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3 Replies Latest reply: May 11, 2012 9:38 AM by 936584 RSS

Generic Methods

936584 Newbie
Currently Being Moderated
Hi,

I've created a class with a generic method as follows:
public class Box<T> {
     
     public static <U> void someGenericMethod(U u, U v) {
          System.out.println("u: " + u.getClass().getName());
          System.out.println("v: " + v.getClass().getName());
     }
     
}
And then wrote some code to test it:
Box.someGenericMethos(new Integer(10), "some text");
This works perfectly. The compiler infers the type for each parameter u and v, one is an
Integer and the other one is a String.

I modified the generic method as follows:
     public static <U> void someGenericMethos(U u, List<Box2<U>> boxes) {
          System.out.println("u: " + u.getClass().getName());
          System.out.println("v: " + boxes.getClass().getName());
     }
And wrote some code to test it again:
          List<Box2<String>> boxes = new ArrayList<Box2<String>>();
          Box2.someGenericMethos(new Integer(10), boxes); // Compiler error
Now I have a compiler error.

My question is, Why the compiler doesn't infer the type for both arguments as in the first case?

Thanks in advance.

Edited by: EJP on 11/05/2012 09:34: added {noformat}
{noformat} tags: please use them. Code is basically illegible without them.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
  • 1. Re: Generic Methods
    EJP Guru
    Currently Being Moderated
    Now I have a compiler error.
    What compile error?

    Posting statements like that is a complete waste of time.
  • 2. Re: Generic Methods
    jtahlborn Expert
    Currently Being Moderated
    The problem is that since your are using an Integer and String, the only possible value for the type U is Object. that means the method is expecting a "List<Box2<Object>>", but you are passing it a "List<Box2<String>>", which is not the same type. nested generics are a little trickier. generally, the solution to such situations is to add (one or more) "? extends " to the nested type in your method parameter. i believe that in your case, you will need "List<? extends Box2<? extends U>>".
  • 3. Re: Generic Methods
    936584 Newbie
    Currently Being Moderated
    Hi,

    First of all thanks a lot for your answers.

    I'm sorry I didn't say what the compile error was, it won't happen again.

    This is the message error:
    The parameterized method <Integer>addBox(Integer, List<Box<Integer>>) of the type Box is not applicable for the arguments (Integer, List<Box<String>>)

    But now I understand why I'm getting this error, or at least that's what I think.

    The solution is a little bit harder to understand right now, but I'm gonna study and think about it.

    Thanks again for your answers.

    PD: Where can I find all the tags I can use to write my posts?

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