How to generate strings that share the same hashcode in Java?

Question

An existing system written in Java uses the hashcode of a string as its routing strategy for load balancing.

Now, I cannot modify the system but ne

Answer 1

I was wondering if there was a "universal" solution; e.g. some constant string XYZ, such that

    s.hashCode() == (s + XYZ).hashCode() 

for any string s. Finding such a string involves solving a fairly complicated equation ... which was beyond my rusty mathematical ability. But then it dawned on me that h == 31*h + ch is always true when h and ch are both zero!

Based on that insight, the following method should create a different String with the same hashcode as its argument:

    public String collider(String s) { 
        return "\0" + s;
    }

If NUL characters are problematic for you, prepending any string whose hashcode is zero would work too ... albeit that the colliding strings would be longer than if you used zero.