why we override hashcode and equal methods

[ramesh gosala]

Hi....I have doubt regading Hashcode and equal methods

why we override this two methods....
why we override hashcode method when we are overriding equal method,
i would very thankful to give answser

Thank you
Ramesh

[Gowher Naik]

class Equal{
private int id;//2
Equal(int id){
this.id=id;
}
}
public class EqualTest{
public static void main(String []args){
Equal e1=new Equal(1);//9
Equal e2=new Equal(1);//10
if(e1.equals(e2)){//11
System.out.print("Equal");
}else{
System.out.print("Not Equal");
}
}
}

Check code Above
in above code i have created two instances of Equal class
at line 9 and 10.
I as developer decided that two objects of Equal class are equal if both
class have same id(line 2)
Now when i use equals method(line 11) it shows that e1 and e2 are
different(see o/p).
I get o/p "Not Equal" because i have not override equal method in
Equal class.
So if i want two class with same id should be equal according
to equal method then i should override equal method in my class.

hashCode method is Linked with equal method first understand
equal method then see why we need to override hashCode method using maps.

[William Yan]

When you make new class, say Dog.

 
class Dog {
   int weight=10;
   Dog(){}
   Dog(int i){weight=i;}
}

When you make 2 objects with the same weight, and invoke equals method

Dog a = new Dog(5);
Dog b = new Dog(5);
a.equals(b);

it will give you false. Because the equals is a method you inherit from Object. in equals of Object, it simple estimate if the two references refer to the same object. And a and b refer to different Dog objects. So you get false.
To solve that, you should override the equals method.
say:

public boolean equals(Object o) {
   return (o instanceof Dog)&&(this.weight==(Dog)o.weight);
}

hashCode method is required to be overriden when you override equals,
that is a bit complex.. understand the equals first.