Avoid focus on first screen field in blackberry

I've add some customized buttonfield follow by some label field. When opening the app, 开发者_StackOverflow社区the first buttonfield gets focus, but I don't need the focus when opening the screen. How to avoid the focus on opening the screen?

use NullField like the code below:

NullField field = new NullField();
field.setFocus();

This will avoid the focus on any field in the screen