//運用list來保存們每一個activity是關鍵  private List<Activity> mList = new LinkedList<Activity>();  //為了實現每次使用該類時不創建新的對象而創建的靜態對象  private static MyApplication instance;  //構造方法  public MyApplication() {  }  //實例化一次  public synchronized static MyApplication getInstance() {    if (null == instance) {      instance = new MyApplication();    }    return instance;  }  //添加要退出的Activity  public void addActivity(Activity activity) {    mList.add(activity);  }  //關閉每一個list內的activity  public void exit() {    try {      for (Activity activity : mList) {        if (activity != null)          activity.finish();      }    } catch (Exception e) {      e.printStackTrace();    } finally {      System.exit(0);    }  }  //殺進程  public void onLowMemory() {    super.onLowMemory();    System.gc();  }

MyApplication.getInstance().addActivity(this);

  /**   * 兩次返回退出   */  private long mExitTime;  public boolean onKeyDown(int keyCode, KeyEvent event) {    if (keyCode == KeyEvent.KEYCODE_BACK) {      if ((System.currentTimeMillis() - mExitTime) > 3000) {        Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();        mExitTime = System.currentTimeMillis();      } else {        MyApplication.getInstance().exit();      }      return true;    }    return super.onKeyDown(keyCode, event);  }