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Java Functional Interfaces

An Interface that contains exactly one abstract method is known as functional interface. It can have any number of default, static methods but can contain only one abstract method. It can also declare methods of object class.

Functional Interface is also known as Single Abstract Method Interfaces or SAM Interfaces. It is a new feature in Java, which helps to achieve functional programming approach.


Example 1

Test it Now

Output:

Hello there

A functional interface can have methods of object class. See in the following example.

Example 2

Test it Now

Output:

Hello there

Invalid Functional Interface

A functional interface can extends another interface only when it does not have any abstract method.

Output:

compile-time error

Example 3

In the following example, a functional interface is extending to a non-functional interface.

Test it Now

Output:

Hello there
Do it now

Java Predefined-Functional Interfaces

Java provides predefined functional interfaces to deal with functional programming by using lambda and method references.

You can also define your own custom functional interface. Following is the list of functional interface which are placed in java.util.function package.


Interface Description
BiConsumer<T,U> It represents an operation that accepts two input arguments and returns no result.
Consumer<T> It represents an operation that accepts a single argument and returns no result.
Function<T,R> It represents a function that accepts one argument and returns a result.
Predicate<T> It represents a predicate (boolean-valued function) of one argument.
BiFunction<T,U,R> It represents a function that accepts two arguments and returns a a result.
BinaryOperator<T> It represents an operation upon two operands of the same data type. It returns a result of the same type as the operands.
BiPredicate<T,U> It represents a predicate (boolean-valued function) of two arguments.
BooleanSupplier It represents a supplier of boolean-valued results.
DoubleBinaryOperator It represents an operation upon two double type operands and returns a double type value.
DoubleConsumer It represents an operation that accepts a single double type argument and returns no result.
DoubleFunction<R> It represents a function that accepts a double type argument and produces a result.
DoublePredicate It represents a predicate (boolean-valued function) of one double type argument.
DoubleSupplier It represents a supplier of double type results.
DoubleToIntFunction It represents a function that accepts a double type argument and produces an int type result.
DoubleToLongFunction It represents a function that accepts a double type argument and produces a long type result.
DoubleUnaryOperator It represents an operation on a single double type operand that produces a double type result.
IntBinaryOperator It represents an operation upon two int type operands and returns an int type result.
IntConsumer It represents an operation that accepts a single integer argument and returns no result.
IntFunction<R> It represents a function that accepts an integer argument and returns a result.
IntPredicate It represents a predicate (boolean-valued function) of one integer argument.
IntSupplier It represents a supplier of integer type.
IntToDoubleFunction It represents a function that accepts an integer argument and returns a double.
IntToLongFunction It represents a function that accepts an integer argument and returns a long.
IntUnaryOperator It represents an operation on a single integer operand that produces an integer result.
LongBinaryOperator It represents an operation upon two long type operands and returns a long type result.
LongConsumer It represents an operation that accepts a single long type argument and returns no result.
LongFunction<R> It represents a function that accepts a long type argument and returns a result.
LongPredicate It represents a predicate (boolean-valued function) of one long type argument.
LongSupplier It represents a supplier of long type results.
LongToDoubleFunction It represents a function that accepts a long type argument and returns a result of double type.
LongToIntFunction It represents a function that accepts a long type argument and returns an integer result.
LongUnaryOperator It represents an operation on a single long type operand that returns a long type result.
ObjDoubleConsumer<T> It represents an operation that accepts an object and a double argument, and returns no result.
ObjIntConsumer<T> It represents an operation that accepts an object and an integer argument. It does not return result.
ObjLongConsumer<T> It represents an operation that accepts an object and a long argument, it returns no result.
Supplier<T> It represents a supplier of results.
ToDoubleBiFunction<T,U> It represents a function that accepts two arguments and produces a double type result.
ToDoubleFunction<T> It represents a function that returns a double type result.
ToIntBiFunction<T,U> It represents a function that accepts two arguments and returns an integer.
ToIntFunction<T> It represents a function that returns an integer.
ToLongBiFunction<T,U> It represents a function that accepts two arguments and returns a result of long type.
ToLongFunction<T> It represents a function that returns a result of long type.
UnaryOperator<T> It represents an operation on a single operand that returnsa a result of the same type as its operand.
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