Java 8 - Functional Interfaces

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.

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.

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 −

C:\JAVA>javac Java8Tester.java

Now run the Java8Tester as follows −

C:\JAVA>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

Source: tutorialspoint.com