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Java Swing BoxLayout layout of outer panel changes when I specify layout of inner panel

I'm very new to Java Swing. I'm using y-axis BoxLayout on an "outer" panel that contains 2 inner panels. Here you can see with this basic code, the 2 inner panels get an equal share of the height avaiable in the box layout (first inner panel yellow, second innner panel orange)
// main test frame
public class MainFrame extends JFrame {
public MainFrame()
{
this.setSize(500, 500);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setLayout(new GridLayout(1,1));
// outer panel: groupWrapPanel with BoxLayout
JPanel groupWrapPanel = new JPanel();
groupWrapPanel.setBackground(Color.blue);
groupWrapPanel.setLayout(new BoxLayout(groupWrapPanel, BoxLayout.Y_AXIS));
// inner panel 1: headerPanel (yellow)
JPanel headerPanel = new JPanel();
headerPanel.setBackground(Color.yellow);
groupWrapPanel.add(headerPanel);
// inner panel 2: headerNotesWrap (orange)
JPanel headerNotesWrap = new JPanel();
headerNotesWrap.setBackground(Color.orange);
groupWrapPanel.add(headerNotesWrap);
this.add(groupWrapPanel);
this.setVisible(true);
}
}
But then when I add seperate layout manager to one of the inner panels, the layout of the outer panel changes, and the 2 inner panels no longer get an equal share of the height of the outer panel. I can do this by adding a single line of code:
headerPanel.setLayout(new BorderLayout());
Can someone please explain why the layout of the outer panel changes when I specify a layout manager for the inner panel, and how I can prevent this.
Thank you.
EDIT
This is a related question and is in response to a comment someone made.
"A BoxLayout will determine the preferred size of each component. If there is extra space available it will then allocate the space equally up to the maximum size of each component. That would explain why each panel is 50/50 in the first case."
If that's the case, how come if I change the 2 inner components from panels to labels, they no longer stretch to fill the available space inside out the outer panel? like so:
public class MainFrame extends JFrame {
public MainFrame()
{
// we're going to create what shall be known as a "Group Box",
// which groups things together
this.setSize(500, 500);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setLayout(new GridLayout(1,1));
// outer panel: groupWrapPanel with BoxLayout
JPanel groupWrapPanel = new JPanel();
groupWrapPanel.setBackground(Color.blue);
groupWrapPanel.setLayout(new BoxLayout(groupWrapPanel, BoxLayout.Y_AXIS));
// inner label 1
JLabel label1 = new JLabel("Label 1");
label1.setBackground(Color.yellow);
label1.setOpaque(true);
groupWrapPanel.add(label1);
// inner label 2
JLabel label2 = new JLabel("Label 2");
label2.setBackground(Color.orange);
label2.setOpaque(true);
groupWrapPanel.add(label2);
this.add(groupWrapPanel);
this.setVisible(true);
}
}
Thanks again

I added the following to your code:
setVisible(true);
System.out.println(headerPanel.getPreferredSize());
System.out.println(headerPanel.getMaximumSize());
System.out.println(headerNotesWrap.getPreferredSize());
System.out.println(headerNotesWrap.getMaximumSize());
and got the following output when using the BorderLayout:
java.awt.Dimension[width=0,height=0]
java.awt.Dimension[width=2147483647,height=2147483647]
java.awt.Dimension[width=10,height=10]
java.awt.Dimension[width=32767,height=32767]
You can see that the BorderLayout maximum size is much larger than that of the FlowLayout.
So when the extra space is allocated proportionally the BorderLayout panel gets a lot more.
A few possible solutions:
Override the getMaximumSize() method of your panels that use a BorderLayout to return an appropriate value.
Use a GridBagLayout. You can also stack panels in a single column.
Try using the Relative Layout which can function similar to a BoxLayout and just stack panels.

Java Layout for scrollable panels

I am trying to create a JPanel that is resizable & scrollable and contains x smaller inner panels. Each inner panel can be as wide as it wants/needs. BUT the depth should be a preferred size.
Like:
So far my code is:
public class TestSize {
public static void main(String[] args) {
JFrame F = new JFrame();
F.setVisible(true);
JPanel P = new JPanel();
P.setLayout(new BorderLayout());
JScrollPane scrollPane = new JScrollPane(P);
scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED);
scrollPane.setPreferredSize(new Dimension(900,900));//.setBounds(50, 30, 300, 50);
JPanel S = new JPanel();
S.setBackground(Color.GREEN);
S.setPreferredSize(new Dimension(900,200));
JPanel S2 = new JPanel();
S2.setBackground(Color.GREEN);
S2.setPreferredSize(new Dimension(900,200));
P.add(S,BorderLayout.NORTH);
P.add(S2,BorderLayout.NORTH);
F.add(scrollPane);
F.pack();
F.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}
But when I have added a single inner panel it fills all the space vertically - which is not what I want:

