Inheritance and private instance variables

Question

I'm having trouble figuring out how to access a private instance variable of the super class. I'm writing an equals method in the Dog class that compares to see if the name and breeds are the same, but name is a private instance variable inside Pet (that Dog inherits).

Here's my code:

public class Pet {

    private String name;

    public Pet(){
        name = "";
    }
    public Pet(String name){
        this.name = name;
    }

    public boolean equals(Pet other){
        return this.name.equals(other.name);
    }
}

and my Dog class:

public class Dog extends Pet {
    private String breed;
    public Dog(String name, String breed) {
        super(name);
        this.breed = breed;
    }

    public Dog(){
        breed = "";
    }

    @Override
    public boolean equals(Object obj){

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

        if(obj.getClass() != this.getClass()){
            return false;
        }else{
            Pet p = (Pet)obj;
            Pet q = (Pet)this;
            Dog temp = (Dog)obj;
            boolean name = q.equals(p);
            boolean bred = breed.equals(temp.breed);
            return name && bred;
        }
    }
}

In my main class:

Dog d1 = new Dog("Harry", "Puggle");
Dog d2 = new Dog("Harry", "Pug");
System.out.println(d1.equals(d2));

For some reason it keeps using my Pet class's equal method.

Thanks

Do you mean equals of Dog class is not used at all? If not, its obvious from your code. — nawfal
– nawfal, Commented Dec 15, 2012 at 1:16

Stephen C · Accepted Answer · 2012-12-15 01:34:47Z

1

@Pshemo has identified the immediate cause of your problem. Your Pet.equals(Object) does not override `Dog.equals(String) because the signatures don't match. And your d1.equals(d2) call is binding to the most closely matching method signature, which is the one with a Pet formal parameter rather than an Object formal parameter.

But once you have corrected that, there is another problem in the Dog.equals(String) method:

        Pet p = (Pet)obj;
        Pet q = (Pet)this;
        boolean name = q.equals(p);

When you fix the signature of Pet.equals, that is going to result in a recursive call to Dog.equals ... and a StackOverflowError. (Dog.equals will call Dog.equals, which will call Dog.equals, which ...). Basically, q.equals is the same method as the one that is currently executing. The type casts aren't doing anything ...

Change it to this:

        Pet p = (Pet)obj;
        boolean name = super.equals(p);

The super keyword used this way invokes the overridden version of the equals method.

I'm having trouble figuring out how to access a private instance variable of the super class.

This is a different issue to what is causing your problem. But the answer is that if you want a child classes method to be able to access a private variable in a parent class, then you need to either add getter and/or setter methods to the parent class, or change the access of the variable to (typically) protected.

edited Dec 15, 2012 at 1:34

answered Dec 15, 2012 at 1:10

Stephen C

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3 Comments

nawfal Over a year ago

//there is another problem in the Dog.equals(String)// where is such a method in OP's q? Also what can cause stackoverflow in his question?

vtshadow Over a year ago

I'm having trouble figuring out why q.equals(p) would throw an exception since I'm just calling the equals method inside Pet?

vtshadow Over a year ago

@StephenC Yeah, I've tried what you said and it works, but I'm just trying to figure out why it works and why my way throws an exception.

Pshemo · Accepted Answer · 2012-12-15 01:19:03Z

1

If you look carefully your Dog class have two equals methods:

equals(Pet other) form super class
equals(Object obj) from Dog class .

Since you are using d1.equals(d2) where d1 and d2 are Dog instances Java will use equals(Pet other) method from super class in which you only check name equality.

edited Dec 15, 2012 at 1:19

answered Dec 15, 2012 at 1:13

Pshemo

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