Running 2 classes together

Question

I have 2 classes that i want to run together. They both have
GUI components to run there separate methods. The code is long
so i've shown the stripped down basic idea below.
public class Class2 extends JFrame
     public Class2(String name)
            super(name);
          //Code for GUI
     public Class2()
          //Default constructor to access the class
     public void createPopupFrame()
          //code to show the Internal Frame
          frame.show();
     public static void main(String[] args)
          //Runs the GUI
          new Class2("Class2");
} i want to call the method for the internal Frame ( createPopupFrame() ) from Class1
while Class2 is running and therefore popUp that frame in class2.
class Class1 extends JFrame
     //Various GUI components
       public Class1(String name)
             super(name);
            //Code for GUI
            //Button to call the method callPopUP()
       public Class1()
            //Default constructor to access the class
     public void callPopUP()
          Class2 dis = new Class2();
          dis.createPopupFrame();
     public static void main(String[] args)
          //Runs the GUI
          new Class1("Class1");
     }//end main
}I'm having trouble implementing this in java. I searched around the api
and the forums and found some stuff about Runnable interface but i'm not really
any the wiser. I've tested the createPopupFrame() method and i works fine from within
class2.
When i call it from class1 while class2 is running i get a NullPointerException. Although
i expected this because i knew, that would just be too easy.
Any help appreciated.

Answer 1

Line 171 in class2I assumed line 171 referenced an object created
somewhere in line 170.I believe this to be an incorrect assumption.
and line 7 in class1I fail to see the null reference here by virtue of
line 6.Typically , a stack trace shows a call sequence. Since line 7 calls the method containing line 171, if frame is null, the NPE stack sequence would show:
NPE in line 171
called from line 7
OP. Try the following:
public void createPopupFrame()
    //code to show the Internal Frame
    if ( frame != null )
        frame.show();
    else
        System.out.println( "The frame object has not been instantiated." );
}Good luck.
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