What is hash collision in JAVA, please explain in detail?

Question

This was asked today in interview any pointer.

"Design a unique hash function for every tweet in Twitter"

Answer 1

if two keys has same hashCode? If multiple keys has same hashCode, than during put() operation collision had occurred, which means multiple Entry object stored in a bucket location. Each Entry keep track of another Entry, forming a linked list data structure there. Now, if we need to retrieve value object in this situation, following steps will be followed :

1) Call hashCode() method of key to find bucket location.

2) Traverse thought linked list, comparing keys in each entries using keys.equals() until it return true.

So, we use equals() method of key object to find correct entry and than return value from that.
For Coding Example ..follow this link - https://myshadesofgray.wordpress.com/2013/06/20/java-hash-collision-in-hashmap/

Answer 2

Hash tables deal with collisions in one of two ways.

Option 1: By having each bucket contain a linked list of elements that are hashed to that bucket. This is why a bad hash function can make lookups in hash tables very slow.

Option 2: If the hash table entries are all full then the hash table can increase the number of buckets that it has and then redistribute all the elements in the table. The hash function returns an integer and the hash table has to take the result of the hash function and mod it against the size of the table that way it can be sure it will get to bucket. so by increasing the size it will rehash and run the modulo calculations which if you are lucky might send the objects to different buckets.

Java uses both option 1 and 2 in its hash table implementations.