CustomForm for Singleton

import com.app.gui.Midlet;
import javax.microedition.lcdui.Form;

public abstract class CustomForm extends Form{

    protected CustomForm prevForm;
    protected CustomForm thisForm;

    public CustomForm() {
        super("App Name");
        thisForm = this;
    }

    abstract protected void initComponents();

    abstract protected void addComponents();

    public boolean setAsDisplay (CustomForm prevForm) {
        thisForm.prevForm = prevForm;
        return setAsDisplay();
    }
    public boolean setAsDisplay () {
        Midlet.getDisplay().setCurrent(this);
        return true;
    }

    protected String fillIfNull (String str) {
        return fillIfNull(str, " - ");
    }

    String fillIfNull (String str, String nullString) {
        if (str == null || str.equals("null")) {
                str = nullString;
        }
        return str;
    }

}