Using Integer as a key with HashMap in Java

Question

Recently I was looking for good implementation of hashCode() method in Java API and looked through Integer source code. Didn\'t expect that, but th

Answer 1

Integer is the worst data type candidate for a key when used with HashMap, as all consecutive keys will be places in one bin

No, that statement is wrong.

In fact, the implementation of Integer's hashCode() is the best possible implementation. It maps each Integer value to a unique hashCode value, which reduces the chance of different keys being mapped into the same bucket.

Sometimes a simple implementation is the best.

From the Javadoc of hashCode() in the Object class:

It is not required that if two objects are unequal according to the java.lang.Object.equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

Integer is one of the few classes that actually guarantees that unequal objects will have different hashCode().