Question

I would like to add elements to a list box on a jframe, from a different class however it doesn’t seem to work no matter what i try… i do not get an error or any feedback on whats wrong with this… but heres the layout i have, the listbox is using DefaultListModel lm2

I know how to add elements to a jList in the jframe class, but for some reason i am unable to add elments from another class even when adding this inside the jframe class:

  public void log(String str) {
      lm2.addElement(str);
    }

and on my “other class”

  frmMain doit = new frmMain();

  doit.log("add to list box");

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More details add- — >

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I have 3 classes and here they all are:

frmMain.class

  // jFrame class which builds the jframe (from jframe template)
    public class frmMain extends javax.swing.JFrame {

        /** Creates new form frmMain */
        public frmMain() {
            initComponents();
        }



        public void log(String str) {
          lm2.addElement(str);
        }



         public DefaultListModel lm2 = new DefaultListModel();
    }

RequestInfo.class

// RequestInfo.class, which is trying to add an item to the 
// jlist but it doesn't add anything or error

public class RequestInfo {

  public void ProcessReturnedInfo(String sData, boolean bWithLabel) {


            frmMain fm = new frmMain();
            fm.log("test test");



}

RS232Example.class

     // My main class which sets the jFrame to visible
  public class RS232Example {
      public static void main(String[] args) throws Exception {


        frmMain form = new frmMain();
        form.setVisible(true);

    }

  }

I understand maybe i need to set form to visible on the RequestInfo.class, however i can’t do this, because it will continuously open the form multiple times, because this class method is called multiple times from an event…

Answer 1

if your listBox-model is set right (like listBox.setModel(this.lm2);)
I guess the following should work:

public static void main(String[] args) throws Exception {
  frmMain form = new frmMain();
  form.setVisible(true);
  form.log("Hallo");
}

If you are wondering why this works and the code within RequestInfo does not, keep in mind, that you create a new frmMain with it’s own listModel for every call of ProcessReturnedInfo

If you want to have only one Frame updated try to use the frame as singleton:

Change the constructor of frmMain from public to private and add this to the class:

private static frmMain instance = null;

public static frmMain getInstance() {
  if (instance == null) {
    instance = new frmMain();
  }
  return instance;
}

Instead of calling new frmMain() you must now use frmMain.getInstance() in RequestInfo and RS232Example

That’s how you will always work on the same frame.

Good Luck.