Why does Java Map<K, V> take an untyped parameter for the get and remove methods?

Question

I ran into a bug in my code where I was using the wrong key to fetch something from a Java map that I believed was strongly typed using Java generics. When looking at the Map Javadocs, many of the methods, including get and remove, take an Object as the parameter instead of type K (for a Map defined as Map). Why is this? Is there a good reason or is it an API design flaw?

Answer 1

I think this is for backwards compatibility with older versions of the Map interface. It's unfortunate that this is the case however as you're right, it would be much better if this took the correct type.

Answer 2

Because the map will return a value if the object passed to the get method is equal to any key stored in the map. Equal does not mean that they have to be of the same type, but that the key's and the passed object's equal methods are implemented in such a way, that the different object types are mutually recognized as equal.

The same applies of course to the remove method.

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Example of valid code, which would break (not compile) if the get method only allowed parameters of type K:

LinkedList<Number> k1 = new LinkedList<Number>();
k1.add(10);

ArrayList<Integer> k2 = new ArrayList<Integer>();
k2.add(10);

Map<LinkedList<Number>, String> map = new HashMap<LinkedList<Number>, String>();

map.put(k1, "foo");
System.out.println(map.get(k2));