In objected oriented (OO) design, objects are aware of the contents inside of themselves.

In Java, the object can access its attributes with the this keyword.
In Python, we commonly use the self variable (though it is arbitrary and up to you).

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class MyObject {
    private int someNumber;

    public MyObject() {
        this.someNumber = 2
    }

    public int getNumber() {
        return this.someNumber;
    }
}

//

MyObject obj = new MyObject();
obj.getNumber();

In this example, we can see that we are using the this keyword to access the someNumber attribute in our object.

Removing this this keyword

Sometimes we can also get rid of the this keyword, as the Java language does not explicitly require it.

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class MyObject {
    private int someNumber;

    public MyObject() {
        someNumber = 2
    }

    public int getNumber() {
        return someNumber;
    }
}

//

MyObject obj = new MyObject();
obj.getNumber();

When this is required

There are times when we must use the this keyword, however.
For example, when we have arguments in the function signature.

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  private int myNumber;
  public int setNumber(int myNumber) {
      // This won't work!
      myNumber = myNumber;

      // We have to use `this`
      this.myNumber = myNumber;
  }

We cannot use this in the context of a class method (In Java, these would be static methods)

Summary

  • As good practice, always use the this keyword when an attribute is accessed.
  • this will not work in static / class methods