# Functional Interfaces in Java Programming

Java 8Object Oriented ProgrammingProgramming

Functional interfaces have a single functionality to exhibit. For example, a Comparable interface with a single method 'compareTo' is used for comparison purpose. Java 8 has defined a lot of functional interfaces to be used extensively in lambda expressions. Following is the list of functional interfaces defined in java.util.Function package.

Given below is the list of interfaces in Java8.

Sr.No. Interface & Description
1

BiConsumer<T,U>

Represents an operation that accepts two input arguments, and returns no result.

2

BiFunction<T,U,R>

Represents a function that accepts two arguments and produces a result.

3

BinaryOperator<T>

Represents an operation upon two operands of the same type, producing a result of the same type as the operands.

4

BiPredicate<T,U>

Represents a predicate (Boolean-valued function) of two arguments.

5

BooleanSupplier

Represents a supplier of Boolean-valued results.

6

Consumer<T>

Represents an operation that accepts a single input argument and returns no result.

7

DoubleBinaryOperator

Represents an operation upon two double-valued operands and producing a double-valued result.

8

DoubleConsumer

Represents an operation that accepts a single double-valued argument and returns no result.

9

DoubleFunction<R>

Represents a function that accepts a double-valued argument and produces a result.

10

DoublePredicate

Represents a predicate (Boolean-valued function) of one double-valued argument.

11

DoubleSupplier

Represents a supplier of double-valued results.

12

DoubleToIntFunction

Represents a function that accepts a double-valued argument and produces an int-valued result.

13

DoubleToLongFunction

Represents a function that accepts a double-valued argument and produces a long-valued result.

14

DoubleUnaryOperator

Represents an operation on a single double-valued operand that produces a double-valued result.

15

Function<T,R>

Represents a function that accepts one argument and produces a result.

16

IntBinaryOperator

Represents an operation upon two int-valued operands and produces an int-valued result.

17

IntConsumer

Represents an operation that accepts a single int-valued argument and returns no result.

18

IntFunction<R>

Represents a function that accepts an int-valued argument and produces a result.

19

IntPredicate

Represents a predicate (Boolean-valued function) of one int-valued argument.

20

IntSupplier

Represents a supplier of int-valued results.

21

IntToDoubleFunction

Represents a function that accepts an int-valued argument and produces a double-valued result.

22

IntToLongFunction

Represents a function that accepts an int-valued argument and produces a long-valued result.

23

IntUnaryOperator

Represents an operation on a single int-valued operand that produces an int-valued result.

24

LongBinaryOperator

Represents an operation upon two long-valued operands and produces a long-valued result.

25

LongConsumer

Represents an operation that accepts a single long-valued argument and returns no result.

26

LongFunction<R>

Represents a function that accepts a long-valued argument and produces a result.

27

LongPredicate

Represents a predicate (Boolean-valued function) of one long-valued argument.

28

LongSupplier

Represents a supplier of long-valued results.

29

LongToDoubleFunction

Represents a function that accepts a long-valued argument and produces a double-valued result.

30

LongToIntFunction

Represents a function that accepts a long-valued argument and produces an int-valued result.

31

LongUnaryOperator

Represents an operation on a single long-valued operand that produces a long-valued result.

32

ObjDoubleConsumer<T>

Represents an operation that accepts an object-valued and a double-valued argument, and returns no result.

33

ObjIntConsumer<T>

Represents an operation that accepts an object-valued and an int-valued argument, and returns no result.

34

ObjLongConsumer<T>

Represents an operation that accepts an object-valued and a long-valued argument, and returns no result.

35

Predicate<T>

Represents a predicate (Boolean-valued function) of one argument.

36

Supplier<T>

Represents a supplier of results.

37

ToDoubleBiFunction<T,U>

Represents a function that accepts two arguments and produces a double-valued result.

38

ToDoubleFunction<T>

Represents a function that produces a double-valued result.

39

ToIntBiFunction<T,U>

Represents a function that accepts two arguments and produces an int-valued result.

40

ToIntFunction<T>

Represents a function that produces an int-valued result.

41

ToLongBiFunction<T,U>

Represents a function that accepts two arguments and produces a long-valued result.

42

ToLongFunction<T>

Represents a function that produces a long-valued result.

43

UnaryOperator<T>

Represents an operation on a single operand that produces a result of the same type as its operand.

## Functional Interface Example

Predicate <T> interface is a functional interface with a method test(Object) to return a Boolean value. This interface signifies that an object is tested to be true or false.

Create the following Java program using any editor of your choice in, say, C:\> JAVA.

## Example

Live Demo
import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;

public class Java8Tester {

public static void main(String args[]) {
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);

// Predicate<Integer> predicate = n -> true
// n is passed as parameter to test method of Predicate interface
// test method will always return true no matter what value n has.

System.out.println("Print all numbers:");

//pass n as parameter
eval(list, n->true);

// Predicate<Integer> predicate1 = n -> n%2 == 0
// n is passed as parameter to test method of Predicate interface
// test method will return true if n%2 comes to be zero

System.out.println("Print even numbers:");
eval(list, n-> n%2 == 0 );

// Predicate<Integer> predicate2 = n -> n > 3
// n is passed as parameter to test method of Predicate interface
// test method will return true if n is greater than 3.

System.out.println("Print numbers greater than 3:");
eval(list, n-> n > 3 );
}

public static void eval(List<Integer> list, Predicate<Integer> predicate) {

for(Integer n: list) {

if(predicate.test(n)) {
System.out.println(n + " ");
}
}
}
}


Here we've passed Predicate interface, which takes a single input and returns Boolean.

### Verify the Result

Compile the class using javac compiler as follows −

C:\JAVA>javac Java8Tester.java


Now run the Java8Tester as follows −

C:\JAVA>java Java8Tester


It should produce the following output −

## Output

Print all numbers:
1
2
3
4
5
6
7
8
9
Print even numbers:
2
4
6
8
Print numbers greater than 3:
4
5
6
7
8
9

Published on 03-Dec-2018 10:27:59