private long exitTime = 0; 

    @Override

    public boolean onKeyDown(int keyCode, KeyEvent event) {

    if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN){

    if((System.currentTimeMillis()-exitTime) > 2000){

    Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();

    exitTime = System.currentTimeMillis();

    } else {

    finish();

    System.exit(0);

    }

    return true;

    }

    return super.onKeyDown(keyCode, event);

    }