Question

This is the problem, I have a class MainWindow that extends JFrame in one of the setup routines I set the layout of the class to a new CardLayout(). This all works fine but when I go to request the the layout from the JFrame and cast what it returns to a CardLayout I get errors because it is returning a BorderLayout. I think you get the picture from hear.

Please find some code below:

public MainWindow()
{
    initWindow();
}

public void actionPerformed(ActionEvent e)
{
    CardLayout m = (CardLayout)super.getLayout();
    m.next(this);
}

private void initWindow()
{
    super.getContentPane().setLayout(_mainLayout);
    super.setTitle(_WINDOW_NAME);
    super.setSize(_DEFAULT_WINDOW_SIZE);
    super.setLocationRelativeTo(null);
    super.setAlwaysOnTop(true);
    super.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
    super.setResizable(false);

    init_buttons_panel();
    super.add(_buttons_panel, "bpanel");
}

I see that the code is not perfect this is because I have been trying everything to trap the problem. Yes I realize casting like that is not good but this is just test code to prove the point. Oh and _mainLayout is you guessed it a CardLayout.

I have also read how to use card layout and my code does not do anything out of the ordinary.

Answer 1

First, you should not cast a layout like that, at least without making any verifications. You should use the instanceof keyword first, to check if it really is a CardLayout, to prevent the program from finishing abruptly.

Second, the problem is here:

public void actionPerformed(ActionEvent e)
{
    CardLayout m = (CardLayout)super.getContentPane().getLayout();
    m.next(this);
}

Basicly you were getting the layout of the JFrame, and not from the JFrame’s content pane, which is the one you actually set to be a CardLayout.