CODEWITHSUNDEEP
javainheritanceequalsoverriding
user2938723
user2938723

Reputation: 1150

calling super.equals() in derived class while overriding equals

I have a base class with age and name as instance members and derived class with bonus. I am overriding equals in Derived class. I know how equals works in Java when there is a single base class. But I am not able to understand how it works in case of inheritance. I want to check if two derived objects are equal.

I was expecting the output to be
This class = Base, Other class = Derived
instead the output is This class = Derived, Other class = Derived

What is super in equals method of derived class exactly doing?Isnt it referring to Base?

<br/>

public class Base{

    private int age;
    private String name;

    public Base(int age, String name){
        this.age = age;
        this.name = name;
    }

    public int getAge(){
        return age;
    }

    public String getName(){
        return name;
    }

    @Override
    public boolean equals(Object otherBase){

        //Check if both the references refer to same object
        if(this == otherBase){
            return true;
        }

        if(otherBase == null){
            return false;
        }

        System.out.println("This class       ="+this.getClass().getCanonicalName()+", Other class = "+otherBase.getClass().getCanonicalName());

        if(this.getClass() != otherBase.getClass()){
            return false;
        }

        if(! (otherBase instanceof Base)){
            return false;
        }

        Base other = (Base) otherBase;

        return this.age == other.age && this.name.equals(other.name);
       }


    public static void main(String[] args){
        Derived d1 = new Derived(10,6,"shri");
        Derived d2 = new Derived(10,5, "shri");
        if(d1.equals(d2)){
            System.out.println("Equal");
        }else{
            System.out.println("Not Equal");
        }
    }
}

class Derived extends Base{

    private int bonus;

    public int getBonus(){
        return bonus;
    }

    public Derived(int bonus, int age, String name){
        super(age, name);
        this.bonus = bonus;
    }

    @Override
    public boolean equals(Object otherDerived){
        if(!(super.equals(otherDerived))){
            return false;
        }

        Derived derived = (Derived) otherDerived;
        return bonus == derived.bonus;
    }
}

Upvotes: 1

Views: 3532

Answers (2)

M A
M A

Reputation: 72874

What is super in equals method of derived class exactly doing?Isnt it referring to Base?

super is used to call the overriden equals method (the one defined in Base). More generally, super references the superclass of the type in which it is used, so in this case yes it refers to Base. However, the referenced object's type at runtime is still Derived.

In the Base#equals() method, the expression this.getClass().getCanonicalName() returns the name of the class for the object at runtime. Even if the expression is called in the equals method of the base class, the actual type is Derived. This is what the getClass method does as mentioned in the Javadocs:

Returns the runtime class of this Object.

If you ever need to get the name of the superclass, you can use this.getClass().getSuperclass().getCanonicalName().

Upvotes: 1

Crazyjavahacking
Crazyjavahacking

Reputation: 9707

So the equals() method is implemented correctly and it works in the following way:

  1. delegate the comparison of the age and name to the Base class
  2. if it is not the same, return false
  3. otherwise compare the value of the bonus field in the Derived class

The call of super.equals() will call the equals() from super class (Base), but this still represents the real instance, e.g. Derived in your case.

Upvotes: 1

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