Simple Java Validation -

i web app programmer (mostly php/javascript/ajax etc). there payroll appplication want code in java. needed know find tutorials on how basic validation in java e.g. checking if textfield null, making sure integers allowed etc.

i have basic program runs , created jframe form:

private void jbutton1actionperformed(java.awt.event.actionevent evt) {          double num1, num2, result;          num1 = double.parsedouble(jtextfield1.gettext());          num2 = double.parsedouble(jtextfield2.gettext());          result = num1 + num2;          jlabel4.settext(string.valueof(result));     } 

how can instance validate jtextfield's 1 , 2 not left blank. i.e., return error box lets user know both fields cannot left blank?

i trying tester:

 if(num1 == 0 && num2 == 0)         {             joptionpane.showmessagedialog(null, "you must fill in fields");         } 

but doesnt work @ all

if belongs swing check

jtextfield1.gettext().trim().length > 0 && jtextfield2.gettext().trim().length > 0  

or

!jtextfield1.gettext().equals("") && !jtextfield2.gettext().equals("") 

also read tutorial on swing components.