Re: Problem with generics and dynamic array copy

From:

lewbloch <lewbloch@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 22 Jul 2011 15:49:04 -0700 (PDT)

Message-ID:

<8d967dd6-a043-4e28-a9a3-6a158791ecba@d8g2000prf.googlegroups.com>

On Jul 22, 1:37 pm, Sebastian <sebast...@undisclosed.invalid> wrote:

Am 22.07.2011 21:27, schrieb Robert Klemme:

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#ge=

t...>

public Class<?> getComponentType()

Returns the Class representing the component type of an array. If thi=

class does not represent an array class this method returns null.

well, yes, but given that src.getClass() must give one the class objec=

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. I=

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#co...=

[],%20int%29

Cheers

robert

Thanks. Java has a confusing variety of ways to copy an array:
  - the copyOf and copyOfRange methods in Arrays
  - System.arraycopy
  - clone and do a cast

I'll go with your recommendation.

'clone()' and cast is the least optimal. 'System.arraycopy()' is the
"old" way. 'Arrays.copyOf()' is the "new" way and most typesafe, and
is somewhat easier to use than the others.

--
Lew