Predicate in Java 8

In Java, a predicate is a functional interface introduced in the Java 8

A predicate represents a boolean-valued function that takes an argument and returns true or false.

The Predicate interface has a single abstract method called test, which evaluates the predicate.

Here’s the basic structure of the Predicate interface:

@FunctionalInterface
public interface Predicate<T> {
    boolean test(T t);
}

Here, T is the type of input to the predicate. You can use the test() method to define the condition you want to check.

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Let’s look at a simple example. Suppose you have a list of integers, and you want to filter out the even numbers using a predicate

package com.java8features.predicate;

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

public class PredicateDemo {

    public void predicateExample() {

        System.out.println("*** filter out the even numbers using a predicate**");
        
        List<Integer> list = Arrays.asList(1, 2, 3, 4, 5,6,7,8);
        Predicate<Integer> predicate = n -> n % 2 == 0;

        for (Integer val : list) {
            if (predicate.test(val)) { // test method called to cecheck condition
                System.out.println(val);
            }
        }

        String[] std = {"runa", "muna", "kuna", "kanha", "miku"};
        Predicate<String> names = (name) -> name.startsWith("u", 1);
        for (String s : std) {
            if (names.test(s)) { // test method called to cecheck condition
                System.out.println(s);
            }
        }

        System.out.println("*** filter out the odd numbers using a predicate**");

        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
        List<Integer> evenlist = numbers.stream().filter(val -> val % 2 != 0).toList();
        for (Integer val : evenlist) {
            System.out.println(val);
        }
    }

    public static void main(String[] args) {
        System.out.println("This is predicate example");
        PredicateDemo predicateDemo = new PredicateDemo();
        predicateDemo.predicateExample();
    }
}

This is a predicate example with a stream API filter

package com.java8features.predicate;

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

public class PredicateDemo {

    public void predicateExample() {

        Predicate<Integer> isEven = n -> n % 2 == 0;
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

        List<Integer> evenNumbers = filterList(numbers, isEven);
        System.out.println(evenNumbers);
    }

    private static List<Integer> filterList(List<Integer> numbers, Predicate<Integer> predicate) {
        return numbers.stream().filter(predicate).toList();
    }

    public static void main(String[] args) {
        System.out.println("This is predicate  example with stream API filter");
        PredicateDemo predicateDemo = new PredicateDemo();
        predicateDemo.predicateExample();
    }
}

In this example, the isEvenpredicate checks whether a given integer is even or not. The filterList method takes a list and a predicate, and it returns a new list containing only the elements that satisfy the condition specified by the predicate.

Output Filtered list of even numbers

Original List: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Even Numbers: [2, 4, 6, 8, 10]

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