Comparing one variable only between two instance

Question

i'm studying Java and they gave me an exercise which i'm really struggling with. I'll give you the full exercise (it's translated from my native language) instead of explaining it, so you can understand it. I really can't understand how should i do it. I don't pretend the full code (even tho i'd appreciate that, since a well made code could help me improve and better understand how things should be done), but at least i'd like to know where should i start looking for the solution, cause right now i'm hitting a wall, and it hurts.

My first idea was to override the .equals method, but that doesn't seems right. I thought i could make a subclass but in the example they are using the same class for both the main instance and the 3 derived instances, so that's a no.

Exercise :

Create an Animal class, characterized in this way:

Number Of Legs
type (Birds, Cats, etc.)
average weight

Two animals are considered equal if they have the same number of legs, they belong to the same type and have the same average weight. The class must implement three methods, which allow to convert the Animal so that only one of the characteristics is compared (one method for each characteristic).

Example :

    Animal cat = new Animal (4, "Cat", 5);
    Animal dog = new Animal (4, "Dog", 5);

    Animal catNumberOfLegs = cat.compareNumberOfLegs();
    Animal catType = cat.compareType ();
    Animal catWeigth = cat.compareWeigth();

    Animal dogNumberOfLegs = dog.compareNumberOfLegs ();
    Animal dogType = dog.compareType ();
    Animal dogWeigth = dog.compareWeigth ();

    System.out.println (cat.equals (dog)); // false
    System.out.println (catNumberOfLegs.equals (dogNumberOfLegs)); // true
    System.out.println (catType.equals (dogType)); // false
    System.out.println (catWeigth.equals (dogWeigth)); // true

------------

The values of the variables must remain unchanged.
Java is an object-oriented language: remember it!

EDIT : I'm editing this cause i see some confusion in the comments, don't know if it's cause the exercise is written poorly or if it's cause of the translation.

The point is, i have two instances of Animal class. One is called Cat, the other is called Dog. I have this 3 separate methods to implements that, when used on an instance, return me a new instance that will only take one of the variables into account when using .equals(). Let's take the method "compareNumberOfLegs ();" as an example. Animal dogTWO = dog.compareNumberOfLegs (); Animal catTWO = cat.compareNumberOfLegs (); This means that "dogTWO" will be a new instance that will mantain everything of "dog", but if compared to "catTWO" using .equals(), i'll get 'True' cause only "numberOfLegs" will be compared.

The example has been writed by you or comes in the exercise? Because it seems wrong to me. — epsilonmajorquezero, Mar 13 '19 at 16:06
I think there are typos in your example. catNumberOfLegs , catType and so on cant be the Animal class. — Sergei Podlipaev, Mar 13 '19 at 16:07
Possible duplicate of [Why do I need to override the equals and hashCode methods in Java?](https://stackoverflow.com/questions/2265503/why-do-i-need-to-override-the-equals-and-hashcode-methods-in-java) — GBlodgett, Mar 13 '19 at 16:13
catNumberOfLegs and catType are instances of Animal, that's what's bothering me — Giulio De Rossi, Mar 13 '19 at 16:27

score 5 · Answer 1 · answered Mar 13 '19 at 16:07

In your Animal class you must implements equals() method.

equals() are implemented by default because is a method on Object but you must override its behaviour for your aim,

so you have in your Animal class:

public boolean equals(Object value) {
    if (value == null) return false;
    if  (!value instanceof Animal) return false;
    Animal other = (Animal)value;

    if (this.numberOfLegs != other.getNumberOfLegs()) return false;
    if (this.type.equals(other.getType()) return false;
    if (this.weigth != other.getWeigth()) return false;
    return true;
}

So you have your object instances cat and dog, you can call cat.equals(dog);

I get this, but sadly, that's not how they want it to be done. I'll edit the first post to try and explain it better. — Giulio De Rossi, Mar 13 '19 at 16:30

Thomas Francois · Accepted Answer · 2019-03-13T17:06:45.467

You could achieve this by extending Animal, so that each subclass overrides equals to check only one attribute. Each compareX method then returns the specialized subclass.

The following code can give you an idea as to where to start:

class Animal {
    // Class attributes

    class AnimalWithLegs extends Animal {
        public AnimalWithLegs(Animal base) { /* ... */ }

        @Override
        public boolean equals(Object o) { /* ... */ }
    }


    class AnimalWithType extends Animal {
        public AnimalWithLegs(Animal base) { /* ... */ }

        @Override
        public boolean equals(Object o) { /* ... */ }
    }


    class AnimalWithWeight extends Animal {
        public AnimalWithLegs(Animal base) { /* ... */ }

        @Override
        public boolean equals(Object o) { /* ... */ }
    }

    public Animal compareNumberOfLegs() {
        return new AnimalWithLegs(this);
    }

    public Animal compareType() {
        return new AnimalWithType(this);
    }

    public Animal compareWeight() {
        return new AnimalWithWeight(this);
    }

    @Override
    public boolean equals(Object o) { /* ... */ }
}

This could actually be what i was looking for! I'll make some tests while waiting for more answers, but i like this one for now. — Giulio De Rossi, Mar 13 '19 at 17:04

Comparing one variable only between two instance

2 Answers2