代码如下

private long exitTime;

@Override

public boolean onKeyDown(int keyCode, KeyEvent event) {

if (keyCode == KeyEvent.KEYCODE_BACK

&& event.getAction() == KeyEvent.ACTION_DOWN) {

if ((System.currentTimeMillis() - exitTime) > 2000) // System.currentTimeMillis()无论何时调用，肯定大于2000

{

Toast toast = Toast.makeText(getApplicationContext(),

getResources().getString(R.string.press_again_exit),

Toast.LENGTH_SHORT);

toast.setGravity(Gravity.CENTER, 0, 0);

toast.show();

exitTime = System.currentTimeMillis();

} else {

finish();

System.exit(0);

}

return true;

}

return super.onKeyDown(keyCode, event);

}