IEqualityComparer interface in Java

Question

More specifically I want an interface to compare objects which can be only compared for equality [e.g. complex numbers] but don\'t have total order on them. It should have [Not

Answer 1

In the end I opted to write what I do in similar cases. If I need special equality/hash - like for example keeping weak references. You can wrap the key like that. Overall it's not very different from the interface approach but it creates dumb instances (like HashMap/Hashtable for bucket entries). You might need extra unwrapping for keySet() and so on...

package t1;

public abstract class KeyX implements java.io.Serializable {
    private static final long serialVersionUID = 0l;

    final Key key;
    final int hash;
    protected KeyX(Key key){
        this.key = key;
        this.hash = hashCode(key);
    }

    protected abstract int hashCode(Key key);

    //Key, Key will be way too strict and it'd required, key.getClass().isInstance(y) prior calling
    protected abstract boolean equals(Key x, Object y);

    @Override
    public final boolean equals(Object obj) {
        if (obj==this)
            return true;
        if (!(obj instanceof KeyX)){
            return false;
        }
        final KeyX other = (KeyX) obj;
        return this.key==other.key ||  (hash==other.hash && other.key!=null  && equals(this.key, other.key)); 

    }

    @Override
    public final int hashCode() {
        return hash;
    }

    public final Key unwrap(){
        return key;
    }
}