If you want a function that will return the same garbage for the same input string, this would work fine.
public String rubbish(String input)
{
    String result = "";
    long seed = 0;
    long size = 0;
    for(int i = 0; i < input.length(); i ++)
    {
        seed += input.charAt(i);
    }
    seed %= Long.MAX_VALUE;
    size = seed % 3000;
    Random rand = new Random(seed);
    for(int i = 0; i < size; i ++)
    {
        result += (char)rand.nextInt(256);
    }
    return result;
}
Additionally, if you want the string to be different each time, you can change new Random(seed) to new Random() and it’ll randomize the string.
solved Convert string to rubbish