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...");
}
}
For your question on how java knows how to compare objects, you need to override the equals method
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/
You're not overriding the
Object#equals
method, but overloading it. In your method declaration you useAnimal
type instead ofObject
:A good overriding of the method would be using the
instanceof
operator. Showing an example:More info on the subject:
Every class inherits the
Object
class silently. And theObject
class has aequals
method. So if any class doesn't override the equals method then it will use the default implementation ofObject.equals.
From the doc
From the source code of Object.equals
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