Java this

Summary: in this tutorial, you will learn about Java this keyword, and how to use it effectively in your classes.

Introduction to Java this keyword

A class is a template for creating objects. To refer to the current object inside the class, you use the this keyword. In other words, the this keyword is a reference to the current instance of the class.

Since the this keyword references the current instance of the class, you only can use it within the methods of the class.

Suppose you have a Person class with two properties name and age, and a method sayHi():

class Person {
    String name;
    int age;

    void sayHi() {
        System.out.printf("Hi. It's %s.", name);
    }
}Code language: Java (java)

Because this references the current instance of the Person class, you can explicitly access the name property in the sayHi() method like this:

this.name;Code language: Java (java)

The Person class will become:

class Person {
    String name;
    int age;

    void sayHi() {
        System.out.printf("Hi. It's %s.", this.name);
    }
}Code language: Java (java)

When using Java this keyword

The this keyword is useful in some scenarios.

1) Differentiating between property and method parameters with the same name

You can the this keyword to differentiate between properties and method parameters that have the same name.

Suppose you want to define a method that sets the new name for a person object:

class Person {
    String name;
    int age;

    void sayHi() {
        System.out.printf("Hi. It's %s.", this.name);
    }

    void setName(String name) {
        // ...
    }
}Code language: Java (java)

In this example, the setName() method has the name parameter which is the same as the name property of the Person class.

How do you reference the name property inside the setName() method? To do that, you use the this keyword as follows:

class Person {
    String name;
    int age;

    void sayHi() {
        System.out.printf("Hi. It's %s.", this.name);
    }

    void setName(String name) {
        this.name = name;
    }
}Code language: Java (java)

In the setName() method, this.name references the name property of the current Person object while the name references the method parameter.

Similarly, you can add the setAge() method using the this keyword to reference the age property of the current object:

class Person {
    String name;
    int age;

    void sayHi() {
        System.out.printf("Hi. It's %s.", this.name);
    }

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

    void setAge(int age) {
        this.age = age;
    }
}Code language: Java (java)

2) Chaining methods

Because the this keyword references the current object, you can return it from a method, allowing for method chaining.

For example, we can modify the setName() and setAge() method to return a Person object and add a statement that returns this:

class Person {
    String name;
    int age;

    void sayHi() {
        System.out.printf("Hi. It's %s.", this.name);
    }

    Person setName(String name) {
        this.name = name;
        return this;
    }

    Person setAge(int age) {
        this.age = age;
        return this;
    }
}Code language: Java (java)

The following creates the person object and calls setName(), setAge() and sayHi() methods:

public class App {
    public static void main(String[] args) {
        var person  = new Person();
        
        person.setName("John");
        person.setAge(22);
        person.sayHi();
    }
}Code language: Java (java)

Since the setName() and setAge() method returns the current Person object, you can chain the method calls like this:

public class App {
    public static void main(String[] args) {

        var person = new Person();

        person.setName("John")
                .setAge(22)
                .sayHi();

    }
}Code language: Java (java)

Summary

• The Java this keyword references the current instance of a class.
• Use Java this keyword to differentiate between properties and method parameters that have the same names and to enable method chaining.