hashCode

[Eric Sexton]

Is there a way to convert a has code to string? I would like calculate a hashCode for the string and then convert it back.

[Joe Ess]

A hash code is an integer (primitive). You can convert it to a String by using String.valueOf(int i).

[Eric Sexton]

Dang, I wasn't clear. Sorry for the confusion.
I want to take a string such as "Hello World", and display it's hashCode.
That part is easy enough. Now I want to convert that hashCode back into the String "Hello World". Is that possible?

[Joe Ess]

Originally posted by Eric Sexton:

Now I want to convert that hashCode back into the String "Hello World". Is that possible?

[/QB]
No. A hash code is intended to give one a good distribution when storing objects in a hash table. There is no way to tell from a hash code what type an object is or what its value(s) is/are.

[John Smith]

There is no way to tell from a hash code what type an object is or what its value(s) is/are.
Well, there is a way to reconstruct a String value from its hashcode, but it's very ugly and not reliable. The only purpose of doing it may be as an exercise to see how the hashCode() works in a particular JDK implementation, or to have it formulated as a pure and theoretical math problem. So, while the reconstruction code below works, don't use it, -- for all intents and purposes, the street from an object to its hash code is a one way street, just like Joe indicated.

package test;

/**
 * Bad Java, used for demo purposes only
 *
 * Reconstructs a 3 character lower case String value from its hash code.
 */
public class StringReconstruction {
    public static final char FIRST = 'a';
    public static final char LAST = 'z';

    // The hashCode for a String is computed by s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
    public static final int BASE = 31;

    public static String reconstruct(int hashCode) {
        String recString = "not found";

        for (char ch1 = FIRST; ch1 <= LAST; ch1++) {
            for (char ch2 = FIRST; ch2 <= LAST; ch2++) {
                for (char ch3 = FIRST; ch3 <= LAST; ch3++) {
                    double code = ch1 + ch2 * Math.pow(BASE, 1) + ch3 * Math.pow(BASE, 2);
                    if (hashCode == (int) code) {
                        recString = "" + ch3 + ch2 + ch1;
                        break;
                    }
                }
            }
        }

        return recString;
    }

    public static void main(String[] args) {
        String s = "cat";
        int hashCode = s.hashCode();
        String reconstructedString = reconstruct(hashCode);
        System.out.println("original string is " + s);
        System.out.println("reconstructed string is " + reconstructedString);
    }
}

Output:
original string is cat
reconstructed string is cat

[ October 06, 2003: Message edited by: Eugene Kononov ]