Java Generics - Using the generic type parameter of the enclosing class in a method parameter

Question

I've a simple generic class follows which accepts a generic type parameter, which is the same as the one declared as a type parameter of the class:

import java.lang.reflect.AnnotatedElement;
import java.lang.reflect.Method;

public abstract class SimpleClass<T extends AnnotatedElement>
{

   public void operate()
   {
      Method m = null;
      this.doSomething(m); // Error : SimpleClass.java:[34,10] doSomething(T) in SimpleClass<T> cannot be applied to (java.lang.reflect.Method)
   }
   protected abstract void doSomething(T annotatedElement);
}

This code fails to compile at the following line:

this.doSomething(m);

with this error:

Error : SimpleClass.java:[34,10] doSomething(T) in SimpleClass<T> cannot be applied to (java.lang.reflect.Method)

Am I missing something here? The type parameter T is marked as T extends AnnotatedElement. As such, I would expect the call to doSomething with a java.lang.reflect.Method argument to compile successfully.

First thoughts: Method isn't a subclass of AnnotatedElement, but rather a class that implements it. — Powerlord
– Powerlord, Commented Mar 3, 2011 at 15:06
@JeremyHeller: I realized that after reading the Java spec, but SO was screwing up and wouldn't let me delete that comment. — Powerlord
– Powerlord, Commented Mar 3, 2011 at 15:20

David Yaw · Accepted Answer · 2011-03-03 15:08:40Z

2

Method implements AnnotatedElement, but that doesn't require that T is a method. What if the class is declared as SimpleClass<Constructor>? That satisfies <T extends AnnotatedElement>, but doesn't support conversion from Method.

answered Mar 3, 2011 at 15:08

David Yaw

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5 Comments

Java User Over a year ago

I see what you are saying. But to take it a bit further, if I change my class and method definition to:

public abstract class SimpleClass<T extends AnnotatedElement> {     public void operate()    {       Method m = null;       this.doSomething(m); // Compiles now!    }    protected abstract <U extends AnnotatedElement> void doSomething(U annotatedElement); }

it starts compiling fine!

Jeremy Over a year ago

@Java User: That is because you are defining a method-level generic, which doesn't depend on the generic defined on the class.

Java User Over a year ago

Sorry about that formatting, I'm having a hard time getting it to show up correctly

Java User Over a year ago

and @Jeremy, thanks for explaining it. I now understand what's going on. Also, I guess I need to do some reading around generics to really wrap my head around it. Thanks again.

David Yaw Over a year ago

@Java: Yes, it compiles, but within that method, you now have two generic types: T and U, which both implement AnnotatedElement, but may or may not be related.

Erik · Accepted Answer · 2011-03-03 15:10:56Z

0

There's no way to know that T is a Method. It could just as well be e.g. a Package, and then your operate() wouldn't make sense, trying to pass a Method to something expecting Package

answered Mar 3, 2011 at 15:10

Erik

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Comments

Jeremy · Accepted Answer · 2011-03-03 15:14:45Z

0

This is because T doesn't matter here. All you know is that the type is AnnotatedElement.

The following change would make it compile:

 protected abstract void doSomething(AnnotatedElement annotatedElement);

answered Mar 3, 2011 at 15:14

Jeremy

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Comments

Umesh Kacha · Accepted Answer · 2011-03-03 15:16:57Z

0

Your generic method is not implemented correctly. Change your generic method to

protected abstract<T extends AnnotatedElement> void doSomething(T annotatedElement);

Above statement creates correct generic method. This will remove compilation error.

answered Mar 3, 2011 at 15:16

Umesh Kacha

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