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

see a test method, basically, so long as you match, a1*31+b1 = a2*31 +b2, which means (a1-a2)*31=b2-b1

public void testHash()
{
    System.out.println("A:" + ((int)'A'));
    System.out.println("B:" + ((int)'B'));
    System.out.println("a:" + ((int)'a'));

    System.out.println(hash("Aa".hashCode()));
    System.out.println(hash("BB".hashCode()));
    System.out.println(hash("Aa".hashCode()));
    System.out.println(hash("BB".hashCode()));


    System.out.println(hash("AaAa".hashCode()));
    System.out.println(hash("BBBB".hashCode()));
    System.out.println(hash("AaBB".hashCode()));
    System.out.println(hash("BBAa".hashCode()));

}

you will get

A:65
B:66
a:97
2260
2260
2260
2260
2019172
2019172
2019172
2019172

edit: someone said this is not straightforward enough. I added below part

    @Test
    public void testN() throws Exception {
        List l = HashCUtil.generateN(3);
        for(int i = 0; i < l.size(); ++i){
            System.out.println(l.get(i) + "---" + l.get(i).hashCode());
        }
    }
AaAaAa---1952508096
AaAaBB---1952508096
AaBBAa---1952508096
AaBBBB---1952508096
BBAaAa---1952508096
BBAaBB---1952508096
BBBBAa---1952508096
BBBBBB---1952508096

below is the source code, it might be not efficient, but it work:

public class HashCUtil {

    private static String[] base = new String[] {"Aa", "BB"};

    public static List generateN(int n)
    {
        if(n <= 0)
        {
            return null;
        }

        List list = generateOne(null);
        for(int i = 1; i < n; ++i)
        {
            list = generateOne(list);
        }

        return list;
    }


    public static List generateOne(List strList)
    {   
        if((null == strList) || (0 == strList.size()))
        {
            strList = new ArrayList();
            for(int i = 0; i < base.length; ++i)
            {
                strList.add(base[i]);
            }

            return strList;
        }

        List result = new ArrayList();

        for(int i = 0; i < base.length; ++i)
        {
            for(String str: strList)
            {   
                result.add(base[i]  + str);
            }
        }

        return result;      
    }
}

look at String.hashCode()

   public int hashCode() {
    int h = hash;
    if (h == 0) {
        int off = offset;
        char val[] = value;
        int len = count;

            for (int i = 0; i < len; i++) {
                h = 31*h + val[off++];
            }
            hash = h;
        }
        return h;
    }