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

  1. #1
    Member snowguy13's Avatar
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    Question 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?
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  2. #2
    Member snowguy13's Avatar
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    Default Re: BoxLayout: Sharing ISN'T Caring... :(

    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?
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    Default Re: BoxLayout: Sharing ISN'T Caring... :(

    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.

  4. The Following User Says Thank You to helloworld922 For This Useful Post:

    snowguy13 (December 23rd, 2011)

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    Member snowguy13's Avatar
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    Default Re: BoxLayout: Sharing ISN'T Caring... :(

    @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?
    Last edited by snowguy13; December 23rd, 2011 at 07:36 PM.
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    Super Moderator helloworld922's Avatar
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    Default Re: BoxLayout: Sharing ISN'T Caring... :(

    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.

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    Default Re: BoxLayout: Sharing ISN'T Caring... :(

    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!
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