What does :: mean in Java?

The :: operator refers to a method reference.

 A method reference is a simplified way of writing a lambda expression in order to call a method. Method references allow you to call a method by mentioning its name. The syntax for a method reference is as follows:

Object :: methodName

There are four ways to use a method reference:

1. A method reference to a static method.
2. A method reference to an instance method of an object.
3. A method reference to instance methods of an arbitrary object of a particular type.
4. A method reference to a constructor.

METHOD REFERENCE TO A STATIC METHOD

Let's take a look at the code snippet below:

import java.util.function.BiFunction;

class Maths {

    public static int printAddition(int x, int y){
        return x + y;
    }

}
public class Main {

    public static void main(String[] args) {
        BiFunction<Integer, Integer, Integer> addition = Maths::printAddition;

        int result = addition.apply(2, 4);
        System.out.println("The result is: " + result);

    }

}

The outcome is:

The result is: 6

1. We have a non-static inner class called Maths
2. Inside this inner class we have a static method that given x and y returns back the addition between x and y
3. Then we have the Main public class where we declare the main method so that we can execute our program
4. Inside the main method we have the BiFunction functional interface.

• To explain this better, BiFunction is a functional interface that takes three parameters: T, U, and R. In our case, T and U are 2 and 4 respectively, which are both Integers. Our R is 6, which is again an Integer. In short, we are saying take a first Integer argument, then take a second Integer argument, and give me a result of Integer type. You can read more about BiFunction and other functional interfaces here.
• This is where we encounter the method reference. We take the name of the inner class (Maths), the :: operator, and the name of the method printAddition.

• The BiFunction uses the method apply()to apply the method to the two arguments. In our case, our arguments are 2 and 4. We assigned this operation to the result variable.

In the end, we print the outcome to the console.

METHOD REFERENCE TO AN INSTANCE METHOD OF AN OBJECT

@java.lang.FunctionalInterface
interface Maths {

    void printAddition();

}
public class Main {

    public void printMessage(){
        System.out.println("Method reference to an instance method of an object");
    }

    public static void main(String[] args) {

        Main main = new Main();

        Maths reference = main::printMessage;

       reference.printAddition();

    }

}

The outcome is:

Method reference to an instance method of an object

1. We start our program with the functional interface Maths, which contains a printAddition() method.
2. We have the Main public class where there is a printMessage() method that prints out a message on the console.
3. In our main method, we instantiate an object called main.
4. We use the method reference on the object that we created earlier.
5. We call the printAddition() via the reference variable of type Maths.

METHOD REFERENCE TO AN INSTANCE METHOD OF AN ARBITRARY OBJECT OF A PARTICULAR TYPE

Let's consider the following program:

package com.maddy;

public class Subject {

    private String name;
    private String assignment;


    public Subject(String name, String assignment) {
        this.name = name;
        this.assignment = assignment;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAssignment() {
        return assignment;
    }

    public void setAssignment(String assignment) {
        this.assignment = assignment;
    }

    public void print(){
        System.out.println("The name of the subject is: " + name + " and your assignment is: " + assignment);
    }
}

And this one as well:

package com.maddy;

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

public class SubjectDemo {

    public static void main(String[] args) {

        List<Subject> subjects = new ArrayList<Subject>();

        subjects.add(new Subject("English", "Shakespeare"));
        subjects.add(new Subject("Geography", "Europe"));
        subjects.add(new Subject("History", " The Romans"));

        subjects.forEach(Subject::print);

    }
}

The outcome is:

The name of the subject is: English and your assignment is: Shakespeare
The name of the subject is: Geography and your assignment is: Europe
The name of the subject is: History and your assignment is: The Romans

Let's examine what is happening here:

1. We created a class Subject that has two fields, the name of the subject, an assignment, a constructor and getters and setters. We also added a method that prints the name of the subject and its assignment.
2. Then we have a class called SubjectDemo where we created a list of Subject objects with some values.
3. We used the method reference to say that we want to apply the print method from the Subject class on each subject. A method reference can be called on an arbitrary method because in this case, we called the print method on all the subjects, not a particular one.

METHOD REFERENCE ON A CONSTRUCTOR

Let's look at the following code snippet:

package com.maddy;

@java.lang.FunctionalInterface
interface Geography {
    void print(String message);
}

class Test{

    public Test(String test) {
        System.out.println(test);
    }
}

public class GeographyTest {

    public static void main(String[] args) {

        Geography geography = Test::new;
        geography.print("Hello student! You've been assigned a Geography test. :) ");
    }
}

The outcome is:

Hello student! You've been assigned a Goography test. :)

Here, we're:

1. Creating a functional interface called Geography in which there is a method that prints a message;
2. An inner class called Test with a constructor that initializes a test String and prints out the test itself when an object is created.
3. In our main method, we use a method reference on the Test class constructor.
4. Finally, we print out a message.