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Make a panel resize dynamically in Swing

I'm creating an interface composed out of a box layout which contains panels inside every space.
In this specific case I've got a "cascade" of panels, the first is supposed to be a panel with a FlowLayout as layout manager, underneath it there's a GridLayout and under it there's supposed to be another label.
The thing is that I'd need the first panel to dynamically resize as the window get resized itself.
Here's the problem: I need the first panel to have a specific size in relation to the absolute size.. the thing is that I can't set my preferred size.. in the class I do the following but the panel stays the exact same size as before..
this.setPreferredSize(new Dimension(windowWidth, 50))
I'll send the code as soon as I get home, for now the situation is the one written above.

I need the first panel to have a specific size in relation to the absolute size
Your main panel should be a BorderLayout (which is the default layout of a JFrame), not a BoxLayout.
Then you add the panel that needs to be a specific size to the BorderLayout.NORTH of the frame.
Then your second panel can use a BoxLayout and add this panel to BorderLayout.CENTER of the frame. Now this panel will get all the extra space as the frame is resize.

Is it possible to have two gridbag layouts side-by-side inside a frame?

I'm using an extended JFrame class, called MFrame, to initialize two objects that extend JPanel. One JPanel has buttons, called ButtonPanel, and the other has a tree generated via reading in XML file, called TreePanel. I'd like to have them in two separate classes because I'll be adding a lot of functionality and I want to have them be as minimal as possible.
Inside both ButtonPane and TreePanel, gridbag is used to establish layout. I'm trying to display both within the same JFrame, but whichever one is added last, via ex. this.add(ButtonPanel), covers up the JPanel class behind it.
Has anyone been able to display two JPanel classes with Gridbag side-by-side within a JFrame? Any help would be appreciated.

Here is how:
Create a MPanel and add it to MFrame
Set its Layout to BoxLayout (horizontal in your case)
Add the TreePanel and ButtonPanel MPanel
You have to add a Panel because the default for JFrame is BorderLayout so add(...) is essentialy the same as add(..., BorderLayout.CENTER) thus only the last componet you add is visible.

Make buttons unresizable

So I was trying to google how to set a default size to JButtons so that they don't grow as the JFrame is resized. I didn't see a setDefaultSize method but the closest one I could find that does a similar job is setMaximumSize(). However, it doesn't seem to work in my situation and I'm guessing it's because I'm using Grid Layout for positioning my buttons in the frame, here's a small piece of my code:
rightPanel.add(ButtonA);
rightPanel.add(ButtonB);
rightPanel.add(ButtonC);
outerPanel.add(leftPanel);
outerPanel.add(rightPanel);
getContentPane().add(outerPanel);
Here's a picture of what happens:
I would also like to have my buttons in the middle of the right panel when I'm resizing (just like they are now but a lot smaller). Any idea of how I can fix this? I'm assuming that I have to use another layout or something.
Thanks
EDIT: I modified my code to use BoxLayout but it does not seem to put the buttons in the middle. The X Alignment is working but Y Alignment is not doing anything:
ButtonA.setAlignmentX(CENTER_ALIGNMENT);
ButtonA.setAlignmentY(CENTER_ALIGNMENT);
ButtonB.setAlignmentX(CENTER_ALIGNMENT);
ButtonB.setAlignmentY(CENTER_ALIGNMENT);
ButtonC.setAlignmentX(CENTER_ALIGNMENT);
ButtonC.setAlignmentY(CENTER_ALIGNMENT);
JPanel rightPanel = new JPanel();
rightPanel.setLayout(new BoxLayout(rightPanel, BoxLayout.Y_AXIS));
rightPanel.add(ButtonA);
rightPanel.add(ButtonB);
rightPanel.add(ButtonC);
outerPanel.add(leftPanel);
outerPanel.add(rightPanel);
getContentPane().add(outerPanel);
EDIT2: Fixed with vertical glue.

A GridLayout will always resize the components to fill the space available.
Try using a vertical BoxLayoutinstead. See the section from the Swing tutorial on How to Use Box Layout for more information and examples.

Encapsulate each JButton in a JPanel with a FlowLayout, and then add those FlowLayout JPanels to the rightPanel instead of the JButtons themselves. This will allow you to keep your evenly spaced buttons, but won't make them expand to take up the entire space that the parent container has available.
If you don't want them evenly spaced, but to be three consecutive buttons one after another top down, you can make the right panel have a BorderLayout, add a sub panel to the north area of the BorderLayout with the original GridLayout that the right panel had, and then add those FlowLayout panels containing the JButtons.

Why does only the last JPanel I add to a JFrame resize?

So I am working on a GUI application using the swing framework. In short, I have 3 JPanels that act as different views of my application. Now the problem is that no matter the order I add the JPanels to my JFrame, only the final JPanel I add resizes when I switch to that view.
Some relevant bits of code:
When creating the window, I first create each individual JPanel, and add it to the JFrame:
JPanel newPanel = new SomeClassExtendingJPanel();
this.jframe.add(newPanel);
Next, whenever I switch between views of the application, I hide the panel that is currently active:
jframe.validate();
jframe.repaint();
oldPanel.setVisible(false);
And then activate the to be shown panel:
jframe.validate();
jframe.repaint();
newPanel.setVisible(true);
Does anyone know what could be wrong?

To resize the JFrame with each swap, you could call pack() on it, but this is kludgy having a GUI resize all the time. A better solution is to use the mechanism that Swing has for swapping views -- a CardLayout. This will size the container to the largest dimension necessary to adequately display all of the "card" components.
Check out the CardLayout tutorial and the CardLayout API for more on this.

JFrame's ContentPane has implemented BorderLayout by Default, and there is possible to put only one JComponents to the one Area,
you have to change used LayoutManager or put another JPanels to the other Areas

Is it possible to have 2 JPanels in a Border layout at the same location?

I'm writing a game which uses a border layout with a JPanel using BorderLayout.CENTER. What I'd like to be able to do is sometimes hide this panel and replace it with another panel with different information. I added both to the container and set visibility of one of them to false.
Then later I try:
panel1.setVisible(false);
panel2.setVisible(true);
but this doesn't display the new panel. I just see gray. Any ideas?
TIA

Use a nested JPanel with a CardLayout for that.