Four Kinds of Method References

What is a Method Reference

A method reference (Class::method) is a shortcut for a lambda that only calls one existing method. s -> s.toUpperCase() becomes String::toUpperCase.

Kind	Syntax	Equivalent lambda
Static method	`Integer::parseInt`	`s -> Integer.parseInt(s)`
Instance method of a particular object	`System.out::println`	`x -> System.out.println(x)`
Instance method of an arbitrary object of a type	`String::length`	`s -> s.length()`
Constructor	`ArrayList::new`	`() -> new ArrayList<>()`

All Four in Action

Example

import java.util.*;
import java.util.function.*;

public class Main {
    public static void main(String[] args) {
        // 1. static method
        Function<String, Integer> parse = Integer::parseInt;
        System.out.println(parse.apply("42") + 8);

        // 2. instance method of a particular object
        String greeting = "Namaste";
        Predicate<String> startsWithGreeting = greeting::startsWith;   // "Namaste".startsWith(x)
        System.out.println(startsWithGreeting.test("Nam"));

        // 3. instance method of an arbitrary object: first argument becomes the receiver
        BiFunction<String, String, Boolean> eqIgnore = String::equalsIgnoreCase;
        System.out.println(eqIgnore.apply("JAVA", "java"));

        // 4. constructor
        Function<String, StringBuilder> sbMaker = StringBuilder::new;
        System.out.println(sbMaker.apply("abc").reverse());

        List.of("x", "y", "z").forEach(System.out::println);
    }
}

Output

50
true
true
cba
x
y
z