Java Generic type get call with invalid class causes no compiler error

Question

Why doesn't this Java code generate any compiler error?

Map<String, String> m = new HashMap<String, String>();
m.get(1);

I try to find an Integer in a map which maps Strings to Strings. This is clearly a programming mistake.

Similarly, this code does not generate an error either:

Map<Integer, String> m = new HashMap<Integer, String>();
m.get("dd1");

So Java compiler does not seem to check the type of the argument to Map.get(Object) method. Any ideas?

Also take a look at http://stackoverflow.com/questions/857420/what-are-the-reasons-why-map-getobject-key-is-not-fully-generic — René Link, Dec 03 '13 at 07:38
"This is clearly a programming mistake." No it is not. Objects of one class can be equal to objects of another class. It happens to not be the case for `Integer` and `String`, but it can happen in general. — newacct, Dec 04 '13 at 11:07

user207421 · Answer 1 · 2013-12-03T09:04:49.293

4

Java compiler does not seem to check the type of the argument to Map.get(Object) method.

Certainly it does. It checks that it's an Object. There's nothing else for it to check. The only way there can be an 'invalid class' is if you are using a primitive type.

edited Dec 03 '13 at 09:04

answered Dec 03 '13 at 07:40

user207421

305,947
44
307
483

Suresh Atta · Answer 2 · 2013-12-03T08:58:14.893

3

Map interface get method takes Object as a argument.

No error in both the cases. The only thing is it must be an Object and if the equals method satisfied with passed Object, then the value returns.

edited Dec 03 '13 at 08:58

answered Dec 03 '13 at 07:32

Suresh Atta

120,458
37
198
307

It doesn't have anything whatsoever to do with Generics. The same applied in 1998. The argument is an `Object,` it is checked to be an `Object.` – user207421 Dec 03 '13 at 08:50
@EJP What I mean is For example if OP using `get(T)` , then he might realizes that it's a benefit of making it as a Object. What you told is mentioned in my first point. – Suresh Atta Dec 03 '13 at 08:52
There is no such thing as `get(T).` The method is defined as `get(Object).` – user207421 Dec 03 '13 at 08:54
@EJP Where T is object and my sentence might misleading. Just edited. – Suresh Atta Dec 03 '13 at 09:00
You're beating about the bush. It didn't just 'mislead', it was *wrong.* There is no `T,` and there are no Generics. – user207421 Dec 03 '13 at 09:03
@EJP Got it, What you just said. Thanks for the correction. – Suresh Atta Dec 03 '13 at 09:06

Ruchira Gayan Ranaweera · Answer 3 · 2013-12-03T07:43:57.157

2

m.get(Object); argument here is valid for any object type in your map. So that will not give you compile error.

edited Dec 03 '13 at 07:43

answered Dec 03 '13 at 07:37

Ruchira Gayan Ranaweera

34,993
17
75
115

score 0 · Answer 4 · answered Dec 03 '13 at 07:32

0

Generally it check at the time of insertion. in put method.

when no wrong data inserted in map then not required to check while getting data using get method

answered Dec 03 '13 at 07:32

shreyansh jogi

2,082
12
20

score 0 · Answer 5 · answered Dec 03 '13 at 07:38

0

As you can see in the documentation of (Java-Map) the get method does not use the generic injection, but instead Object as input parameter. So you will get a ClassCastException at Runtime. This differs from the put method where the generic type injection is used.

answered Dec 03 '13 at 07:38

flex36

146
1
3

"So you will get a ClassCastException at Runtime." No you won't. It can't. Read the contract of the `get` method. – newacct Dec 04 '13 at 11:08

user3059794 · Answer 6 · 2013-12-03T07:42:11.010

-1

it will check your put action and the return value（ public V get(Object key) ）, not all of the action.

edited Dec 03 '13 at 07:42

answered Dec 03 '13 at 07:34

user3059794

41
3

question has nothing to do with `put` – newacct Dec 04 '13 at 11:09

Java Generic type get call with invalid class causes no compiler error

6 Answers6