Question

Looking through the implementation of the Java HashMap here : http://www.docjar.com/html/api/java/util/HashMap.java.html I noticed the following :

The internal data structure used is an array which at each index stores the reference to the first entry in a linked list. The array index is based on the key’s hashcode and the linked list represents the bucket for that particular hashcode.
What I found interesting is the method indexFor(int h, int length) which, for a given key, determines what bucket in the array to look in. But the implementation, return h & (length – 1) looks odd in the sense that for an indeterminate number of hashcodes which do not coincide with a given array index the method will return 0. So, no matter what unique hashcode you implement for your object, the 0 bucket in the array will most likely be full of objects and thus you don’t benefit from what a unique hashcode is supposed to offer you, that is faster data access.

Am I missing something?

Cristian

Answer 1

You are missing the following Javadoc from the HashMap source code:

/**
 * The table, resized as necessary. Length MUST Always be a power of two.
 */
transient Entry<K,V>[] table;

This means that table.length-1 will always be a sequence of 1’s.