JAVA .. validating input when textfields lose focus

Question

I'm (a beginner) creating a sign up form in netbeans. What I really want to happen is, a JOptionPane will appear when the textfield loses focus if a users input is wrong . I've been searching for the answer on the internet for the whole day but the answers I find don't satisfy me.

Answer 1

You need to implement a FocusListener, add it to your textField and in your focusLost implementation of the FocusListener you need to do whatever you want it to do when focus is lost.

http://docs.oracle.com/javase/tutorial/uiswing/events/focuslistener.html

I'm not experienced with NetBeans but I'm assuming you use the graphical tool to create your form. I think you will need to start coding to implement the listener.