java.lang.IllegalArgumentException: adding a window to a container

Question

I get 'java.lang.IllegalArgumentException: adding a window to a container' when I call frame.add(this). What am I doing wrong, and how do I fix the error. Thanks in advance.

public class mainclass extends JFrame{

private static final long serialVersionUID = 1L;

private int width = 400;
private int height = 400;

public static JFrame frame;

public static void main(String args[]) {
frame = new JFrame();

    mainclass mainclass = new mainclass();
    mainclass.createFrame();
}

public void createFrame() {
    frame.setSize(new Dimension(width, height));
    frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
    frame.add(this); // this is where the error occurs
    frame.setResizable(false);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
    frame.setIconImage(new ImageIcon("res/icon.png").getImage());
}
}

Stacktrace:

Exception in thread "main" java.lang.IllegalArgumentException: adding a window to a container
at java.awt.Container.checkNotAWindow(Unknown Source)
at java.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at javax.swing.JFrame.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at test.mainclass.createFrame(mainclass.java:27)
at test.mainclass.main(mainclass.java:21)

Answer 1

The simple answer is that you can't do this. Your question would benefit from describing why you would want to do this - what end result are you trying to achieve?

If you're trying to add another container then you should use JPanels. If you are trying to create an MDI-like app, then you should look at JInternalFrames. If you want a popup frame, you need JDialogs.

For a little more information, JFrames are designed to be top-level containers - they contain a JRootPane as its only child. When you want to add something to a frame you are in effect adding to the frame's root pane, referred to as the content pane. The correct way is to call frame.getContentPane().add().

This was a constant source of frustration because a lot of developers instinctively wanted to call frame.add() which is how practically all the other Swing components work. Therefore as a convenience frame.add() has been overridden to call frame.getContentPane().add().

So if you think about what's happen in your example now, you are trying to add a JFrame to a frame's root content pane. Understandably root panes cannot have other top-level containers as child elements, eg JFrames as they possess their own root pane.

Answer 2

mainclass extends from JFrame. You can not a window to a container, that's just the way it is.

Try making mainclass extend from something like JPanel instead.

I'm also concerned about new ImageIcon("res/icon.png"). This looks very much like the icon.png is an embedded resource rather than a file on the file system. You may find you need to use new ImageIcon(getClass().getResource("/res/icon.png")) instead, but I don't have enough context to 100% sure