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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.

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

 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 −

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