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Re: Problem with generics and dynamic array copy

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From	Robert Klemme <shortcutter@googlemail.com>
Newsgroups	comp.lang.java.programmer
Subject	Re: Problem with generics and dynamic array copy
Date	Fri, 22 Jul 2011 21:27:06 +0200
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On 22.07.2011 20:36, Sebastian wrote:
> Am 22.07.2011 20:23, schrieb markspace:
>> On 7/22/2011 10:30 AM, Sebastian wrote:
>>
>>>> public static final <T> T[] arraycopy( T[] src )
>>>> {
>>>> Class<T> componentType = src.getClass().getComponentType(); // !!!!
>>
>>
>> RTFM. getComponentType() returns Class<?>, not Class<T>.
>>
>> <http://download.oracle.com/javase/6/docs/api/java/lang/Class.html#getComponentType%28%29>
>>
>>
>>
>> public Class<?> getComponentType()
>>
>> Returns the Class representing the component type of an array. If this
>> class does not represent an array class this method returns null.
>>
>
> well, yes, but given that src.getClass() must give one the class object
> for arrays with component type T, why is the compiler not smart enough
> to infer that the unknown class parameter in the return value of
> getComponentType() must be T? As a human I can see that, that's why I
> can cast to Class<T>, but I don't believe that I'm smarter than javac...

The compiler has no idea what the semantics of getComponentType() is. 
It could be implemented as

public Class<?> getComponentType() { return Object.class; }

and still be conformant to the declaration.  Hence it cannot do any 
automatic inference based on the fact you know that the array is T[]. 
Btw, you can actually pass B[] where B is a subclass of T.

Since Array.newInstance() accepts Class<?> you should simply use that - 
that cast to T[] is needed anyway.

Of course, even better you scrap your implementation and use

http://download.oracle.com/javase/6/docs/api/java/util/Arrays.html#copyOf%28T[],%20int%29

Cheers

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

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Problem with generics and dynamic array copy Sebastian <sebastian@undisclosed.invalid> - 2011-07-22 19:26 +0200
  Re: Problem with generics and dynamic array copy Sebastian <sebastian@undisclosed.invalid> - 2011-07-22 19:30 +0200
    Re: Problem with generics and dynamic array copy markspace <-@.> - 2011-07-22 11:23 -0700
      Re: Problem with generics and dynamic array copy Sebastian <sebastian@undisclosed.invalid> - 2011-07-22 20:36 +0200
        Re: Problem with generics and dynamic array copy Robert Klemme <shortcutter@googlemail.com> - 2011-07-22 21:27 +0200
          Re: Problem with generics and dynamic array copy lewbloch <lewbloch@gmail.com> - 2011-07-22 13:33 -0700
          Re: Problem with generics and dynamic array copy Sebastian <sebastian@undisclosed.invalid> - 2011-07-22 22:37 +0200
            Re: Problem with generics and dynamic array copy lewbloch <lewbloch@gmail.com> - 2011-07-22 15:49 -0700

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