OpenJDK 1.23 · java.util.function · APIdia, the JavaDoc alternative

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 java.io.FileFilter, are defined in the packages where they are used.

The interfaces in this package are annotated with java.lang.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 java.util.function.Function (unary function from T to R), java.util.function.Consumer (unary function from T to void), java.util.function.Predicate (unary function from T to boolean), and java.util.function.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 java.util.function.BiFunction (binary function from T and U to R).
There are additional derived function shapes which extend the basic function shapes, including java.util.function.UnaryOperator (extends Function) and java.util.function.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 java.util.function.ToIntFunction. Otherwise, type arguments are specialized left-to-right, as in java.util.function.DoubleConsumer or java.util.function.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 java.util.function.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 java.util.function.ObjIntConsumer).

Interface Summary

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

Package java.util.function

Interface Summary