Why not allow an external interface to provide hashCode/equals for a HashMap?

Question

With a TreeMap it\'s trivial to provide a custom Comparator, thus overriding the semantics provided by Comparable objects added to the

Answer 1

Note: As noted in all other answers, HashMaps don't have an explicit ordering. They only recognize "equality". Getting an order out of a hash-based data structure is meaningless, as each object is turned into a hash - essentially a random number.

You can always write a hash function for a class (and often times must), as long as you do it carefully. This is a hard thing to do properly because hash-based data structures rely on a random, uniform distribution of hash values. In Effective Java, there is a large amount of text devoted to properly implementing a hash method with good behaviour.

With all that being said, if you just want your hashing to ignore the case of a String, you can write a wrapper class around String for this purpose and insert those in your data structure instead.

A simple implementation:

public class LowerStringWrapper {
    public LowerStringWrapper(String s) {
        this.s = s;
        this.lowerString = s.toLowerString();
    }

    // getter methods omitted

    // Rely on the hashing of String, as we know it to be good.
    public int hashCode() { return lowerString.hashCode(); }

    // We overrode hashCode, so we MUST also override equals. It is required
    // that if a.equals(b), then a.hashCode() == b.hashCode(), so we must
    // restore that invariant.
    public boolean equals(Object obj) {
        if (obj instanceof LowerStringWrapper) {
            return lowerString.equals(((LowerStringWrapper)obj).lowerString;
        } else {
            return lowerString.equals(obj);
        }
    }

    private String s;
    private String lowerString;
}