSupplier
() → int	IntSupplier
() → long	LongSupplier
() → double	DoubleSupplier
() → A	Supplier<A> or java.lang.Callable<A>

Whether to use Supplier or Callable, it’s mostly a matter of taste and semantics. Supplier has a method called get, Callable has call. This suggests that functions with larger side-effects should be Callable, and functions with small side-effects (mostly lazily-initialized values) should be Suppliers. Of course, this is only a suggestion.

---

Unary functions

function type	Java type
int → void	IntConsumer
long → void	LongConsumer
double → void	DoubleConsumer
A → void	Consumer<A>
int → boolean	IntPredicate
long → boolean	LongPredicate
double → boolean	DoublePredicate
A → boolean	Predicate<A>
int → int	IntUnaryOperator
long → int	LongToIntFunction
double → int	DoubleToIntFunction
A → int	ToIntFunction<A>
int → long	IntToLongFunction
long → long	LongUnaryOperator
double → long	DoubleToLongFunction
A → long	ToLongFunction<A>
int → double	IntToDoubleFunction
long → double	LongToDoubleFunction
double → double	DoubleUnaryOperator
A → double	ToDoubleFunction<A>
A → A	UnaryOperator<A>
A → B	Function<A, B>

Note the following:

• The primitive type of the argument is attached directly, the primitive type of the result uses the prefix To.
• If the result is boolean, then the function is called a predicate. It’ it’s void, then a consumer.
• If the parameter type and the result type are the same, the function is called a unary operator.
• There are no specialized interfaces from a primitive to a reference.

---

Binary functions

function type	Java type
(A, int) → void	ObjIntConsumer<A>
(A, long) → void	ObjLongConsumer<A>
(A, double) → void	ObjDoubleConsumer<A>
(A, B) → void	BiConsumer<A, B>
(A, B) → boolean	BiPredicate<A, B>
(int, int) → int	IntBinaryOperator
(long, long) → long	LongBinaryOperator
(double, double) → double	DoubleBinaryOperator
(A, A) → A	BinaryOperator<A>
(A, B) → int	ToIntFunction<A, B>
(A, B) → long	ToLongBiFunction<A, B>
(A, B) → double	ToDoubleBiFunction<A, B>
(A, B) → C	BiFunction<A, B, C>

---

Ternary functions and above

None.