Thread: Extending JFrame + having multiple windows.

Each instance of the class extending JFrame would have its own window.
Create a class that extends JFrame, create multiple instances of that class and set them all visible.
Something like this:
new ClassThatExtendsJFrame().setVisible(true); // first window
new ClassThatExtendsJFrame().setVisible(true); // second window
new ClassThatExtendsJFrame().setVisible(true); // third window

Define new classes as needed, not just to have more classes.

Window is the name of a Java SE class. It will be better if you use a unique name vs one used by Java.

to call a new window/JFrame

   public static void main( String[] args ) {
       MyWindow gui1 = new MyWindow() ;     // first
       MyWindow gui2 = new MyWindow() ;     // second 
       MyWindow gui3 = new MyWindow() ;     // third
   }

The constructors could be passed args they could use to make unique contents for their windows. Or there could be unique classes for each window. Its up to the programmer.

Originally Posted by RedCloudTsunami

Would it be a good idea to have the other classes in different .java files that all extend JFrame?

It is always a good idea to put other classes in their own .java files. Though this does not mean you need multiple classes...

Originally Posted by RedCloudTsunami

... However, that means all three windows will be identical as they'll all be instances of the same constructor method, so I assume that new classes will need to be introduced.

Not necessarily. You can (as Norm stated) send different arguments to the constructor to get unique windows. You can also have multiple constructors in the same class and end up with very different windows with very different functionality.

Edit: Depending on exactly what is going on, it may be a good idea to end up with different classes. Just because they all extend the same class does not mean you should have just one class. If the windows will be very different for very different uses it may make sense to use multiple classes which all extend the same class, or even extend one or more of your other classes etc. It really gets project specific at this point.