Reference To An Instance method Of a Class (Method References)

What is a reference to an instance method of a class?

This type is also called "a reference to an instance method of an arbitrary object of a particular type."

This method reference refers to an instance method of an object of a particular type determined at runtime.

Syntax

ClassName::instanceMethodName

Examples

Some examples will help you understand this kind of method reference.

A) List of strings

public class Person {
  private final String name;

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

  public String getName() {
    return name;
  }
}

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

public class ListOfObjects {
  public static void main(String[] args) {
    List<Person> people =
        Arrays.asList(
            new Person("Alice"),
            new Person("Bob"),
            new Person("Charlie")
            );

    List<String> names =
        people
            .stream()
            .map(Person::getName)
            .collect(Collectors.toList());

    System.out.println(names);
  }
}

Person class contains a single instance variable and a method getName that returns the name.

On line 7 of Example class, we created a collection of items and added few Person objects.

On line 15, we have a list of persons chained with the .stream() method on line 16^th.

On line 17, we get the list of person names and collect them to a list of strings on line 20.

Similarly, the method reference String::concat would invoke the method a.concat(b)

B) Sorting integers

This is a classic sorting example where we make use of compareTo  method in Integer class.

import java.util.Arrays;
import java.util.List;

public class SortingIntegers {
  public static void main(String[] args) {
    List<Integer> integerValues = Arrays.asList(11, 4, 2, 8, 9, 10, 32, 22, 20, 17);
    
    integerValues // list of integers
        .stream() // stream of integers
        .sorted(Integer::compareTo) // method reference
        .forEach(System.out::println); // print on console
  }
}

On line 6, we have a list of integer items randomly stored.

On line 9, we chained it with .stream() to convert the list of integer values into a stream of integer values.

Line 10 contains an intermediate operation .sorted that takes Integer::compareTo that sorts the integer values.

Line 11 prints each of the integer on the console.