private void showTips(){
AlertDialog alertDialog = new AlertDialog.Builder(Activity.this)
.setTitle("退出程序")
.setMessage("是否退出程序")
.setPositiveButton("確定", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which)
Activity.this.finish();
}
})
.setNegativeButton("取消",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which)
return;
}}).create(); //創建對話框
alertDialog.show(); // 顯示對話框
}

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if(keyCode==KeyEvent.KEYCODE_BACK && event.getRepeatCount()==0){
this.showTips();
return false;
}
return false;
}

private long exitTime = 0;

/**
 * 捕捉返回事件按鈕
 *
 * 因為此 Activity 繼承 TabActivity 用 onKeyDown 無響應，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() {
  // 判斷2次點擊事件時間
  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();
  }
}