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

@FunctionalInterface
interface sayable{
	void say(String msg);
}
public class FunctionalInterfaceExample implements sayable{
	public void say(String msg){
		System.out.println(msg);
	}
	public static void main(String[] args) {
		FunctionalInterfaceExample fie = new FunctionalInterfaceExample();
		fie.say("Hello there");
	}
}

Test it Now

Output:

Hello there

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

Example 2

	
@FunctionalInterface
interface sayable{
	void say(String msg);	// abstract method
	// It can contain any number of Object class methods.
	int hashCode();
	String toString();
	boolean equals(Object obj);
}
public class FunctionalInterfaceExample2 implements sayable{
	public void say(String msg){
		System.out.println(msg);
	}
	public static void main(String[] args) {
		FunctionalInterfaceExample2 fie = new FunctionalInterfaceExample2();
		fie.say("Hello there");
	}
}

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.

interface sayable{
	void say(String msg);	// abstract method
}
@FunctionalInterface
interface Doable extends sayable{
	// Invalid '@FunctionalInterface' annotation; Doable is not a functional interface
	void doIt();
}

Output:

compile-time error

Example 3

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

interface Doable{
	default void doIt(){
		System.out.println("Do it now");
	}
}
@FunctionalInterface
interface Sayable extends Doable{
	void say(String msg);	// abstract method
}
public class FunctionalInterfaceExample3 implements Sayable{
	public void say(String msg){
		System.out.println(msg);
	}
	public static void main(String[] args) {
		FunctionalInterfaceExample3 fie = new FunctionalInterfaceExample3();
		fie.say("Hello there");
		fie.doIt();
	}
}

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.