Hello,
I have created a radio button yes/no on VF page. There is text box called "comment" on the page. I need a help on how to create an error message while saving the form(VF). If a user selects the option "Yes" he/she must put value on "comment box" or in another word coment box is not equal to Null. I am new to the VF page. If you could provide me the code, that would be great help. Thank you in advance.

It says cannot change field which is used in Apex class or trigger.

Hello,
I have created a radio button yes/no on VF page. There is text box called "comment" on the page. I need a help on how to create an error message while saving the form(VF). If a user selects the option "Yes" he/she must put value on "comment box" or in another word coment box is not equal to Null. I am new to the VF page. If you could provide me the code, that would be great help. Thank you in advance.