Using lambda expressions predicates to get a certain values from a list

Starting with Java 8, you can use lambda expressions & predicates.

Example: Use a lambda expressions & a predicate to get a certain value from a list. In this example every person will be printed out with the fact if they are 18 and older or not.

Person Class:

public class Person {
    private String name;
    private int age;

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

    public int getAge() { return age; }
    public String getName() { return name; }
}

The built-in interface Predicate from the java.util.function.Predicate packages is a functional interface with a boolean test(T t) method.

Example Usage:

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

public class LambdaExample {
    public static void main(String[] args) {
        List<Person> personList = new ArrayList<Person>();
        personList.add(new Person("Jeroen", 20));
        personList.add(new Person("Jack", 5));
        personList.add(new Person("Lisa", 19));

        print(personList, p -> p.getAge() >= 18);
    }

    private static void print(List<Person> personList, Predicate<Person> checker) {
        for (Person person : personList) {
            if (checker.test(person)) {
                System.out.print(person + " matches your expression.");
            } else {
                System.out.println(person  + " doesn't match your expression.");
            }
        }
    }
}

The print(personList, p -> p.getAge() >= 18); method takes a lambda expression (because the Predicate is used a parameter) where you can define the expression that is needed. The checker’s test method checks if this expression is correct or not: checker.test(person).

You can easily change this to something else, for example to print(personList, p -> p.getName().startsWith("J"));. This will check if the person’s name starts with a “J”.

Found a mistake? Have a question or improvement idea? Let me know.

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