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Java Method References

A method reference names an existing method without calling it. The double colon :: joins the owner to the method name.

System.out::println feeds a forEach

forEach needs an action for each element. The reference hands it the println method directly.

import java.util.List;

public class Main {
  public static void main(String[] args) {
    List<String> names = List.of("Ana", "Ben", "Cy");
    names.forEach(System.out::println);
  }
}

String::toUpperCase as a Function

The reference fits a Function<String, String>. Each incoming string becomes the object toUpperCase runs on.

import java.util.function.Function;

public class Main {
  public static void main(String[] args) {
    Function<String, String> upper = String::toUpperCase;
    System.out.println(upper.apply("stream"));
    System.out.println(upper.apply("java"));
  }
}

A static helper by class name

Shouter::addMark names a static method on the Shouter class. The Function call runs that method.

import java.util.function.Function;

public class Main {
  public static void main(String[] args) {
    Function<String, String> shout = Shouter::addMark;
    System.out.println(shout.apply("Go"));
    System.out.println(shout.apply("Done"));
  }
}
class Shouter {
  static String addMark(String text) {
    return text + "!";
  }
}

Lambda and reference print the same lines

The first loop passes a lambda that prints each word. The second loop passes the reference form of the same rule.

import java.util.List;

public class Main {
  public static void main(String[] args) {
    List<String> words = List.of("pen", "cup");
    words.forEach(word -> System.out.println(word));
    words.forEach(System.out::println);
  }
}

Example

import java.util.List;
import java.util.function.Function;

public class Main {
  public static void main(String[] args) {
    List<String> cities = List.of("Oslo", "Lima");
    Function<String, String> upper = String::toUpperCase;
    for (String city : cities) {
      System.out.println(upper.apply(city));
    }
    cities.forEach(System.out::println);
  }
}

Key points

  • A method reference stands where a functional interface is expected.
  • System.out::println names the println method of the standard output object.
  • String::toUpperCase uses the incoming string as the object the method runs on.
  • A static method is referenced with its class name, two colons and the method name.
  • A lambda and a method reference can carry the same rule into the same call.

In this example, String::toUpperCase fills a Function that prints each city in capitals. System.out::println then prints the same list in its stored form. The two pairs of lines show one list read by two references.

Note

A constructor reference writes the type name, two colons and new, as in Student::new.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

What does the reference System.out::println name?