public void onBackPressed() {
    Intent setIntent = new Intent(Intent.ACTION_MAIN);
    setIntent.addCategory(Intent.CATEGORY_HOME);
    setIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    startActivity(setIntent);
  }

@Override
  public boolean onKeyDown(int keyCode, KeyEvent event) {
     //当按下键盘上返回按钮，给出退出对话框
     if (keyCode == KeyEvent.KEYCODE_BACK) {
    // System.exit(0);
     //直接退出程序
       /*Intent setIntent = new Intent(Intent.ACTION_MAIN);
       setIntent.addCategory(Intent.CATEGORY_HOME);
       setIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
       startActivity(setIntent);
       */
       onBackPressed();
       //不退出程序仅仅返回桌面
       return false;
     } else {
       return super.onKeyDown(keyCode, event);
     }
   }