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JDK1.5 Generic Class Cast problem..

Posted on 2006-10-20
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Last Modified: 2011-04-14
Map<?, ?> map = (Map<?, ?>) req.getParameterMap();

Set<String> set = (Set<String>) map.keySet();

=========================

I received the following compilatio warning:

warning: [unchecked] unchecked cast
found   : java.util.Set<capture of ?>
required: java.util.Set<java.lang.String>
                Set<String> set = (Set<String>)map.keySet();

===========
How do I get rid of this warning??
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Question by:zollen
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7 Comments
 
LVL 14

Expert Comment

by:hoomanv
ID: 17773748
Because of Map<?, ?>
You can only say
Set<?> set = (Set<?>) map.keySet();
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LVL 86

Expert Comment

by:CEHJ
ID: 17773792
Try

java.util.Enumeration<String> enum = req.getParameterNames();

or

java.util.Enumeration<String> enum = (java.util.Enumeration<String>)req.getParameterNames();
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LVL 14

Expert Comment

by:hoomanv
ID: 17773981
CEHJ there is no Generics support in javaee according to the documentation
http://java.sun.com/javaee/5/docs/api/javax/servlet/ServletRequest.html#getParameterMap()
All collections are being used like traditional non-generic form
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Expert Comment

by:CEHJ
ID: 17774007
That's true
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LVL 9

Expert Comment

by:shinobun
ID: 17774061
Since the J2EE specs do not return generalized (?) Maps, the best you can do is annotate your method with SuppressWarnings to tell the compiler to ignore the warning.

http://java.sun.com/j2se/1.5.0/docs/api/java/lang/SuppressWarnings.html
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Expert Comment

by:shinobun
ID: 17774081
I'm not saying that it will work at runtime, though...  You still might get a ClassCastException.
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Accepted Solution

by:
shinobun earned 50 total points
ID: 17774143
@SuppressWarnings("unchecked")
void yourMethod() {
    Map<?, ?> map = (Map<?, ?>) req.getParameterMap();
    Set<String> set = (Set<String>) map.keySet();
}

cf. http://java.sun.com/docs/books/tutorial/java/javaOO/annotations.html
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