How to set focus?

Hi,

I have 3 input field in an ABAP program and need to do validation on these 3 mandatory fields. When the user click on the submit pushbutton, the code behind will need to validate the 3 mandatory fields, if one of the field is empty, the validation code must set focus on that particular field and prompt a message to alert the user. How to do that?

Best Regards

Rayden