Android點擊返回鍵back時彈出對話框Dialog

public boolean onKeyDown(int keyCode, KeyEvent event) {  
    if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {  
        ExitDialog(MainActivity.this).show();  
        return true;  
    }  
      
    return super.onKeyDown(keyCode, event);  
}  

private Dialog ExitDialog(Context context) {  
    AlertDialog.Builder builder = new AlertDialog.Builder(context);  
    builder.setIcon(R.drawable.icon);  
    builder.setTitle("彈出對話框");  
    builder.setMessage("確定要××××嗎，O(∩_∩)O哈哈~");  
    builder.setPositiveButton("確定",  
            new DialogInterface.OnClickListener() {  
                public void onClick(DialogInterface dialog, int whichButton) {           
                   //××××操作代碼
                }  
            });  
    builder.setNegativeButton("取消",  
            new DialogInterface.OnClickListener() {  
                public void onClick(DialogInterface dialog, int whichButton) {  
                }  
            });  
    return builder.create();  
}