Two instances having the same hashcode but not equal

Question

I was reading the paragraph quoted below from an article entitled- Java theory and practice: Hashing it out - Defining hashCode() and equals() effectively and correctly

Defining equality The Object class has two methods for making inferences about an object's identity: equals() and hashCode(). In general, if you override one of these methods, you must override both, as there are important relationships between them that must be maintained. In particular, if two objects are equal according to the equals() method, they must have the same hashCode() value (although the reverse is not generally true).[emphasis added by me]

My question relates to the latter bit of the paragraph "although the reverse is not generally true". How is it possible for two different instances of a class to have the same hashCode but not be equal?

Answer 1

You can consider hashes to be a bucket..

• If two objects are equal, they will go into the same bucket (have same hashcodes)
• But, if the two objects go into the same bucket (have same hashcode), that doesn't mean that they must be equal
• Also note that, if two objects are not equal, even then they can have the same hash code.. Obviously, this infers from the above two points..

So, hashcode is nothing but the hash-value for that Bucket.. Any number of objects can have same hashcode, depending upon the algorithm used to calculate the hashcodes..

An ideal algorithm is the one, which generates different hashcodes for different objects. So, there is ideally 1 object per bucket.. Of course this is the perfect case, which might not be possible..

A bucket may of course contain several objects, based on some property..

Answer 2

In simple terms hashcode () is a function to generate hash by some formula, so there can be some collisions, two different values can turn out to have same hashcode.

If I simply calculate the hashcode by taking mod by 6, then two different values might be having same hashcode.