Java 8 Predicate interface Example

In this tutorial, we will learn how to use Predicate functional interface with an example.

java.util.function.Predicate is a functional interface that can be used as an assignment target for a lambda expression. The Predicate interface represents an operation that takes a single input and returns a boolean value.

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Predicate Implementation Code

This is the internal implementation of the Predicate interface:

@FunctionalInterface
public interface Predicate<T> {

    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);
}

T – Type of the input to the predicate

Predicate Interface Example #1

The following example shows how to use the test() method of the Predicate interface using lambda.

package com.javaguides.java.functionalinterfaces;

import java.util.function.Predicate;

public class PredicateDemo {
    public static void main(String[] args) {
        Predicate < Integer > predicate = (t) -> {
            if (t % 2 == 0) {
                return true;
            } else {
                return false;
            }
        };

        System.out.println(predicate.test(10));
    }
}

Output:

true

Predicate.and() Method Example #2

This example demonstrates how to use Predicate.and() with filter() method:

package com.javaguides.java.functionalinterfaces;

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

public class PredicateDemo {
    public static void main(String[] args) {
        // example 2
        List < Integer > list1 = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

        Predicate < Integer > predicate2 = (x) -> x > 5;
        Predicate < Integer > predicate3 = (x) -> x < 8;
        list1.stream().filter(predicate2.and(predicate3)).collect(Collectors.toList()).forEach(System.out::println);
    }
}

Output:

6
7

Predicate.or() Method Example #3

This example demonstrates how to use Predicate.or() with filter() method:

package com.javaguides.java.functionalinterfaces;

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

public class PredicateDemo {
    public static void main(String[] args) {
        // example 4
        Predicate < String > lengthIs3 = x -> x.length() == 3;
        Predicate < String > startWithA = x -> x.startsWith("A");

        List < String > list = Arrays.asList("A", "AA", "AAA", "B", "BB", "BBB");

        List < String > collect = list.stream()
            .filter(lengthIs3.or(startWithA))
            .collect(Collectors.toList());

        System.out.println(collect);
    }
}

Output:

[A, AA, AAA, BBB]

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Predicate Interface Example #4

In this example, we will check if a person's age is greater than 30 using Predicate.test() method and return the result in boolean value:

package com.java.tutorials.functionalinterfaces;

import java.util.function.Predicate;

public class PredicateExample {
    public static void main(String[] args) {
        Predicate < Person > predicate = (person) -> person.getAge() > 30;
        boolean result = predicate.test(new Person("Ramesh", 31));
        System.out.println(result);
    }
}

class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

Output:

true