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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.

S. 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.

To get more clarity on this, write the following program in an code editor and verify the results.

Java8Tester.java

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 −

$javac Java8Tester.java

Now run the Java8Tester as follows −

$java Java8Tester

It should produce the following 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