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Make children of JLayeredPane fill available space of container

I have two JPanel instances in a JLayeredPane, on different z-orders. I want both of my child JPanels to always fill the space of the LayeredPane.
The idea is for me to toggle the display of a 2nd panel over top of the first to display a modal-like dialog. Yes, I could just use a JDialog, but I thought it would be fun to try and create some transparancy overtop of the covered JPanel for a nice effect.
I find that using a layout manager on the JLayeredPane, like BorderLayout, and trying to set both children to CENTER conflicts since both panels can't be in the Center.
Is there a trick that I'm not seeing?

The idea is for me to toggle the display of a 2nd panel over top of the first
The easiest way to do this is to use a Glass Pane.
Check out the Disabled Glass Pane for an example of this approach.

There are two ways to create some "Glass Panel like" overlay for JPanels with JLayeredPane:
Add a ComponentListener to the JLayeredPane and update the sizes of all child components whenever the size of the JLayeredPane changes
Create a simple FillLayout, which expands the size of its child Components to the size of the Layout Container (In our case the JLayeredPane). You need to keep a list of children Components. During layoutContainer you copy the dimensions of the Container to these child Components. I wrote this and its really simple, but unfortunately I can't post it, since it's corporate. But if anyone is interested just ask in the comments. The implementation basically consists of one-liners.
For both solutions you need to make sure, that the panels on top are transparent, by setting setOpaque to false. This ensures that underlying panels render their content.

WindowBuilder - Component size

Im trying to make a fullscreen application.
I would like to know how to make the component that I add on the JFrame to occupy a part of the screen on every resolution.
Design tab mode:
When I run on fullscreen i get this:
how can I make this following interface adjust to full screen?

Here's an idea of the some of the basic layout managers and which ones respect preferred sizes (meaning if preferred size is respected, as container expands, component will not expand with it, and vice versa)
That being said, you may want to use a BorderLayout, and place your top component at the BorderLayout.PAGE_START. Ad the screen increases, so will the component contained in that layout position. (Note: the example's main container uses a BorderLayout)
As far as the image, if you want to stretch, I'd take a look at StretchIcon from Darryl Burke. This will keep the image to a relative size.
Also a common practice is to nest panels with different layout managers, to get your desired result. You can see an example here by Andrew Thompson
Also see more about layout managers at Visual Guide to Layout Managers

How to make BorderLayout dynamic?

I have java swing aplication where I use Borderlayout as layout. I put elements in EAST,CENTER and WEST. I want to change size of elements with mouse just like in NetBeans.
For example when I move mouse on the edge of the CENTER I need to show mouse resize cursor. And when I drag mouse on the edge, the CENTER will change size accordingly.
What is the best approach to do it?

You would need to use a JSplitPane. Simply nest them in the border layout. This is a good resource for examples.

JXMultiSplitPane going bonkers when I switch tabs

I'm using JXMultiSplitPane (from SwingX 1.6.2) to implement a three-pane horizontal interface. In the center pane is a JTabbedPane with two tabs: one with a JTextArea (in a JScrollPane, of course) used for entering Markdown code and the other a JEditorPane (again, in a scroll pane) for displaying a rendered HTML preview. When the user switches to the preview pane, the text in the editor is processed and displayed in the preview pane.
My problem is that if I enter text in the editor with long lines, and then switch to the preview, the center pane will expand. Sometimes it's just by a little bit, other times it'll take up more room than is actually on the screen. But if I move one of the resize handles manually, everything will snap back in place.
I've found only two ways to deal with this before it happens:
Manually resize one of the panes before entering any text.
Give the center pane a weight of 1 in the MultiSplitLayout model.
I can't use the second one since it will expand the center pane to take up almost the whole window by default.
Is there a way to fix this?
Update
After a little more testing, even technique (2) doesn't keep the size constant; switching between the two tabs changes the size of the center pane slightly.
I now believe that the problem is partly with the tabbed pane. The JTextArea and the JEditorPane do not have the same size and that JTabbedPane is resizing when I switch between them (since I'm resetting the JEditorPane text every time. This wouldn't be a problem except that JXMultiSplitPane will keep automatically resizing the center pane until the user forces a specific size by resizing manually.
So I should be able to fix the issue by making the size of the JTabbedPane fixed, but still able to be resized by the handle bars. Any tips on doing that?

The MultiSplitLayout is .. a LayoutManager, so you have to understand how it works (me too, not overly familiar with it myself :-)
The basic layout happens according to the component's prefSize, the weights are for distributing excess/missing space relative to the pref. By default, the dividers are "floating", that is they are positioned between the components as layouted by the basic mechanism. The moment a user touches a divider, dividers are "not-floating", comp sized to fit in-between the dividers. That's the reason for you not seeing the size-greed after moving the divider once. So one ways out is to
setup the JXMultiSplitPane as usual, add the components and realize the frame
fix the dividers after the manager has done its initial layout
String layout = "(ROW " +
"(LEAF name=selector weight=0.15)" +
"(LEAF name=center weight=0.7)" +
"(LEAF name=list weight=0.15)" +
")";
JXMultiSpitPane pane = new JXMulitSplitPane((MultiSplitLayout.parseModel(layout))
// add components and realize the frame
...
pane.getMultiSplitLayout().setFloatingDividers(false);
Alternatively, give more weight to the weights - force the layoutManager to use them for the layout itself (instead of only for the distribution of excess/missing space). A side-effect is that the prefSize of the comps might be set (by the layout, which is a no-no-never, but who's perfect ;-)
pane.getMulitSplitLayout().setLayoutByWeights(true);
Not sure which way I would prefer or if/how that could be made easier in the multisplit ..

Centering the JTabbedPane but not the actual tabs

How would I go about making the length of the tabs automatically resize based on how much room is left in that row of tabs.
Picture:
As you can see the tab's width is based off the text in the tab.
If you need me to explain what I want better then just ask me because I don't know if I made it clear enough.

You can use a custom component and set it's preferred size. For example, in ButtonTabComponent of TabComponentsDemo:
label.setPreferredSize(new Dimension(...));
You have to choose an appropriate dimension based on other aspects of your layout, so it won't be automatic.
I want to define a size for the actual tabbed pan.
The size of the JTabbedPane is a function of the dimensions and LayoutManager of the Container to which it has been added. In the example cited, the default layout of the frame's content pane is BorderLayout, and add(pane) adds it to the center by default.
To accomplish your goal, I see two approaches:
Divide the current width of the enclosing Container among the existing tabs and repaint the tabbed pane, as shown in this example.
Develop your own implementation of TabbedPaneUI and interpret SCROLL_TAB_LAYOUT accordingly.