How does HashMap work in Java ?

What makes good hash keys ?
Classes which are immutable and implement hashCode() and equals() sensibly, make good hash keys.


Important Points 

•  if two objects are equal according to the equals() method, they must have the same hashCode()value (although the reverse is not generally true).
• Under this default implementation, two references are equal only if they refer to the exact same object.

Why does our root object class need hashCode(), when its discriminating ability is entirely subsumed by that of equals()?

The hashCode() method exists purely for efficiency. The Java platform architects anticipated the importance of hash-based collection classes -- such as Hashtable, HashMap, and HashSet -- in typical Java applications, and comparing against many objects with equals() can be computationally expensive. Having every Java object support hashCode() allows for efficient storage and retrieval using hash-based collections.

The hashmap has a number of "buckets" which it uses to store key-value pairs in. Each bucket has a unique number - that's what identifies the bucket. When you put a key-value pair into the map, the hashmap will look at the hash code of the key, and store the pair in the bucket of which the identifier is the hash code of the key. For example: The hash code of the key is 235 -> the pair is stored in bucket number 235. (Note that one bucket can store more then one key-value pair).

When you lookup a value in the hashmap, by giving it a key, it will first look at the hash code of the key that you gave. The hashmap will then look into the corresponding bucket, and then it will compare the key that you gave with the keys of all pairs in the bucket, by comparing them with equals().

Now you can see how this is very efficient for looking up key-value pairs in a map: by the hash code of the key the hashmap immediately knows in which bucket to look, so that it only has to test against what's in that bucket.

Looking at the above mechanism, you can also see what requirements are necessary on thehashCode() and equals() methods of keys:

• If two keys are the same (equals() returns true when you compare them), their hashCode()method must return the same number. If keys violate this, then keys that are equal might be stored in different buckets, and the hashmap would not be able to find key-value pairs (because it's going to look in the same bucket).
• If two keys are different, then it doesn't matter if their hash codes are the same or not. They will be stored in the same bucket, but the hashmap will use equals() to tell them apart.

Reference :

• http://www.ibm.com/developerworks/java/library/j-jtp05273/index.html
• http://stackoverflow.com/questions/6493605/how-does-java-hashmap-work
• http://javarevisited.blogspot.in/2011/02/how-hashmap-works-in-java.html