@Override    public boolean onKeyDown(int keyCode, KeyEvent event) {        // TODO Auto-generated method stub                if(keyCode==KeyEvent.KEYCODE_BACK){            //弹出确定退出对话框            new AlertDialog.Builder(this)            .setTitle("退出")            .setMessage("确定退出吗？")            .setPositiveButton("确定", new DialogInterface.OnClickListener() {                                @Override                public void onClick(DialogInterface dialog, int which) {                    // TODO Auto-generated method stub                    Intent exit = new Intent(Intent.ACTION_MAIN);                    exit.addCategory(Intent.CATEGORY_HOME);                    exit.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);                    startActivity(exit);                    System.exit(0);                }            })            .setNegativeButton("取消", new DialogInterface.OnClickListener() {                                @Override                public void onClick(DialogInterface dialog, int which) {                    // TODO Auto-generated method stub                    dialog.cancel();                }            })            .show();            //这里不需要执行父类的点击事件，所以直接return            return true;        }        //继续执行父类的其他点击事件        return super.onKeyDown(keyCode, event);    }