Java: Composite key in hashmaps

Question

I would like to store a group of objects in a hashmap , where the key shall be a composite of two string values. is there a way to achieve this?

i can simply concate

Answer 1

public int hashCode() {
    return (str1 + str2).hashCode();
}

This seems to be a terrible way to generate the hashCode: Creating a new string instance every time the hash code is computed is terrible! (Even generating the string instance once and caching the result is poor practice.)

There are a lot of suggestions here:

How do I calculate a good hash code for a list of strings?

public int hashCode() {
    final int prime = 31;
    int result = 1;
    for ( String s : strings ) {
        result = result * prime + s.hashCode();
    }
    return result;
}

For a pair of strings, that becomes:

return string1.hashCode() * 31 + string2.hashCode();

That is a very basic implementation. Lots of advice through the link to suggest better tuned strategies.