BitmapField click does not work on Blackberry application

i arranged 7 bimapfield in horizontal if i click the bitmapfield it want to push the another screen Here is my code but i did't get the another screen can any one help me whats wrong in this code

BitmapField bitmap1 = new BitmapField(
    Bitmap.getBitmapResource("profile_n.png"),FOCUSABLE | DrawStyle.HCENTER)
{
    protected void onFocus(int direction)
    {
        rollno=1开发者_运维技巧;
        this.getScreen().invalidate();
    }
    protected void onUnfocus()
    {

    }   
    protected boolean navigationClick(int status, int time){
           UiApplication.getUiApplication().popScreen(getScreen());
            UiApplication.getUiApplication().pushScreen(new AccMainScreen());
        return true;
    }
};

You can use a custom field that hold a image and behave like a button instead of bitmapfield. Here is the code i suggest:

import net.rim.device.api.system.Bitmap;
import net.rim.device.api.ui.Field;
import net.rim.device.api.ui.Graphics;
public class CustomButton extends Field{

    protected Bitmap icon;
    protected int fieldWidth;
    protected int fieldHeight;

    public CustomButton (String iconSource,long style) {
        super(style);
        icon = Bitmap.getBitmapResource(iconSource);
        fieldHeight = icon.getHeight();
        fieldWidth = icon.getWidth();
    }

    public int getPreferredWidth() {
        return fieldWidth;
    }

    public int getPreferredHeight() {
        return fieldHeight;
    }
    protected void layout(int arg0, int arg1) {
        setExtent(getPreferredWidth(), getPreferredHeight());
    }
    protected void drawFocus(Graphics graphics, boolean on){ }

    protected void paint(Graphics graphics) {
        graphics.fillRect(0, 0, fieldWidth, fieldHeight);
        graphics.drawBitmap(0,0, fieldWidth, fieldHeight, icon, 0, 0);
    }
    protected boolean navigationClick(int status, int time) {
        fieldChangeNotify(0);
        return true;
    }
}

you can use this button like a default button and change listener of it by using setChangeListener function.

CustomButton aButton = new CustomButton ("graphics/someIcon.png");
aButton.setChangeListener(new FieldChangeListener() {
    public void fieldChanged(Field field, int context) {
        UiApplication.getUiApplication().popScreen(getScreen());
        UiApplication.getUiApplication().pushScreen(new AccMainScreen());   
    }
});