Problem with generics and dynamic array copy

[Sebastian]

Hello there,

how can I make the compile error go away? The marked line is wrong:
Type mismatch: cannot convert from Class<capture#2-of ?> to Class<T>

/**
* Copies the specified array to a new array with the same component
* type and length as the given array.
* @param <T> the type of the array elements
* @param src the array to be copied
* @return the copy
*/
public static final <T> T[] arraycopy( T[] src )
{
Class<T> componentType = src.getClass().getComponentType(); // !!!!
@SuppressWarnings("unchecked")
T[] dest = (T[]) Array.newInstance( componentType, src.length );
System.arraycopy( src, 0, dest, 0, src.length );
return dest;
}

I'd be grateful for a hint.
-- Sebastian

 

[Sebastian]

Am 22.07.2011 19:26, schrieb Sebastian:

Hello there,

how can I make the compile error go away? The marked line is wrong:
Type mismatch: cannot convert from Class<capture#2-of ?> to Class<T>

/**
* Copies the specified array to a new array with the same component
* type and length as the given array.
* @param <T> the type of the array elements
* @param src the array to be copied
* @return the copy
*/
public static final <T> T[] arraycopy( T[] src )
{
Class<T> componentType = src.getClass().getComponentType(); // !!!!
@SuppressWarnings("unchecked")
T[] dest = (T[]) Array.newInstance( componentType, src.length );
System.arraycopy( src, 0, dest, 0, src.length );
return dest;
}

I'd be grateful for a hint.
-- Sebastian

to be mor exact: of course I could just cast componentType to Class<T>.
But why should that be necessary in view of the declaration of src?
Is the thinking behind the method wrong?

-- Sebastian

 

[markspace]

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()>

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.

 

[Sebastian]

Am 22.07.2011 20:23, schrieb markspace:

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()>


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...

-- Sebastian

 

[Robert Klemme]

Am 22.07.2011 20:23, schrieb markspace:

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()>



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(T[], int)

Cheers

robert

 

[lewbloch]

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#get....>
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#co...[], int)

I'd say _best_ yet. In general, unless it really, really doesn't do
what you need, stick to the standard API instead of reinventing it.

This does, of course, require that you be aware of the API, but things
like 'java.util.Arrays' had better be part of your standard vocabulary
in any event.

Otherwise you have some upgrading to do on your skills.

 

[Sebastian]

Am 22.07.2011 21:27, schrieb Robert Klemme:

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()>




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(T[], int)


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.

-- Sebastian

 

[lewbloch]

Am 22.07.2011 21:27, schrieb Robert Klemme:

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#get...>
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#co...[], int)

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.