Question

我试图通过为我的应用程序使用多个类来使我的代码更整洁表格选项。目前，在尝试setCurrent时，我一直收到空指针异常。

这是我的主类，当我调用另一个类时，错误在我的命令监听器中开始。

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();
          }
      }         
  }
}

这是被调用的类。

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

发生NullPointerException是因为SelectDate类中的display为null。

要解决此问题，您可以从那里删除它，而是添加到方法参数：

// ...
public static void BuildCalendar(Display display) // added parameter

然后，当您从CalFrontEnd调用上述方法时，将display的实例传递到那里：

                      // ...
                      SelectDate.BuildCalendar(display); //Error will go away

在多个类中设置可显示的对象（J2ME）

1 个答案: