correct java syntax for generics with extends

Question

this is a piece of code i'm struggling with.

public class Channel<T extends Something>{
   public Channel(){}
   public void method(T something){}
}

public class Manager{
   private static ArrayList<Channel<? extends Something>> channels
         = new ArrayList<Channel<? extends Something>>();
   public static <T extends Something> void OtherMethod(T foo){
      for(Channel<? extends Something> c : channels)
        c.method(foo);  // this does not work
   }
}

The line that does not work gives me compiler error:

The method method(capture#1-of ? extends Something) in the type Channel<capture#1-of ? extends Something> is not applicable for the arguments (T)

I don't understand this error. If I remove all the generics in Manager class it is working but type unsafe. How should I do this in correct Java?

Kalenda · Accepted Answer · 2014-12-03 15:36:01Z

1

You need a type parameter for your method public <T extends Something> void method(T foo)

public class Channel<T extends Something> {
  public Channel() {
  }

  public <T extends Something> void method(T foo) {
  }
}

public class Manager {
   private static ArrayList<Channel<? extends Something>> channels = new ArrayList<Channel<? extends Something>>();

   public static <T extends Something> void OtherMethod(T foo) {
     for (Channel<? extends Something> c : channels)
        c.method(foo); // this does not work
   }
}

answered Dec 3, 2014 at 15:36

Kalenda

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Comments

SLaks · Accepted Answer · 2014-12-03 15:10:24Z

1

That's inherently unsafe.

What happens if you add a Channel<MyThing> to the list, then call OtherMethod() with a YourThing?

You should make the entire class generic (and make the members non-static), and use the same T for the channels and the parameter.

answered Dec 3, 2014 at 15:10

SLaks

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2 Comments

ArcticLord Over a year ago

Never happens. I simplified the code. OtherMethod would search in his channels for a Channel<YourThing> and would use this. Please don't tell me that my code is not ellegant. Please tell me why it is not correct.

SLaks Over a year ago

@ArcticLord: The way your code is written, that is not true. If you aren't showing your actual code, I can't help you.

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