J2ME: subclassing lcdui.game.Layer

Question

hello,
am trying to subclass from javax.microedition.lcdui.game.Layer (part of Sun's J2ME distribution), which is an abstract class that has no constructors.
i get a compiler error saying "cannot resolve symbol: constructor Layer()".
what am i doing wrong?
this is my non-working code:
import javax.microedition.lcdui.game.*;
public class PlayfieldLayer extends Layer {     
  public void paint(Graphics g) {     }

Answer 1

Better late than never!
The problem is that you are implicitely making a call
to the default constructor Layer() of the super class
javax.microedition.lcdui.game.Layer which does not exist.
The Layer class has one constructor Layer(int width, int height)
but this constructor has the 'package' scope,
which means it is not visible outside package
javax.microedition.lcdui.game.
This is taken from the source code from Sun:
* Creates a new Layer with the specified dimensions.
* This constructor is declared package scope to
* prevent developers from creating Layer subclasses
* By default, a Layer is visible and its upper-left
* corner is positioned at (0,0).
* @param width The width of the layer, in pixels
* @param height The height of the layer, in pixels
Layer(int width, int height) {
setWidthImpl(width);
setHeightImpl(height);
The only solution is to extend TiledLayer or Sprite instead.
It is not a very nice solution because
these classes were obviously not intended for this.
This problem is mentionned in the
Nokia Series 60 Developer Platform 2.0 Specification (p.91):
" 2.1.16.1 Layer-class subclassing
The implementation must have only the package-protected constructor:
Layer(int width, int height)
This leads to the application not being allowed to create
new classes that inherit the Layer abstract class.
This requirement means that the Sprite and TiledLayer classes
are the only classes that directly inherit the Layer class.
Any other classes must be inherited from either
the Sprite or TiledLayer class."
I tried to add my own Layer extension class in package
javax.microedition.lcdui.game, but the compiler will not allow this:
"Cannot create class in system package".
It was worth a try...
What has motivated the decision to
'prevent developers from creating Layer subclasses'?
I think it is probably a last minute decision
that is not consistent with the original intended design.
Why make the Layer abstract class public if it has no
accessible constructor?