Why does this code with generics compile?

Question

This seems like a stupid question, but I'm tripping over it at the moment. Why does this compile?

import java.util.*;

public class Test {
        public static void main (String[] argv) throws Exception {
                Map<String,String> map = new HashMap<String,String>();
                map.get(new ArrayList<String>());
        }
}

Shouldn't it be illegal to call get with something that's not compatible with "String"?

Answer 1

See this Also this

Answer 2

From the Javadocs for Map:

V get(Object key)

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

get is simply a method which takes in ANY object, and will (if it exists) return an object that was mapped to it. So passing it a non-string is legal, however, since all the keys have to be strings, you'll always get null if you pass in a non-string.

Answer 3

The get() method for Map just takes an Object, not the generic type K.

The code will compile, but will never get anything out of the Map.

Answer 4

Map.get takes an Object, not a generic type, cf. the documentation.

get(Object key): Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

The important thing is that it returns a generic type, so you do not have to cast the return value.