How can I make a JDialog to make main Frame to "sleep"

I have one question. I noticed that when I use a FileDialog, every command that follows making the FileDialog to be visible only continues only after I close the FileDialog.
But I am looking to make the same with a JDialog window. It is bad that sometimes I use the Dialog to set a value that the commands that follow making it visible need that value, and you guess.. I get a lot of NullPointerExceptions. How can I make the main Frame to "sleep" until I close the Dialog?

Make the JDialog modal so that execution of its caller waits until it returns.