BlackBerry - Exception when creating a graphics object from a bitmap - exception

Question

I am making the following call in my blackberry application (API ver 4.5)...
public void annotate(String msg, EncodedImage ei)
{
Bitmap bitmap = ei.getBitmap();
Graphics g = new Graphics(bitmap);
g.drawText(msg,0,0);
}
And I keep getting an IllegalArgumentException when I instantiate the Graphics object. Looking at the documentation for Graphics is confusing as it leaves many things unstated.
What does it mean by 'default type of the device'?
How do you know if the type of 'bitmap' is not supported? Does this mean that there are different types of bitmaps? Can different encodedImages generate different types of bitmaps?
Is there another way to add my string to the associated encoded image?
public Graphics(Bitmap bitmap)
Constructs a Graphics object for drawing to a bitmap.
Parameters:
bitmap - Bitmap to draw into. Must be Bitmap.COLUMNWISE_MONOCHROME or the default type of the device.
Throws:
IllegalArgumentException - If the type of 'bitmap' is not supported, or the bitmap is readonly.

Answer 1

Are you sure that your Bitmap is mutable? You can't create Graphics objects from immutable Bitmaps. That is one cause of an IllegalArgumentException. You can set the decode mode for your EncodedImage (EncodeImage.setDecodeMode). There are different modes that allow you to specify whether the file is native or readonly...along with other modes that can be combined.
The size of the bitmap might be another IllegalArgumentException. Of course, this is relevant to the target device.

Answer 2

I'd imagine that the default type depends on the graphics chip and hardware. (If you have a monochrome screen, the default would probably be different than if you had a color one.)
Bitmap has a static method getDefaultType(), which "Queries the default Bitmap type for the device". There's also a non-static method getType(). It seems to be telling you the rule is that for the code above to work then either:
bitmap.getType() == Bitmap.getDefaultType()
...or...
bitmap.getType() == COLUMNWISE_MONOCHROME
And presumably neither of these conditions are true. You can do a sanity check on that, and maybe print out the result of getDefaultType() so you know what your target is.
Looks like you'll have to convert the bitmap or get it from somewhere else.

Answer 3

The Graphics object isn't normally constructed explicitly. Rather, you are given an instance of it in the paint() method, if you've overridden it.
I suspect what you want to do is create a subclass of BitmapField and override the paint() method to include your code for drawing text on the bitmap.