Appropriate hashCode method implements with equals method

[Abimaran Kugathasan]

How to find whether the hashCode method is appropriate to the given equals method? Any suggestion??? (I don't mean the contract between them) Thanks in Advanced!

[Minhaj Mehmood]

the appropriate equals & hashcode methods should use same instance variables in both methods.

[Ankit Garg]

M M kaimkhani wrote:the appropriate equals & hashcode methods should use same instance variables in both methods.

That's not necessary, the hashCode and equals contract can still be fulfilled with different instance variables. To fulfill the contract hashCode must be the same for equal objects. So even this fulfills the hashCode and equals contract

class HashEqualsTry {
    String name;
    public int hashCode() {
        return 1;  //inefficient but still fulfills the contract
    }
    public boolean equals(Object other) {
        if(! other instanceof HashEqualsTry)
            return false;
        HashEqualsTry het = (HashEqualsTry)other;
        return this.name.equals(het.name);
    }
}

[Tim McGuire]

Abimaran Kugathasan wrote:How to find whether the hashCode method is appropriate to the given equals method? Any suggestion??? (I don't mean the contract between them) Thanks in Advanced!

If you don't mean the contract, I'm not sure what you mean. If you want to find whether it is appropriate, drop your object in a HashTable and see if you can ever find it again. Then, drop 100,000 other objects into the same HashTable and see if you can find yours quickly enough.

The way it is explained in the books I studied is " a HashTable is a series of buckets each with zero or more objects." The hashcode determines which bucket you look in to get the object. The equals determines which object you pull out of the bucket. It is appropriate if your implementation of equals and hashcode makes it possible and efficient to find the right bucket and then the right object. If you have 200,000 members in your HashTable all with the same hashcode, then your code has to find the object using only the equals method in one big "bucket" of objects. If the members are spread out over 2000 buckets, your code just has to search through 100 items to find the right one with the equals method.

If you are not putting the object in a Hashed collection, then override hashcode becomes less important.

[Mandar B Kulkarni]

A Couple of Rules, that I know,

1) If hashcode method is invoked on an object multiple times it should return the same value, unless the object's state is being modified in such a way that it affects the hashcode calculation.(Never use any random number in "hashcode"( e.g. Math.Random)).

2) If two objects are equal according to the "equals" method then both of these objects must return the same value for "hashcode".

[Muhammad Khojaye]

Abimaran Kugathasan wrote:How to find whether the hashCode method is appropriate to the given equals method? Any suggestion??? (I don't mean the contract between them) Thanks in Advanced!

There are some guidelines available for writing good HashCode function. You can use Joshu Recipe and can also try HashCodeBuilder. Both are listed here
HashCodeBuilder require that you need to use the same instance field that are used in Equal method.

[Jesper de Jong]

Here is a good article which explains hashCode() and equals():

Hashing it out - Defining hashCode() and equals() effectively and correctly

[Abimaran Kugathasan]

Thanks to All.....
Tim McGuire, Muhammad Ali Khojaye, Jesper Young Thanks a lot......