Does the equals method work with objects? If so, how?

Question

I have a program that is zoo and in the zoo there are branched subgroups of animals that are reptiles. When I do an equals method the main program compiles and it runs. I'm confused how does java know to use the equals method if I'm comparing objects and not specifically int or String?

public class Zoo {

    public static void main(String[]args) {
        Animal a=new Animal("Bob");
        Reptile komodo= new Reptile("Snakey");
        komodo.bask();
        a.size=3;
        komodo.size=5;
        System.out.println(a);
        System.out.println(komodo);
        Turtle t= new Turtle("Slowy");
        t.hide();
        t.size=6;
        t.numlegs=4;
        System.out.println(t);
        System.out.println(t.equals(komodo));
    }
}

public class Animal {
    public String name;

    public boolean equals(Animal other) {
        return other.size==this.size;
    }

    public Animal(String s) {
        name=s;
    }
    public void setName(String n) {
        this.name=n;
    }
    public void eat(String meal) {
        System.out.println("chump chump yummy "+meal);
    }
    public int size;

    public String toString() {
        return "I am "+name+" and I'm "+size+" cm long";
    }

}

public class Reptile extends Animal {

    public Reptile(String n) {
        super(n);
        numlegs=0;
    }
    public Reptile(String n, int l) {
        super(n);
        numlegs=l;
    }

    public void bask() {
        System.out.println("basking...");
    }
    public String toString() {
        return super.toString()+numlegs+" legs";
    }

    public int numlegs;
}
public class Turtle extends Reptile {

    public Turtle(String n) {
        super (n,4);
        shellColor="Brown";
    }
    public void hide() {
        System.out.println("you cant see me");
    }
    public String toString() {
        return super.toString()+" and my shell is"+ shellColor;
    }
    public String shellColor;

    public void bask() {
        super.bask();
        System.out.println("turtle is basking...");
    }

}

Answer 1

You're not overriding the Object#equals method, but overloading it. In your method declaration you use Animal type instead of Object:

public boolean equals(Animal other)

A good overriding of the method would be using the instanceof operator. Showing an example:

@Override
public boolean equals(Object other) {
    if(other instanceof Animal) {
        Animal otherAnimal = (Animal)other;
        //comparison logic...
    }
    return false;
}

More info on the subject:

• Best practices regarding equals: to overload or not to overload?
• Overriding Object.equals VS Overloading it

Answer 2

For your question on how java knows how to compare objects, you need to override the equals method

 public boolean equals(Object other){
    // return true or false based on your logic

  }

While comparing, equals method is used. You can have a look at this good tutorial which explains the significance of the equals method.

http://www.thejavageek.com/2013/06/26/what-is-the-significance-of-equals-method-in-java/

Also, only overriding equals is not enough if you are using objects into collections those use hashing. You will find a good tutorial at

http://www.thejavageek.com/2013/06/28/significance-of-equals-and-hashcode/

Answer 3

Every class inherits the Object class silently. And the Object class has a equals method. So if any class doesn't override the equals method then it will use the default implementation of Object.equals.

From the doc

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

From the source code of Object.equals

public boolean equals(Object obj) {
    return (this == obj);
}

So If any object doesn't have it's own implementation of equals then the equals method will simply check if the object reference is same or not.

So get a desired result from equals you need to implement by your own as alread suggested in other answer