android 再按一次后退键退出应用程序

额。。其实这个功能我觉得有点多余的。。。不过，既然设计提出也没办法。。。还是mark下
public class BackActivity extends Activity {

private static final String TAG = "BackActivity";
private long mLastBackTime = 0;
private long TIME_DIFF = 2 * 1000;

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK) {
long now = new Date().getTime();
if (now - mLastBackTime < TIME_DIFF) {
Log.d(TAG, "back, finish");
return super.onKeyDown(keyCode, event);
} else {
Log.d(TAG, "after 2s, toast");
mLastBackTime = now;
Toast.makeText(this, "再点一次将推出", 2000).show();
}
return true;
}
return super.onKeyDown(keyCode, event);
}
}