Java Wildcards

[Brendon McCullum]

Hello all, this is my first post on Java Ranch. I have a very small doubt, regarding Java wildcards.
I am going through the following documentation on wildcards : Javadocs

All's clear but for a certain piece of code :

Collection<?> c = new ArrayList<String>();
c.add(new Object()); // Compile time error

Here as per the author, the type of c is unknown, hence a new Object cannot be added to it, since the type is not known. But, as per first line of this code, the collection refers to an ArrayList of Strings.
Hence, isn't the type String defined for the Collection c. If not, what does line 1 infers to?
I would appreciate help on this. Thanks.

[Campbell Ritchie]

I can never remember why, but you can never add to a Collection<?>. The fact that it is declared as Collection<?> is what matters, not that its runtime type is Collection<String>.

[Campbell Ritchie]

Got to be off now, but


… welcome to the Ranch

[Tony Docherty]

Brendon McCullum wrote:Here as per the author, the type of c is unknown, hence a new Object cannot be added to it, since the type is not known. But, as per first line of this code, the collection refers to an ArrayList of Strings.
Hence, isn't the type String defined for the Collection c. If not, what does line 1 infers to?
I would appreciate help on this. Thanks.

Yes your first line does assign an ArrayList of Strings to 'c' but between that line and and line where a String is being added to 'c' a different Collection (eg ArrayList<Integer>) could have been assigned to 'c'. So just because you originally assigned an ArrayList of Strings to 'c' it doesn't mean it will always refer to an ArrayList of Strings. If you want 'c' to always refer to a Collection of Strings then declare 'c' as Collection<String> rather than using the wildcard.

[Bear Bibeault]

I think generics is too advanced a topic for the beginner's forum; I've promoted this to the General Java forum.

[Henry Wong]

Brendon McCullum wrote:
Here as per the author, the type of c is unknown, hence a new Object cannot be added to it, since the type is not known. But, as per first line of this code, the collection refers to an ArrayList of Strings.
Hence, isn't the type String defined for the Collection c. If not, what does line 1 infers to?

Simply, there is nothing in the Java Specification that says, the compiler should go back a few lines and try to figure out what type it is. And if it did, how? how far should it go back? What to do if that value is not really clear? etc. etc. etc.

It is simply an ArrayList of an unknown type. Period.


Oh, and BTW ... you can't add an Object instance into an ArrayList<String>, so it would be a compile error, even if the compiler can figure it out.

Henry

[Henry Wong]

Campbell Ritchie wrote:I can never remember why, but you can never add to a Collection<?>. The fact that it is declared as Collection<?> is what matters, not that its runtime type is Collection<String>.

Technically, there is one value that can be added to a collection of any generic type, and hence, can be added to a collection of an unknown generic type. And that value is null.

Henry