Error Upon Assigning Layout: Boxlayout Can't Be Shared

error upon assigning Layout: BoxLayout can't be shared

Your problem is that you're creating a BoxLayout for a JFrame (this), but setting it as the layout for a JPanel (getContentPane()). Try:

getContentPane().setLayout(
    new BoxLayout(getContentPane(), BoxLayout.PAGE_AXIS)
);

java.awt.AWTError: BoxLayout can't be shared

When calling setLayout on a JFrame, you're actually adding the layout to the JFrame's contentPane not the JFrame itself since this method is more of a convenience method that transmits the method call to the contentPane. The BoxLayout constructor must reflect this since you can't add the BoxLayout to one container and then pass in as a parameter a different container. So change this:

this.setLayout(new BoxLayout(this,BoxLayout.X_AXIS));

to this:

setLayout(new BoxLayout(getContentPane(), BoxLayout.X_AXIS));

Also: no need for all the this. business since this. is implied, and also there is no actual need here to extend JFrame.

edit: this code below is all that is needed to demonstrate your error and its solution:

import javax.swing.*;

public class BoxLayoutFoo extends JFrame {
   public BoxLayoutFoo() {

      // swap the comments below
      setLayout(new BoxLayout(this, BoxLayout.LINE_AXIS)); // comment out this line
      //setLayout(new BoxLayout(getContentPane(), BoxLayout.LINE_AXIS)); // uncomment this line

      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      pack();
      setVisible(true);
   }

   public static void main(String[] args) {
      new BoxLayoutFoo();
   }
}

Keep getting java.awt.AWTError: BoxLayout can't be shared

Here:

panelMain.setLayout(new BoxLayout(getContentPane(), BoxLayout.X_AXIS));

You need the container that is getting the layout, panelMain, to be the same as the one within the BoxLayout constructor, not getContentPane(). So the correct code would be:

panelMain.setLayout(new BoxLayout(panelMain, BoxLayout.X_AXIS));

Resources:

• BoxLayout API
• Relevant tutorial

How To solve BoxLayout can't be shared error while setting it for JPanel?

BoxLayout needs to be passed the container, but labelpanel is null when you create the layout manager. So you need to create the panel before you can create the layout manager:

this.labelpanel = new JPanel();
labelpanel.setLayout(new BoxLayout(labelpanel, BoxLayout.PAGE_AXIS));

By the way, get rid of the null layout you use in another part of the code - those are a bad idea so it's better to get in the habit of always using layout managers right from the start.

BoxLayout can't be shared

Ok, it took me a while because I am really not familiar with your mother tongue (it would be much simpler for everyone if you posted your code with english names for variables), but the problem comes from here:

aufgabeBeschreibungPanel.setLayout(new BoxLayout(aufgabeBeschreibungLabel, BoxLayout.LINE_AXIS));

Your setting a BoxLayout on aufgabeBeschreibungPanel but you provide aufgabeBeschreibungLabel as a parameter of BoxLayout. You should instead write:

aufgabeBeschreibungPanel.setLayout(new BoxLayout(aufgabeBeschreibungPanel, BoxLayout.LINE_AXIS));

When seeing this issue, the most common cause of this is that you wrote:

y.setLayout(new BoxLayout(x, BoxLayout.XXX));

where y and x are different.

Java BoxLayout returning error BoxLayout can't be shared

I believe that instead of:

wrapper2.setLayout(new BoxLayout(wrapper1, BoxLayout.Y_AXIS));

You want to do:

wrapper2.setLayout(new BoxLayout(wrapper2, BoxLayout.Y_AXIS));

For now, you create a new BoxLayout with wrapper1 for parent and try to "share" it by making it the layout of wrapper2. It's just a mistyping (or most probably some omission after a quick copy/paste) :p

Why can't a BoxLayout be shared whereas a FlowLayout can?

Simple answer

Basically, "BoxLayout can't be shared" in this situation means that you're trying to make your JFrame and its contentPane() share the same BoxLayout object.

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Advanced Explanation

When setting the Layout for a JFrame, it implicitely calls getContentPane().setLayout(manager) so you are actually setting the layout for the contentPane() and not for the frame itself.

This brings us to how the containers are checked within BoxLayout and FlowLayout.

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FlowLayout

This layout does not have a constructor that will take the container as a parameter and thus will not, at the making of the object, consider the container. Neither has this class a container instance variable.

BoxLayout

BoxLayout on the contrary has a constructor that takes the container as a parameter and stocks it within the instance variable target. This is done to check it later within the layoutContainer(container) method. It has a checkContainer(container) method that will verify if the instance variable is equals to the container given in parameter. It throws throw new AWTError("BoxLayout can't be shared"); if it is not the case.

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This was an introduction to the following explaination.

As said within the first paragraph, JFrame.setLayout(LayoutManager) will call JFrame.getContentPane().setLayout(LayoutManager) and thus the Layout is set on the contentPane() which is a JPanel by default.

Check out the constructor of BoxLayout(container, int) and ask yourself :

Now I know that the Layout is set on the JPanel (contentPane()) and not the JFrame itself, which parameter will I give to this constructor?

Is it a good idea to give it the JFrame itself? Knowing that it is not the Component which on the Layout has been set?

The answer is : of course it is not a good idea.

Here is the correct way to achieve this :

JFrame frame = new JFrame();
frame.setLayout(new BoxLayout(frame.getContentPane(), ...);

Why?

Because we know now that the container is the contentPane() and that eventually, when adding a component for example, a check is going to happen and the parameter within the constructor must be the exact same object as the component where on the layout is set which does not happens with FlowLayout for example.

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Sources

BoxLayout

• BoxLayout(Container, int)
• BoxLayout.checkContainer(Container)
• BoxLayout.layoutContainer(Container)

FlowLayout

• FlowLayout.layoutContainer(Container)

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