Setting displayable objects in multiple classes (J2ME) - java-me

Question

I'm trying to make my code neater by using multiple classes for my applications
form options. Currently I keep getting null pointer exceptions when trying to setCurrent.
Here is my main class the error starts in my command listener when I call the other class.
import java.util.*;
import javax.microedition.lcdui.*;
import javax.microedition.midlet.MIDlet;
public class CalFrontEnd extends MIDlet implements CommandListener {
private Display display;
private List list = new List("Please Select a Option", List.IMPLICIT);
private List Blist = new List("Please Select a Browsing Option", List.IMPLICIT);
private Command select = new Command("Select", Command.SCREEN, 1);
private Command exit = new Command("Exit", Command.EXIT, 2);
private Command save = new Command("Save,", Command.SCREEN, 2);
private DateField calendar;
Alert alert;
//
//
//
public CalFrontEnd() {
display = Display.getDisplay(this);
list.append("Select Date", null);
list.append("Add Events", null);
list.append("Remove Events", null);
list.append("Browse Events", null);
list.addCommand(select);
list.addCommand(exit);
list.setCommandListener(this);
}
//
//Start Application Method
//
public void startApp() {
display.setCurrent(list);
}
//
//Pause Application Method
//
public void pauseApp() {
}
//
//Destroy Application Method
//
public void destroyApp(boolean unconditional) {
}
//
//Method creates form which contains calendar
//
/*public void selectDate()
{
calendar = new DateField("Date In :", DateField.DATE, TimeZone.getTimeZone("GMT"));
Form cform = new Form("Calendar");
cform.append(calendar);
cform.addCommand(exit);
display.setCurrent(cform);
}*/
//
//Method creates form which contains adding events
//
public void AddEvents()
{
TextBox aeText = new TextBox("Add Event","", 256, 0);
display.setCurrent(aeText);
}
//
//Method creates form which contains removing events
//
public void RemoveEvents()
{
Form reform = new Form("Remove Event");
reform.append(calendar);
display.setCurrent(reform);
}
//
//Method creates form which contains removing events
//
public void BrowseEvents()
{
Blist.append("Monthly", null);
Blist.append("Daily", null);
Blist.addCommand(select);
Blist.addCommand(exit);
Blist.setCommandListener(this);
display.setCurrent(Blist);
}
//
//but it's better practice to make each form a different class extending CommandListener and it's own commandAction. And leave the display public static in MIDlet class
//...
public void commandAction(Command command, Displayable displayable) {
if (displayable == list) {
if (command == List.SELECT_COMMAND) {
switch (list.getSelectedIndex()) {
case 0: // select date
SelectDate.BuildCalendar(); //Error Here
break;
case 1: //add events
AddEvents();
break;
}
} else if (command == exit) {
destroyApp(false);
notifyDestroyed();
}
}
}
}
And here is the class that is being called.
public class SelectDate
{
private static DateField calendar;
private static Form form = new Form("derb");
private static Command select = new Command("Select", Command.SCREEN, 1);
private static Command exit = new Command("Exit", Command.EXIT, 2);
private static Command save = new Command("Save,", Command.SCREEN, 2);
private static Display display;
public static void BuildCalendar()
{
calendar = new DateField("Date In :", DateField.DATE, TimeZone.getTimeZone("GMT"));
form.append(calendar);
form.addCommand(exit);
display.setCurrent(form);
}
}

Answer 1

The NullPointerException happens because display in SelectDate class is null.
To fix that, you can for example drop it from there and instead, add to method parameters:
// ...
public static void BuildCalendar(Display display) // added parameter
Then, when you invoke above method from CalFrontEnd, pass the instance of display there:
// ...
SelectDate.BuildCalendar(display); //Error will go away