Question

What is the easiest way in Java to map strings (Java String) to (positive) integers (Java int), so that

• equal strings map to equal integers, and
• different strings map to different integers?

So, similar to hashCode() but different strings are required to produce different integers. So, in a sense, it would be a hasCode() without the collision possibility.

An obvious solution would maintain a mapping table from strings to integers, and a counter to guarantee that new strings are assigned a new integer. I’m just wondering how is this problem usually solved. Would also be interesting to extend it to other objects than strings.

Answer 1

This is impossible to achieve without any restrictions, simply because there are more possible Strings than there are integers, so eventually you will run out of numbers.

A solution is only possible when you limit the number of usable Strings. Then you can use a simple counter. Here is a simple implementation where all (2^32 = 4294967296 different strings) can be used. Never mind that it uses lots of memory.

import java.util.HashMap; import java.util.Map;  public class StringToInt {      private Map<String, Integer> map;      private int counter = Integer.MIN_VALUE;      public StringToInt() {         map = new HashMap<String, Integer>();     }      public int toInt(String s) {         Integer i = map.get(s);         if (i == null) {             map.put(s, counter);             i = counter;             ++counter;         }         return i;     } }