private long exitTime = 0;
	
/**
 * 捕捉返回事件按钮
 * 
 * 因为此 Activity 继承 TabActivity 用 onKeyDown 无响应，所以改用 dispatchKeyEvent
 * 一般的 Activity 用 onKeyDown 就可以了
 */

@Override
public boolean dispatchKeyEvent(KeyEvent event) {
  if (event.getKeyCode() == KeyEvent.KEYCODE_BACK) {
    if (event.getAction() == KeyEvent.ACTION_DOWN && event.getRepeatCount() == 0) {
      this.exitApp();
    }
    return true;
  }
  return super.dispatchKeyEvent(event);
}

/**
 * 退出程序
 */
private void exitApp() {
  // 判断2次点击事件时间
  if ((System.currentTimeMillis() - exitTime) > 2000) {
    Toast.makeText(MainActivity.this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
    exitTime = System.currentTimeMillis();
  } else {
    finish();
  }
}