I'm not sure what you're trying to do, but here's something to consider: c(); won't do anything. c is an instance of the class checkbox and not a method to be called. So consider this:
public class FirstWindow extends JFrame { public FirstWindow() { checkbox c = new checkbox(); c.yourMethod(yourParameters); // call the method you made in checkbox } } public class checkbox extends JFrame { public checkbox(yourParameters) { // this is the constructor method used to initialize instance variables } public void yourMethod() // doesn't have to be void { // put your code here } }