Question

I have a JFrame application with some variables and a number of SWING widgets. In it I create an instance of another class and pass the JFrame to the child in the constructor. From the child, I can reference the variables, but not the widgets. Why?

 // My JFrame
 public class Prot2Prom extends JFrame  {

    // My Child
    public Prot2Prom() {
    super( "Protocol To PROM" );
        Child child = new Child(this);

In the Child class my constructor does

 Prot2Prom frame = null;

 public Child(Prot2Prom gui) {
    frame = gui;
 }

The following works:

 frame.<parent variable>=x;

The following does not:

 frame.textArea.append("Hello");

The textArea cannot be resolved. There were all added with “new”. Why can’t I see them?

Answer 1

Some notes and recommendations:

• This has nothing to do with “widgets” or Swing and all to do with visibility of variables. I’m guessing that textArea is not a public field of the Prot2Prom class.
• If variables are public outside classes can “see” them, access them, modify them.
• A possible solution is to in fact make the variables that you want other classes to see, public.
• In general you really don’t want to do this.
• Instead much better is to give a class public methods that allow other classes to call and by doing so alter the original class’s behavior. In other words, your Swing GUI classes should adhere to good OOPs principles just as any Java class should.
• Later you’ll want to read up on the MVC or Model, View, Control design pattern as a way of separating out behaviors of your code into separate logical entities, which can make your code much more flexible and powerful.

Edit 1
Regarding your comment:

The “widgets” are all created by WindowsBuilder Pro. I am trying to use the textArea to create my Eclipse Console for a stand alone (jar) application. How can I print to it from a class instantiated by the Frame?

You’ll want to give the class that holds the textArea variable a public method:

public void appendTextAreaText(String text) {
  textArea.append(text);
}

Then your other classes can append text to the JTextArea. Why is this important? One reason is that if the class that holds textArea will at some times not want to allow other classes the ability to append to this widget, it can have the logic to control this in the method. Thus it gives much more control over the widget to the class that holds it.

e.g.,

public void appendTextAreaText(String text) {
  if (allowTextAreaAppend) {  // a class boolean field 
    textArea.append(text);
  }
}

As an aside, I also recommend that you put the code generation tool to the side and instead create your Swing GUI’s by hand for a bit until you get a firm grasp of Swing and Java fundamentals. This will make your future use of the Swing code generation tool much better and productive.