//P.setLayout(new BorderLayout());
Why did you set the layout to a BorderLayout? You can only add 1 component to the NORTH. Is that what you want? Read the section from the Swing tutorial on Using Layout Managers and pick a more appropriate layout manager. Bookmark the tutorial link as it provides the basics for Swing programming.
scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED);
Why did you set these properties? These are the default values.
scrollPane.setPreferredSize(new Dimension(900,900));
Why would you set the height to be 900. You only want the scrollpane to contain components of height 200? In general you should NOT be setting the preferred size. Let the frame.pack() method do its job.
S.setBackground(Color.GREEN);
Why would you set the background color of both panels to be the same. How can you tell if the two panels get added? Make them different color for an easy visual.

In BoxLayout, there are different methods of using invisible components as filler. I don't think you will want to use a "rigid area", since I think you want to have a variable number of inner panels. You may want to try using vertical glue or custom Box.Filler.
Another solution might be to put a JPanel between your JFrame and your ScrollPane that uses a BorderLayout, and put the scrollpane in the BorderLayout.NORTH of that panel. Components in BorderLayout.NORTH get resized horizontally, but they do not get resized vertically. Essentially, they just get pushed to the top of the panel.
Edit:
I think you will want something like this:
JFrame F = new JFrame();
F.setVisible(true);
F.setLayout(new BorderLayout());
JPanel P = new JPanel(new BoxLayout(P, BoxLayout.PAGE_AXIS));
JScrollPane scrollPane = new JScrollPane(P);
JPanel S = new JPanel();
S.setBackground(Color.GREEN);
JPanel S2 = new JPanel();
S2.setBackground(Color.BLUE);
P.add(S);
P.add(S2);
F.add(scrollPane, BorderLayout.NORTH);
F.pack();
F.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
You said you want it resizeable, so I don't see why you would set the preferred size of the frame. This way it will just fit to the panels inside.

Panel over a GridLayout Panel

Is it possible to show a Panel (e.g. Panel-A which is nearly transparent) over a panel that has a GridLayout of images?
JPanel mainPanel = new JPanel(new BorderLayout());
JPanel gridOfImages = new JPanel(new GridLayout(3,3));//Panel with a grid of images
JPanel nearlyOpaquePanel = new JPanel(); //A panel that is almost transparent.
// assuming that all already have the required properties like size and colour.
mainPanel.add(gridOfImages,Borderlayout.CENTER);
mainPanel.add(nearlyOpaquePanel,BorderLayout.CENTER);
My thought with this idea is that the panels will stack together and the gridOfImages will be shown through the nearlyOpaquePanel, but my result is that I only got the nearlyOpaquePanel to show and I can't see the gridOfImages through it.

See How to Decorate Components with the JLayer Class

JTabbedPane in JPanel?

I have a simple problem when I want to add tabs in my jpanel. The alignment of the tabs get horizontal instead of vertical, wich looks like crap =/.
It looks like this:
If I discard the panel instead and add the tabbedPane directly to the frame, everything works fine.
If you uncomment the three lines of code and remove the getContentPane().add(jtp); you can reproduce my probleme.
working Code:
public class TabbedPane extends JFrame
{
public TabbedPane()
{
setTitle("Tabbed Pane");
setSize(300, 300); // set size so the user can "see" it
JTabbedPane jtp = new JTabbedPane();
// JPanel panel = new JPanel();//uncomment all three lines
// panel.add(jtp);
// getContentPane().add(panel);
getContentPane().add(jtp);//remove me
JPanel jp1 = new JPanel();// This will create the first tab
JPanel jp2 = new JPanel();// This will create the second tab
JLabel label1 = new JLabel();
label1.setText("This is Tab 1");
jp1.add(label1);
jtp.addTab("Tab1", jp1);
jtp.addTab("Tab2", jp2);
JButton test = new JButton("Press");
jp2.add(test);
setVisible(true); // otherwise you won't "see" it
}
public static void main(String[] args)
{
TabbedPane tab = new TabbedPane();
}
}
Thanks a lot!

