Added in API level 24

java.util.function

Functional interfaces provide target types for lambda expressions and method references. Each functional interface has a single abstract method, called the functional method for that functional interface, to which the lambda expression's parameter and return types are matched or adapted. Functional interfaces can provide a target type in multiple contexts, such as assignment context, method invocation, or cast context:

// Assignment context
     Predicate<String> p = String::isEmpty;

     // Method invocation context
     stream.filter(e -> e.getSize() > 10)...

     // Cast context
     stream.map((ToIntFunction) e -> e.getSize())...

The interfaces in this package are general purpose functional interfaces used by the JDK, and are available to be used by user code as well. While they do not identify a complete set of function shapes to which lambda expressions might be adapted, they provide enough to cover common requirements. Other functional interfaces provided for specific purposes, such as FileFilter, are defined in the packages where they are used.

The interfaces in this package are annotated with FunctionalInterface. This annotation is not a requirement for the compiler to recognize an interface as a functional interface, but merely an aid to capture design intent and enlist the help of the compiler in identifying accidental violations of design intent.

Functional interfaces often represent abstract concepts like functions, actions, or predicates. In documenting functional interfaces, or referring to variables typed as functional interfaces, it is common to refer directly to those abstract concepts, for example using "this function" instead of "the function represented by this object". When an API method is said to accept or return a functional interface in this manner, such as "applies the provided function to...", this is understood to mean a non-null reference to an object implementing the appropriate functional interface, unless potential nullity is explicitly specified.

The functional interfaces in this package follow an extensible naming convention, as follows:

There are several basic function shapes, including Function (unary function from T to R), Consumer (unary function from T to void), Predicate (unary function from T to boolean), and Supplier (nullary function to R).
Function shapes have a natural arity based on how they are most commonly used. The basic shapes can be modified by an arity prefix to indicate a different arity, such as BiFunction (binary function from T and U to R).
There are additional derived function shapes which extend the basic function shapes, including UnaryOperator (extends Function) and BinaryOperator (extends BiFunction).
Type parameters of functional interfaces can be specialized to primitives with additional type prefixes. To specialize the return type for a type that has both generic return type and generic arguments, we prefix ToXxx, as in ToIntFunction. Otherwise, type arguments are specialized left-to-right, as in DoubleConsumer or ObjIntConsumer. (The type prefix Obj is used to indicate that we don't want to specialize this parameter, but want to move on to the next parameter, as in ObjIntConsumer.) These schemes can be combined, as in IntToDoubleFunction.
If there are specialization prefixes for all arguments, the arity prefix may be left out (as in ObjIntConsumer).

Interfaces

BiConsumer<T, U>	Represents an operation that accepts two input arguments and returns no result.
BiFunction<T, U, R>	Represents a function that accepts two arguments and produces a result.
BinaryOperator<T>	Represents an operation upon two operands of the same type, producing a result of the same type as the operands.
BiPredicate<T, U>	Represents a predicate (boolean-valued function) of two arguments.
BooleanSupplier	Represents a supplier of `boolean`-valued results.
Consumer<T>	Represents an operation that accepts a single input argument and returns no result.
DoubleBinaryOperator	Represents an operation upon two `double`-valued operands and producing a `double`-valued result.
DoubleConsumer	Represents an operation that accepts a single `double`-valued argument and returns no result.
DoubleFunction<R>	Represents a function that accepts a double-valued argument and produces a result.
DoublePredicate	Represents a predicate (boolean-valued function) of one `double`-valued argument.
DoubleSupplier	Represents a supplier of `double`-valued results.
DoubleToIntFunction	Represents a function that accepts a double-valued argument and produces an int-valued result.
DoubleToLongFunction	Represents a function that accepts a double-valued argument and produces a long-valued result.
DoubleUnaryOperator	Represents an operation on a single `double`-valued operand that produces a `double`-valued result.
Function<T, R>	Represents a function that accepts one argument and produces a result.
IntBinaryOperator	Represents an operation upon two `int`-valued operands and producing an `int`-valued result.
IntConsumer	Represents an operation that accepts a single `int`-valued argument and returns no result.
IntFunction<R>	Represents a function that accepts an int-valued argument and produces a result.
IntPredicate	Represents a predicate (boolean-valued function) of one `int`-valued argument.
IntSupplier	Represents a supplier of `int`-valued results.
IntToDoubleFunction	Represents a function that accepts an int-valued argument and produces a double-valued result.
IntToLongFunction	Represents a function that accepts an int-valued argument and produces a long-valued result.
IntUnaryOperator	Represents an operation on a single `int`-valued operand that produces an `int`-valued result.
LongBinaryOperator	Represents an operation upon two `long`-valued operands and producing a `long`-valued result.
LongConsumer	Represents an operation that accepts a single `long`-valued argument and returns no result.
LongFunction<R>	Represents a function that accepts a long-valued argument and produces a result.
LongPredicate	Represents a predicate (boolean-valued function) of one `long`-valued argument.
LongSupplier	Represents a supplier of `long`-valued results.
LongToDoubleFunction	Represents a function that accepts a long-valued argument and produces a double-valued result.
LongToIntFunction	Represents a function that accepts a long-valued argument and produces an int-valued result.
LongUnaryOperator	Represents an operation on a single `long`-valued operand that produces a `long`-valued result.
ObjDoubleConsumer<T>	Represents an operation that accepts an object-valued and a `double`-valued argument, and returns no result.
ObjIntConsumer<T>	Represents an operation that accepts an object-valued and a `int`-valued argument, and returns no result.
ObjLongConsumer<T>	Represents an operation that accepts an object-valued and a `long`-valued argument, and returns no result.
Predicate<T>	Represents a predicate (boolean-valued function) of one argument.
Supplier<T>	Represents a supplier of results.
ToDoubleBiFunction<T, U>	Represents a function that accepts two arguments and produces a double-valued result.
ToDoubleFunction<T>	Represents a function that produces a double-valued result.
ToIntBiFunction<T, U>	Represents a function that accepts two arguments and produces an int-valued result.
ToIntFunction<T>	Represents a function that produces an int-valued result.
ToLongBiFunction<T, U>	Represents a function that accepts two arguments and produces a long-valued result.
ToLongFunction<T>	Represents a function that produces a long-valued result.
UnaryOperator<T>	Represents an operation on a single operand that produces a result of the same type as its operand.

Interfaces