The following code achieve that when i click the different button, different
string will display on panel.
Here is the problem:
1,for now,when i click the next button,last displayed string will disappear,how to make all strings remain? Just like add one line of string each time when i click the button, but not replace the last string.
2,I use flag to control the display, is there any other better ways to chieve but not use flag?