Issue with accessing application's JFrame component from a Dialog.

[shall]

Hello,

My program has a dialog box that needs to access its parent's JPanel. In the code shown below, Line 11 throws a "Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: javax.swing.JPanel cannot be cast to MazeGame.MazePanel".

My program passes the application's JFrame and modal = true to the GridSizeDialog constructor. The application's JFrame contains a mazePanel, i.e. a JPanel.

What am I doing wrong?

public class GridSizeDialog extends javax.swing.JDialog {

    private int gridSize;
    private final JFrame dialogParent;

    public GridSizeDialog(java.awt.Frame parent, boolean modal) {
        super(parent, modal);
        
        dialogParent = (JFrame) parent;

        MazePanel mazePanel = (MazePanel) dialogParent.getContentPane();

        mazePanel.CreateMaze(gridSize);
     }
}

[Junky]

The error is telling you exactly what is wrong, you cannot cast a JPanel to a MazePanel. Surely you have heard something like: all beagles are dogs but not all dogs are beagles.

Why doesn't it allow it? Suppose a have another class that extends JPanel called OtherPanel which has no relation to MazePanel. Could I do this?

JPanel panel = new OtherPanel();
MazePanel mp = (MazePanel) panel;

Clearly not.

I suggest you rethink your design.

[shall]

I can understand what you said. Essentially, you said a wolf is an animal and a cat is an animal, but a wolf is not a cat.

How can that be similar to my situation?

The JFrame (or parent) my program passed to the the JDialog class contains a mazePanel and so, I would think, getContentPane should return the mazePanel. Actually it returns a container, which should be the mazePanel.

[DarrylBurke]

When in a situation like this, I sysout whatever information I can get, just to check my assumptions. You could assign the value returned by getContentPane() and sysout the value and the result of its getClass().getName().

Adding a component to a frame adds it to the BorderLayout.CENTER of the frame's content pane. It doesn't set the component as the content pane.

db

[DarrylBurke]

Moved from New to Java

db

[shall]

When in a situation like this, I sysout whatever information I can get, just to check my assumptions. You could assign the value returned by getContentPane() and sysout the value and the result of its getClass().getName().

Adding a component to a frame adds it to the BorderLayout.CENTER of the frame's content pane. It doesn't set the component as the content pane.

db

Thanks.

System.out.println(dialogParent.getContentPane().g etClass().getName()) returns javax.swing.JPanel.

I found out that I needed to mazeFrame.setContentPane(mazePanel); instead of mazeFrame.add(mazePanel);.

Once I did that, it returned MazeGame.MazePanel.

[DarrylBurke]

Now that that's sorted out, I'd like to say that I think you have your design back to front. If the dialog needs a reference to the MazePanel, one should be passed as a constructor parameter.

You can use a static method of SwingUtilities to get a reference to the window that contains the MazePanel, to pass to the super implementation.

Also, if your application always uses the GridSizeDialog as only modal or only non-modal, that boolean should be hardcoded into the call to the super implementation and not passed as a parameter. Assuming here that the dialog is always modal, the constructor would look like

public GridSizeDialog(MazePanel mazePanel) {
    super(SwingUtilities.xxxx(mazePanel), true);
    mazePanel.CreateMaze(gridSize);
}

Look up the appropriate method of SwingUtilities in the API.

db

[shall]

Sounds like a good idea.

I think the SwingUtilities method is windowForComponent. I'll try it out.

Thanks