Why using "this" keyword to get superclass methods

Question

I saw some Java examples that using this keyword to get superclass methods. Example: this.superClassMethod(). In usual case we would use super. Could someone help to clarify with example why the developer used this instead of super? Thank you.

Answer 1

There is no difference between this.method() and super.method() until the said method() gets overridden in the caller's class.

For example, with

class SuperClass {

    public void method() {
        System.out.println("SuperClass");
    }

}

class SubClass extends SuperClass {

    public SubClass() {
        method();
        this.method();
        super.method();
    }

}

Calling

new SubClass();

Prints

SuperClass
SuperClass
SuperClass

While with

class SuperClass {

    public void method() {
        System.out.println("SuperClass");
    }

}

class SubClass extends SuperClass {

    @Override
    public void method() {
        System.out.println("SubClass");
    }

    public SubClass() {
        method();
        this.method();
        super.method();
    }

}

Calling

new SubClass();

Prints

SubClass
SubClass
SuperClass

---

In parallel, there is no difference between this.field and super.field until the said field gets hidden in the caller's class.

For example, with

class SuperClass {

    protected String field = "SuperClass";

}

class SubClass extends SuperClass {

    public SubClass(String field) {
        System.out.println(field);
        System.out.println(this.field);
        System.out.println(super.field);
    }

}

Calling

new SubClass("parameter");

Prints

parameter
SuperClass
SuperClass

While with

class SuperClass {

    protected String field = "SuperClass";

}

class SubClass extends SuperClass {

    private String field = "SubClass";

    public SubClass(String field) {
        System.out.println(field);
        System.out.println(this.field);
        System.out.println(super.field);
    }

}

Calling

new SubClass("parameter");

Prints

parameter
SubClass
SuperClass

---

Side note: methods() get overriden while fields get hidden.

Answer 2

Using this does not invoke the superclass method. It is actually superfluous, because it specifically invokes this instance's method. It may be relevant if you want to call another constructor of the same instance, but otherwise it's the same as just calling the method.

It may be useful for variable scoping (for example when there's a local variable with the same name as an instance variable) to make sure the instance variable is used, but it makes no difference when calling methods.

Personally I'd think that the developer wanted to take advantage of code completion and the IDE shows possible method names after entering this. :-)

Answer 3

super is used to access methods of the base class, while this is used to access methods of the current class.

Few references
1) usage of this
2) critique on super on SO

Extending the notion, if you write super(), it refers to constructor of the base class, and if you write this(), it refers to the constructor of the very class where you are writing this code.

class Animal {
  void eat() {
    System.out.println("animal : eat");
  }
}

class Dog extends Animal {
  void eat() {
    System.out.println("dog : eat");
  }
  void anotherEat() {
    super.eat();
  }
}

public class Test {
  public static void main(String[] args) {
    Animal a = new Animal();
    a.eat();
    Dog d = new Dog();
    d.eat();
    d.anotherEat();
  }
}

The output is going to be

animal : eat
dog : eat
animal : eat

The third line is printing "animal:eat" because we are calling super.eat(). If we called this.eat(), it would have printed as "dog:eat".