wildcard operator - one doubt ...

[ankur rathi]

Here�s a simple (no generics) method to print out any list:
private void printList(List list) {    
for (Iterator i = list.iterator(); i.hasNext(); ) {        System.out.println(i.next());    
}
}
 
The above still works in Java 1.5, but now it generates warning messages
Java 1.5 incorporates lint (like C lint) to look for possible problems
You should eliminate all errors and warnings in your final code, so you need to tell Java that any type is acceptable:
private void printListOfStrings(List<?> list) {
for (Iterator<?> i = list.iterator(); i.hasNext(); ) {        System.out.println(i.next());   
}
}
 

I found this from one tutorial , Is this the only use of wildcard operator ... [ when you don't want any specific paramaterized list , or can say list like 1.4's list ]

Thanks a lot .

[Joyce Lee]

when you don't want any specific paramaterized list , or can say list like 1.4's list

There is a difference between List<?> and List.

For example,

List<?> list1 = new ArrayList<Integer>();
list1.add(1); // this statement will cause compile-time error in JDK 5.0
 
List list2 = new ArrayList<Integer>();
list2.add(1); // this statement will cause unchecked warning in JDK 5.0

[marc weber]

If you really want a pre-generics List without compile-time warnings, then you need to specify the type parameter as Object.

[marc weber]

Joyce: I've never seen (nor can I imagine) the wildcard type parameter used outside of generic method arguments. Are there other valid uses for the wildcard?

[Joyce Lee]

Hi Marc,

Some examples of wildcard type parameter used on non-generic method can be seen in Collection interface like addAll, containsAll, removeAll and retainAll and its subclasses.

If you really want a pre-generics List without compile-time warnings, then you need to specify the type parameter as Object.

Are you referring to something like this?

List<Object> list = new ArrayList<Object>();
list.add(1);
Integer i = (Integer) list.get(0); // requires casting 

Joyce
[ March 03, 2005: Message edited by: Joyce Lee ]

[Guddu Jhakas]

rathi ji kya khatru question poochha hain what is ur source of that q.

[ankur rathi]

Abe guddu ,
Abhi-abhi bas dimag me aaya aur type kar diya ....

[marc weber]

Originally posted by Joyce Lee:
... Are you referring to something like this? ...

Exactly. If a person really wanted to do this, specifying the type parameter as Object would "simulate" a pre-generics collection: No type-safety and no implicit casting.

[marc weber]

Originally posted by Joyce Lee:
Some examples of wildcard type parameter used on non-generic method can be seen in Collection interface like addAll, containsAll, removeAll and retainAll and its subclasses.

Thanks for the response!

First let me verify some terminology: When you say "non-generic method," you're talking about a method that does not specify its own type parameter, but still might take arguments of a generic type. For example, public void meth1(List<Integer> inList){} would be non-generic, as opposed to the obviously generic, public <T> void meth2(T t){}. Correct?

My impression from reading Schildt (referenced here) is that both would be considered "generic methods." If this isn't accurate, then the way I phrased my question would have been misleading.

What I'm really asking is this: Are there any valid uses of the wildcard outside of a method's argument list? For example, would it ever make sense to do the following (which, as you point out above, opens the door for some interesting compilation errors)?

List<?> myList = new ArrayList<Integer>();
[ March 03, 2005: Message edited by: marc weber ]

[pooja jain]

rathi ji kya khatru question poochha hain what is ur source of that q.

Abe guddu ,
Abhi-abhi bas dimag me aaya aur type kar diya ....

jyada masti mat karo bacho
varna firangi naraj ho jayenge !!!

[Jim Yingst]

Ahem...

Please use English here at JavaRanch. Thanks.

[Joyce Lee]

For example, public void meth1(List<Integer> inList){} would be non-generic, as opposed to the obviously generic, public <T> void meth2(T t){}. Correct?

Yeah, that's the impression I gathered from the generics tutorial chap 5. Please feel free to comment if you guys think otherwise.

List<?> myList = new ArrayList<Integer>();

For the above statement, I couldn't think of a reason of using it since no element can be added to myList afterwards. I just used it to illustrate the constraint of List<?>. How about using it as a return type?

For example:

method() {
    List<? extends Number> list = getList();
    ...
}
 
...
 
List<? extends Number> getList() {
    ...
    return anyTypeOfNumberlist;
}

[marc weber]

...that's the impression I gathered from the generics tutorial chap 5. Please feel free to comment if you guys think otherwise...

To me, it makes sense that a "generic method" needs to specify its own type parameter, since this parallels the idea of a generic class. But no one asked me. Other material I've read (in particular, Herbert Schildt's Java 2 v5.0 (TIGER) New Features) seems to indicate that any method taking a parameterized argument is a "generic method." Perhaps I'm misinterpreting. I don't really know...

... How about using it as a return type? ...

That also works in the sense that it compiles and runs. But we still end up with a variable that (apparently) can't be dereferenced, so I'm still questioning whether this is a valid use.

[ankur rathi]

Sorry Jim .