Question

I could use a little help. I got this program to work right then I found out I had to use the MVC design. It seems pretty simple but, my little toy program won’t display my forms. HELP!! See the below snipets:

PART OF MIDLET

  public MileageMidlet()
  {

        // First get a blank user form
        form = new Form("Bradford Gas Mileage Calculator");

        startPage = new StartPageView();
        inputScreen = new InputScreen();
        calculateMileage = new CalculateMileage();

        startCmd = new Command ("Start",Command.SCREEN,5);
        clearCmd = new Command ("Clear",Command.SCREEN,1);
        enterCmd = new Command ("Enter",Command.SCREEN,1);
        exitCmd = new Command("Exit", Command.EXIT, 1);

        // Set up event handlers to process user commands
        form.setCommandListener(this);
  }


  public void startApp() {

        startPage.createView(form);            

        form.addCommand(startCmd);
        form.addCommand(exitCmd);

        // Display initial form
        Display.getDisplay(this).setCurrent(form);
 }

START PAGE VIEW CLASS

 import javax.microedition.lcdui.*;

 public class StartPageView
 {

 StringItem strgItm, strgItm2;
 private Command startCmd, exitCmd;

 public StartPageView()
 {

 }

 public void createView(Form form)
 {
        // First get a blank user form
        form.deleteAll();
        form = new Form("Bradford Gas Mileage Calculator");
        strgItm = new StringItem ("","Welcome to the Bradford Mobile Gas Mileage Calculator!");
        strgItm2 = new StringItem ("","To obtain you gas mileage please click the start button.");            
        form.append(strgItm);
        form.append(strgItm2);

 }

I got nothing! Really literally a blue screen.

}

Answer 1

The issue has nothing to do with MIDP or J2ME. The problem is of the semantics of how arguments are passed to methods.

It;s important to remember that arguments to method are passed by value in Java. The consequence is that when an object that is passed to a method, a copy of that reference is passed. Any changes to the reference of the object in the method does not have any affect outside of it.

Please see this article for more information.

So in your code,

form.deleteAll();
form = new Form("Bradford Gas Mileage Calculator");

Comment the above two lines. Everything should be fine.