The main reason for this is that if you call setFulLScreenMode(true) within the constructor of a class - the actual update won’t happen until after the constructor has finished. If you have the calls to getHeight() within the same constructor, or within another constructor in a sequence of class initializations, then it will return the wrong value.

Here is one solution (assuming you use a thread for your game loop):

1) Move your call to setFullScreenMode(true); outside of any constructor preferably move it to the startApp() method for your main MIDlet:

(e.g.)

public void startApp()
{
if(canvas== null)
{
canvas = new MyCanvas(this);
canvas.setFullScreenMode(true);
}

Display.getDisplay(this).setCurrent(canvas);
canvas.start(); // To start the Thread.
}

2) You then need to override the sizeChanged(int w, int h) method of GameCanvas:

(e.g)

public void sizeChanged(int _w, int _h)
{
slx = _w;
sly = _h;

screenUpdated = true;
}

You will notice here that I am using a Boolean flag to identify whether the screen has been updated or not...

3) In your thread's run() method, inside the while loop, add the following call:

while(!screenUpdated)
{
currentThread.yield();
}

This will make your current Thread and wait until the Canvas has been updated and hopefully you will now have the right values