If I discard the panel instead and add the tabbedPane directly to the frame, everything works fine.
The default layout of JPanel is FlowLayout, which "lets each component assume its natural (preferred) size." The default layout of JFrame is BorderLayout, the CENTER of which ignores preferred size. In either case, invoking setSize() precludes the layout from functioning initially; re-size the frame to see the effect. Instead, use pack(), which "Causes this Window to be sized to fit the preferred size and layouts of its subcomponents."
setDefaultCloseOperation(EXIT_ON_CLOSE);
pack();
setVisible(true); // otherwise you won't "see" it

There are many things I would change in that code, starting with the recommendations of #trashgod. OTOH this is the minimal change needed in order to stretch the tabbed pane to the width/height of the parent container.
// give the panel a layout that will stretch components to available space
JPanel panel = new JPanel(new GridLayout());//uncomment all three lines
panel.add(jtp);
getContentPane().add(panel);
//getContentPane().add(jtp);//remove me
For more details see this answer.

Well firstly you can try this:
JPanel panel = new JPanel();//uncomment all three lines
panel.setLayout(new GridLayout());
JPanel jp1 = new JPanel();// This will create the first tab
JPanel jp2 = new JPanel();// This will create the second tab
JLabel label1 = new JLabel();
label1.setText("This is Tab 1");
jp1.add(label1);
jtp.addTab("Tab1", jp1);
jtp.addTab("Tab2", jp2);
JButton test = new JButton("Press");
jp2.add(test);
getContentPane().add(jtp);
and in the main:
TabbedPane tab = new TabbedPane();
tab.pack();
tab.setVisible(true);
May I suggest using MigLayout to set layouts, it will make your life easier. Hope it helps.

Try GridbagLayout. Once you have mastered it, you can design UI of any sort with this layout.

I agree with prasanth regarding the use of GridBagLayout
I have gone through this problem once and I solved it by adding the JTabbedPaneto the panel via GridBagLayout, make sure you add the JTabbedPane using the ipadx and ipady according to your requirements in your GridBagConstraints object
e.g.
JPanel myPanel=new JPanel();
myPanel.setLayout(new GridBagLayout());
JTabbedPane jTP=new JTabbedPane();
jTP.add("Tab1",new JPanel());//substitute your component instead of "new JPanel"
GridBagConstraints myConstraints=new GridBagConstraints();
myConstraints.ipadx=400;//streches the component being added along x axis - 200 px on both sides
myConstraints.ipady=600;//streches the component being added along y axis - 200 px on both sides
myPanel.add(jTP,myConstraints);
You can adjust both these properties according to what is perfect for your need

Automatically size JPanel inside JFrame

I have a JPanel subclass on which I add buttons, labels, tables, etc. To show on screen it I use JFrame:
MainPanel mainPanel = new MainPanel(); //JPanel subclass
JFrame mainFrame = new JFrame();
mainFrame.setTitle("main window title");
mainFrame.getContentPane().add(mainPanel);
mainFrame.setLocation(100, 100);
mainFrame.pack();
mainFrame.setVisible(true);
mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
But when I size the window, size of panel don't change. How to make size of panel to be the same as the size of window even if it was resized?

You can set a layout manager like BorderLayout and then define more specifically, where your panel should go:
MainPanel mainPanel = new MainPanel();
JFrame mainFrame = new JFrame();
mainFrame.setLayout(new BorderLayout());
mainFrame.add(mainPanel, BorderLayout.CENTER);
mainFrame.pack();
mainFrame.setVisible(true);
This puts the panel into the center area of the frame and lets it grow automatically when resizing the frame.

You need to set a layout manager for the JFrame to use - This deals with how components are positioned. A useful one is the BorderLayout manager.
Simply adding the following line of code should fix your problems:
mainFrame.setLayout(new BorderLayout());
(Do this before adding components to the JFrame)

If the BorderLayout option provided by our friends doesnot work, try adding ComponentListerner to the JFrame and implement the componentResized(event) method. When the JFrame object will be resized, this method will be called. So if you write the the code to set the size of the JPanel in this method, you will achieve the intended result.
Ya, I know this 'solution' is not good but use it as a safety net.
;)

From my experience, I used GridLayout.
thePanel.setLayout(new GridLayout(a,b,c,d));
a = row number, b = column number, c = horizontal gap, d = vertical gap.
For example, if I want to create panel with:
unlimited row (set a = 0)
1 column (set b = 1)
vertical gap= 3 (set d = 3)
The code is below:
thePanel.setLayout(new GridLayout(0,1,0,3));
This method is useful when you want to add JScrollPane to your JPanel. Size of the JPanel inside JScrollPane will automatically changes when you add some components on it, so the JScrollPane will automatically reset the scroll bar.

As other posters have said, you need to change the LayoutManager being used. I always preferred using a GridLayout so your code would become:
MainPanel mainPanel = new MainPanel();
JFrame mainFrame = new JFrame();
mainFrame.setLayout(new GridLayout());
mainFrame.pack();
mainFrame.setVisible(true);
GridLayout seems more conceptually correct to me when you want your panel to take up the entire screen.