Method reference

Learn method references with Class::method.

Method reference

A method reference is a short way to use an existing method instead of a lambda. The syntax is Class::method. You can write System.out::println instead of s -> System.out.println(s).

There are four kinds: static method (Math::abs), instance method of a type (String::length), instance method of an object (System.out::println) and constructor (User::new). A method reference cannot always replace a lambda; only when the body is a single method call.

When to use a method reference?

When a lambda body is only a single method call, a method reference is more readable. Writing names.forEach(System.out::println); means the same as names.forEach(s -> System.out.println(s)); but shorter.

But if the body has computation, filtering or transformation, a method reference is not enough. For example s -> s.toLowerCase().trim() calls two methods and must stay as a lambda. This is a trade-off between readability and brevity.

Exercise forEach with a method reference

Declare List<String> names = new ArrayList<>();; add "Ayşe" and "Alex". Print each element with a method reference using names.forEach(System.out::println);.

Show example solution

import java.util.List;
import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Ayşe");
        names.add("Alex");
        names.forEach(System.out::println);
    }
}

Test yourself

Match the method reference kinds

Math::abs

Static method
String::length

Type's instance method
System.out::println

Instance method
User::new

Constructor

Complete the method reference

names.forEach(System.out::____);

Show answer

names.forEach(System.out::println);

Which lambda is the same as System.out::println?
- s -> s.length()
- s -> System.in.read()
- s -> s.toUpperCase()
- s -> System.out.println(s)
Show answer

s -> System.out.println(s) — s -> System.out.println(s) equals System.out::println.

Next lesson: forEach with lambda

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies