Java – exception: add a window to the container How?

I have a jdialog class called preferences This class creates a constructor, such as:

class Preferences extends javax.swing.JDialog {
          Preferences(java.awt.Frame parent,modal)  {
                      super(parent,modal);
                      //......
          }
}

In my program, I want this preferences dialog box to open when I click a button in the JFrame form After registering the action listener on the button, I write the code to:

Frame fr = new Frame();
Preferences p = new Preferences(fr,false);
fr.add(p);
fr.setVisible(true);

When I run this code, I get the following exception (when I click the button):

Exception in thread "AWT-EventQueue-0" 
    java.lang.IllegalArgumentException: adding a window to a container

What does this mean and how can I solve it?

Solution

A top-level container (dialog box) cannot be added to another (frame)

Simply call setVisible (true) in the preferences dialog box instead of adding it.