Function interface in Java

Functional interfaces have a single functionality to exhibit. For example, a Comparable interface with a single method ‘compareTo’ is used for comparison purposes. 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.

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.

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);
      System.out.println("Print all numbers:");
      //pass n as parameter
      eval(list, n->true);
      System.out.println("Print even numbers:");
      eval(list, n-> n%2 == 0 );
      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 + " ");
         }
      }
   }
}

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

Maruthi Krishna

Updated on: 06-Sep-2019

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