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"?

Run FindBugs and it will complain about the bad object type used in get(). — akarnokd, Jul 17 '09 at 19:09
As the asker, I'm voting to close as a dupe of: http://stackoverflow.com/questions/857420/what-are-the-reasons-why-map-getobject-key-is-not-fully-generic — jsight, Jul 17 '09 at 19:14

score 3 · Answer 1 · answered Jul 17 '09 at 19:08

3

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.

answered Jul 17 '09 at 19:08

Falaina

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I think you mean "always" get null. :) – jsight Jul 17 '09 at 19:09
You're quoting from the 1.4.2 docs, while to OP uses 1.5 syntax. – Torsten Marek Jul 17 '09 at 19:13
1

"you'll always get null if you pass in a non-string" Not if the object you pass in has a .equals() method that returns true for strings. – newacct Jul 17 '09 at 19:56
@newacct - Excellent point, and this is a big part of the reason why they defined it the way they did. It's still more annoying than useful, though, imo. – jsight Jul 18 '09 at 04:05

score 1 · Accepted Answer · edited May 23 '17 at 10:33

1

See this Also this

edited May 23 '17 at 10:33

Community

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answered Jul 17 '09 at 19:07

Savvas Dalkitsis

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You should give context when providing links – Michael Donohue Jul 17 '09 at 19:12
Thanks for the links. It looks like the reason is basically that they wanted backwards compatibility even at the cost of making the API more painful. Sadly, this makes converting to generics harder in a lot of cases, rather than easier. – jsight Jul 17 '09 at 19:15
Duly noted... i wont edit since this is question is now voted as closed – Savvas Dalkitsis Jul 17 '09 at 19:16

score 1 · Answer 3 · answered Jul 17 '09 at 19:08

1

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.

answered Jul 17 '09 at 19:08

jjnguy

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score 1 · Answer 4 · answered Jul 17 '09 at 19:11

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.

Why does this code with generics compile?

4 Answers4

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