Duplicate keys for HashMap

Question

Hi,
I have a class I created, call it MyClass. It looks like this:
public String myString;
public final String toString() {
    return myString;
public boolean equals(MyClass mc) {
    return (mc != null && myString.equalsIgnoreCase(mc.toString()));
}When I use objects of type MyClass as the keys to a HashMap when doing a put(), duplicate keys seem to be allowed. And when I query, knowing that I have an entry for a key, I get null. However, when I change my get() and put() to use MyClass.toString() as the key, everything works fine.
It seems to me that the JVM must be assuming that different instances of objects of type MyClass are different and thus a new key+value is created when "putting" instead of the old value being overwritten. But MyClass has an equals() method, so when putting, why doesn't it tell the JVM that the key already exists?
Thanks,
Jeff

Answer 1

True, I missed that.
Were you thinking that the HashMap is going to use the equals you provide that takes MyClass as an arg? It won't. It uses the one that takes Object, which means you'll be using Object's equals, which compares references, so no MyClass can ever be equal to another.
You need to override equals (with the Object argument) and override hashCode.