Java Generics: Why Does Map.get() Ignore Type?

Question

In Java, the Map interface is defined as,

public interface Map<K,V> {
    ...
    V get(Object key);
    ...
}

Why not?

V get(K key);

I just bumped into a nasty bug because wrong type key is used. I thought the purpose of the generics is to catch type error early during compiling. Does this defeat that purpose?

Answer 1

Kevin Bourrillion blogged about this a while ago. Summary:

Uniformly, methods of the Java Collections Framework (and the Google Collections Library too) never restrict the types of their parameters except when it's necessary to prevent the collection from getting broken.

Personally I'm not a fan of that approach, but it does make some sense given the variance approach which has also been taken.

Answer 2

Some good answers can be found here:

What are the reasons why Map.get(Object key) is not (fully) generic

Answer 3

To add to @subtenante, Java is designed so that Java will always be backwards compatible. Generics disallow puts of the wrong type, as this does not break backwards compatibility. The easy way to make sure that the right key is being used is to do this.

K key = null;
V value = null;

Map<K,V>  mapped = new HashMap<K,V>()
.......//set the key and value..........
mapped.put(key, value)
.....
V var = mapped.get(key);

Problem solved.
One more caveat to generics, any child of a class can also be placed into a collection.

Number k= null;
Number v= null;

Map<Number,Number>  mapped = new HashMap<Number,Number>()
.......//set the key and value..........
k = double someDouble;
v = int someInt;
mapped.put(k, v)
.....

This could cause some big bugs!!

Answer 4

an object of one type can be .equals() to an object of another type. get() only requires that the object that you give it be .equals() to the key you are getting