Question

How do I compare Base class object with derived class object in derived class? I need to get true as output from the code below:

class Base {
    int i;
    String str;
}

class Derived extends Base{
    double d;
    String str1;

    public static void main(String []a){
        Base b= new Base();
        Derived d = new Derived();
        System.out.println("Equals! :"+d.equals(b));
    }
}

It always gives me false as output. How can I compare base class with derived class?

Answer 1

Object equality is usually defined for objects of the same class, e.g.: You might want two objects Base b1, b2 to be equal if their str and i values are equal. Then you would need to define the equals() (and, typically, hashCode()) methods to test for this condition. For example:

public boolean equals(Object obj)
{
    // test for identity
    if (this == obj) return true;

    // check that obj is not null, and that it is an instance of Base
    if (obj == null) return false;
    if (getClass() != obj.getClass()) return false;

    // compare attributes
    Base other = (Base) obj;
    if (i != other.i) return false;
    if (str == null && other.str != null) return false;
    if (!str.equals(other.str)) return false;
    return true;
}

In your case, since Base and Derived are different classes, with different attributes (Base has str and i, while Derived has str, i, str1, and d), you would need to define exactly when are a Base object and a Derived object supposed to be equal.