Question

In my answer from yesterday I called the following piece of code “a hack”:

final class MyMap extends HashMap<SomeSuperLongIdentifier, OtherSuperLongIdentifier> {}
// declared MyMap as an alias for readability purposes only

MyMap a = new MyMap();
a.put("key", "val");

Giving it another thought, this does not seem like a bad idea at all, but I might be missing something. Are there any potholes I missed out on? Is this an acceptable (possibly creative) way for declaring aliases in Java?

Answer 1

The drawback would be that you won’t be able to directly use any methods that return a correctly typed Map, because they will never return a MyMap. Even if they could return a Map<SomeSuperLongIdentifier, OtherSuperLongIdentifier>.

For example you wouldn’t be able to use the filter() methods in Maps (provided by Google Collections). They would accept a MyMap instance as input, but they would return only a Map<SomeSuperLongIdentifier, OtherSuperLongIdentifier>.

This problem can be somewhat reduced, by writing your MyMap to delegate to another Map implementation. Then you could pass the return value of such a method into the constructor and still have a MyMap (without copying, even). The default constructor could just set the delegate to a new HashMap instance, so the default usage would stay the same.