如何使用一个继承MIDlet的抽象类
大家好！
  现在发现一个抽象类继承MIDlet,但运行后，自动推出了，然后报错：java.lang.InstantiationException
  想问下，MIDlet不是个抽象类吗？为什么还要写一个抽象类来继承它呢？
   
abstract public class SDAApplication extends MIDlet {

  private static SDAApplication instance;
  protected final SDAApplication Application = this;
  private AppCanvas appCanvas = null;
  private Graphics graphics = null;
  private Display disp = Display.getDisplay(this);
  private SDAForm foucsedForm = null;
  private SDAForm mainForm = null;
  public SDAInputPanel inputPanel = new SDAInputPanel();
  private Vector dispList = new Vector();
  private SDAMessageBox msgBox;
  public static final int SKIN_STANDARD = 0;
  public static final int SKIN_POPULAR = 1;
  private SDASkins Skins = null;
  private int skin = SKIN_STANDARD;
  private boolean doubleBuffered = true;
  private boolean ctrl3d = false;
  private Timer runOneceTimer = new Timer();
  public SDAApplication() {
  instance = this;
  this.appCanvas = new AppCanvas();
  this.appCanvas.setFullScreenMode(true);
  this.disp.setCurrent(appCanvas);
  }

  public final void setSkin(int skin) {
  this.skin = skin;
  if (skin == SKIN_POPULAR) {
  Skins = null;
  Skins = SDASkins.CreateSkins();
  }
  }

  public final void setDoubleBuffered(boolean doubleBuffered) {
  this.doubleBuffered = doubleBuffered;
  }

  public void setCtrl3d(boolean ctrl3d) {
  this.ctrl3d = ctrl3d;
  }

  public void setMainForm(SDAForm mainForm) {
  this.mainForm = mainForm;
  }

  public final boolean isDoubleBuffered() {
  return this.doubleBuffered;
  }

  public final boolean hasPointer() {
  return this.appCanvas.hasPointerEvents();
  }

  protected final Graphics GetGraphics() {
  return this.graphics;
  }

  protected final Canvas GetCanvas() {
  return this.appCanvas;
  }

  protected SDAForm getFoucsedForm() {
  return foucsedForm;
  }
  public int getSkin() {
  return skin;
  }

  public boolean isCtrl3d() {
  return ctrl3d;
  }

  public static SDAApplication GetInstance() {
  return instance;
  }

  public void MessageBox(String Caption, String Text, int flags) {
  MessageBox(Caption, Text, flags, null);
  }

  public void MessageBox(String Caption, String Text, int flags,
  MsgBoxCloseEvent closeEvent) {
  msgBox.closeEvents.addElement(closeEvent);
  msgBox.caption = Caption;
  msgBox.txtLabel.setText(Text);
  msgBox.button1.setVisible(false);
  msgBox.button2.setVisible(false);
  int btnFlags = flags % 10;
  switch (btnFlags) {
  case SDAConsts.MB_OK: {
  msgBox.button1.setVisible(true);
  msgBox.button1.setText("确定");
  break;
   
  }
  case SDAConsts.MB_YESNO: {
  msgBox.button1.setVisible(true);
  msgBox.button2.setVisible(true);
  msgBox.button1.setText("是");