this and super Keywords in Java

What are this and super

this and super are two special keywords. this refers to the current object, and super refers to the parent class. They help you access the right field, method or constructor, especially when names clash between a class and its parent.

this — the current object

class Student {
    String name;
    void setName(String name) {
        this.name = name;   // this.name = field, name = parameter
    }
}

When a parameter has the same name as a field, name means the parameter and this.name means the object's field. this clears up the ambiguity — very common in setters and constructors.

this() for constructors

class Box {
    int w, h;
    Box() { this(1, 1); }          // call the other constructor
    Box(int w, int h) {
        this.w = w; this.h = h;
    }
}

this() calls another constructor in the same class, reusing its setup. It must be the first statement in the constructor.

super — the parent

class Animal {
    void sound() { System.out.println("Some sound"); }
}
class Dog extends Animal {
    void sound() {
        super.sound();      // call parent's version
        System.out.println("Woof");
    }
}

super.sound() calls the parent's method even though the child overrode it. This lets a child add to the parent's behaviour instead of fully replacing it.

super() in constructors

class Person {
    String name;
    Person(String name) { this.name = name; }
}
class Student extends Person {
    int roll;
    Student(String name, int roll) {
        super(name);        // call parent constructor
        this.roll = roll;
    }
}

super(name) runs the parent's constructor first, so the parent sets up its part (name), then the child adds its own (roll). It must be the first line of the constructor.

this vs super

Common mistakes

• Forgetting this when names clash. name = name; assigns the parameter to itself.
• super() not first. It must be the first statement in the child constructor.
• Using both this() and super(). Only one can be first, so you cannot use both.
• Expecting super without inheritance. super only makes sense in a subclass.

Summary

• this refers to the current object; super to the parent class.
• Use this.field = field when a parameter shares a field's name.
• this() calls another constructor in the same class.
• super.method() calls the parent's version; super(...) its constructor.
• Both this() and super() must be the first line, so use only one.

Next, see them in action with inheritance, or set up objects with constructors.