Using HashMap with custom key

Question

Quick Question: If I want to use HashMap with a custom class as the key, must I override the hashCode function? How will it work if I do not override that function?

Answer 1

If you don't override hashCode AND equals you will get the default behaviour which is that each object is different, regardless of its contents.

Answer 2

Technically, you don't have to override the hashCode method as long as equal objects have the same hashCode.

So, if you use the default behaviour defined by Object, where equals only returns true only for the same instance, then you don't have to override the hashCode method.

But if you don't override the equals and the hashCode methods, it means you have to make sure you're always using the same key instance.

E.g.:

MyKey key1_1 = new MyKey("key1");

myMap.put(key1_1,someValue); // OK

someValue = myMap.get(key1_1); // returns the correct value, since the same key instance has been used;

MyKey key1_2  = new MaKey("key1"); // different key instance

someValue = myMap.get(key1_2); // returns null, because key1_2 has a different hashCode than key1_1 and key1_1.equals(key1_2) == false

In practice you often have only one instance of the key, so technically you don't have to override the equals and hashCode methods.

But it's best practice to override the equals and hashCode methods for classes used as keys anyway, because sometime later you or another developer might forget that the same instance has to be used, which can lead to hard to track issues.

And note: even if you override the equals and hashCode methods, you must make sure you don't change the key object in a way that would change the result of the equals or the hashCode methods, otherwise the map won't find your value anymore. That's why it's recommended to use immutable objects as keys if possible.