Thread: BoxLayout: Sharing ISN'T Caring... :(

I'm trying to lay out some radio buttons using BoxLayout, which aligns Components right next to each other vertically or horizontally. In other words, it makes them nice and neat. However, I'm running into the exception:
"BoxLayout can't be shared"

I have been aware of this exception for a long time, but I didn't expect it to arise in the situation it did:

      //... other stuff ...
 
      gameModePanelLabel = new JLabel("Choose the type of game.", SwingConstants.CENTER);
      compVcompModeOption = new JRadioButton("Computer vs. Computer");
      playVcompModeOption = new JRadioButton("Player vs. Computer");
      playVplayModeOption = new JRadioButton("Player vs. Player");
      gameModes = new ButtonGroup();
      gameModes.add(compVcompModeOption);
      gameModes.add(playVcompModeOption);
      gameModes.add(playVplayModeOption);
      gameModePanel = new JPanel(new BoxLayout(gameModePanel, BoxLayout.PAGE_AXIS));
      gameModePanel.setBorder(BorderFactory.createTitledBorder("Game Mode"));
      gameModePanel.add(gameModePanelLabel);   //There exception occurs in this line, but I assume
      gameModePanel.add(compVcompModeOption);  //that the three lines below it will also provoke
      gameModePanel.add(playVcompModeOption);  //an exception
      gameModePanel.add(playVplayModeOption);
 
      //... and yet other stuff ..

I have no idea why I am getting this error, as I am only adding Components to a JPanel with a BoxLayout...

Will someone please point out my silly mistake?

Weeeell, never mind. I got it to work, simply by changing...

gameModePanel = new JPanel(new BoxLayout(gameModePanel, BoxLayout.PAGE_AXIS));

...to...

gameModePanel = new JPanel();
gameModePanel.setLayout(new BoxLayout(gameModePanel, BoxLayout.PAGE_AXIS));

Isn't that weird...

Would someone be able to offer an explanation as to why that worked?

Your confusion is probably due to the way Java manages object references.

Initially, gameModePanel points to some panel (or more likely, null).

In the first example, you pass that old panel (or null) to the BoxLayout constructor, so it's managing the layout of another component. Then when you try to use it, it freaks out because you don't have access rights to the other component.

In the second example, gameModePanel is initialized to the correct container first. Then when you try to set the layout to a BoxLayout, you pass in the already initialized gameModePanel, so that the BoxLayout manages the correct gameModePanel and everything is happy.

@helloworld922

Okay, that explains a lot. Thank you!

Hmm... But that brings another question to my mind: why does this problem only occur with BoxLayout, and not GridLayout, GridBagLayout, or other such LayoutManagers?

GridLayout doesn't take the component as a parameter so this situation can't happen. Same for GridBagLayout. I'm think other Layout Managers are the same way (these are the two main layouts I use).

I am slightly curious as to why BoxLayout even requires the component in the constructor, but I haven't really ever used BoxLayout before.

I am slightly curious as to why BoxLayout even requires the component in the constructor

I share that curiosity. When I first used BoxLayout, I was thrown off by having that parameter in its constructor. It seems redundant.

But perhaps its as you said before. Instead of the component controlling the BoxLayout, perhaps BoxLayout needs to be independent and controls the component from a somewhat third-person-ish perspective...

Anyways, thanks for you help!