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JFrame and JPanel sizing

I have a JFrame and a few JPanels that are displayed depending on where the user goes (to login, homepage etc), is there a way to set all JPanels to the same size without having to manually specify?
public mainApp() {
main = new JFrame("Application");
main.setSize(640, 480);
main.setVisible(true);
}
private JPanel loginScreen() {
login = new JPanel(null);
login.setSize(640, 480);
login.setVisible(true);
}
For example I have 5 different JPanels, and have to specify the size in each, is there a way to set a default size that is the same as the JFrame?

You have different valid options here. Knowing that you want to change from one view to another and keep the same size, it sounds like the best option would be to use a CardLayout allowing you to change between the different views without having to worry about repainting and revalidating stuff (this tutorial from Oracle helped me a lot back when I was learning to use this layout: Oracle - How to use CardLayout.
However, as usual with Swing/AWT this is not the only valid option. For example, you could also use the BorderLayout that is applied by default to the ContentPane from the JFrame and add the desired JPanel to the Center of that BorderLayout. However, you would have to manage the view-changing process in this case.

Elements don't appear in a Panel with a GridLayout or FlowLayout, but with a setBounds they do

I'm doing a program that is composed by multiple panels in a JFrame.
I need to do every elements in differents classes (It's because in my school, we need to have every elements separeated in different classes for clean code), but every example that I see with my kind of problem, they do everything in one class.
And I think that my problem comes from having multile classes so I show you my classes.
I have a panel in wich I need to put 2 panel, here is the code :
public class Inscription extends JPanel{
private PanneauBoutons panneauBoutons = new PanneauBoutons();
private PanneauFormulaire panneauFormulaire = new PanneauFormulaire();
public Inscription(){
this.setLayout(new BorderLayout());
this.setBorder(BorderFactory.createLineBorder(Color.RED, 2));
this.add(panneauFormulaire,BorderLayout.CENTER);
this.add(panneauBoutons,BorderLayout.SOUTH);
this.setVisible(true);
}
}
And here is the Panel panneauFormulaire :
public class PanneauFormulaire extends JPanel{
private JLabel labelMatricule;
private JTextField zoneTexteMatricule;
public PanneauFormulaire(){
this.setLayout(new GridLayout(8,2,10,10));
this.setBorder(BorderFactory.createLineBorder(Color.black));
labelMatricule = new JLabel("Matricule : ");
this.add(labelMatricule);
zoneTexteMatricule = new JTextField(30);
this.add(zoneTexteMatricule);
this.setVisible(true);
}
So the problem Inscription don't appear on the main Frame if I don't do setBounds, but I want a BorderLayout...
(I tested and with a set bounds I can see the borders, so I think that it means the panel are really added to the Frame so why without setBounds I see anything?).
And the other problem is that the panel PanneauFormulaire don't appear on the Inscription panel...
So if I miss something, can you help me? thank you
And here it is the class that extends JFrame :
public class FenetrePrincipale extends JFrame {
private Container cont;
private Inscription inscriptionForm;
public FenetrePrincipale(){
super("IESN");
setBounds(100,100,1200,960);
getContentPane().setLayout(null);
setLocationRelativeTo(null);
setResizable(false);
...
inscription.addActionListener(new ActionListener(){
#Override
public void actionPerformed(ActionEvent e){
cont.removeAll();
inscriptionForm = new Inscription();
inscriptionForm.setOpaque(true);
cont.add(inscriptionForm);
invalidate();
repaint();
}
});

You should NOT be using a null layout and setBounds(). Swing was designed to be used with layout managers.
but when I click on an option in the menu, the current panel need to be change by another one,
Then you should be using a CardLayout.
Read the section from the Swing tutorial on How to Use CardLayout for working examples. So download the example and use it as the starting point of your project. The code will be better structured then what you currently have and it is easier to change working code than it is to fix broken code.

so why without setBounds I see anything?
That is because you set your layout to null in getContentPane().setLayout(null);.
Java containers comes with a default layout which you are allowed to set to a different one. How the components are arranged in the container are dependent on the layout you use. The layout will directly affects the location, alignment, spacing, dimension, preferredSize of the components.
However, if you choose not to use any layout (.setLayout(null)). Swing will not know how you want the components to be arranged, hence you see nothing in your content pane.
Since you wanted "absolute control" over the components, you will be expected to set the bounds (location and dimension) of each added component manually by yourself. This is why you are not seeing any components (even if you already added it) until you set the bounds for them.
Java, elements don't appear in a Panel with a GridLayout or FlowLayout, but with a setBounds they do
Every layout has their own characteristics and for some of them the order of your codes does makes a difference. Hence, I will advise you to go through what each layout can do for you. Then, depending on your needs, choose one (or a combination of a few) and study how to use it.
And here it is the class that extends JFrame :
You probably won't want to extends to a JFrame. You can always make a customized Container like JPanel and add it to the frame.
(Why would you want to paint your paintings on a frame instead of a piece of paper?)

Adding panel with without layout to the NORTH of BorderLayout

Colleagues.
I'm trying to construct simple GUI in Java, where JFrame has Border Layout. I want to put JScrollPane with JTable to CENTER, and JPanel without layout to NORTH.
The problem is that JPanel doesn't visible. There is simple examle of the problem:
JFrame frame = new JFrame("Test frame");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JButton button = new JButton("Test button");
button.setBounds(10, 10, 40, 20);
JPanel panelN = new JPanel(null); // layout = null, panelN without layout
panelN.add(button);
frame.add(panelN, BorderLayout.NORTH);
JTable table = new JTable(new DefaultTableModel(4, 4));
JScrollPane scrollPane = new JScrollPane(table);
frame.add(scrollPane, BorderLayout.CENTER);
frame.setSize(400, 400);
frame.setVisible(true);

You have to use a LayoutManager. It's totally discouraged not using layoutManager, but if you want this you have to set panel.setBounds(..) to the panel too.
By default JPanel has FlowLayout so if you put
JPanel panelN = new JPanel(); // FlowLayout used
panelN.add(button);
frame.add(panelN, BorderLayout.NORTH);
So your frame will look like this.
Layout Managers determines the size and position of the components within a container. Although components can provide size and alignment hints, a container's layout manager has the final say on the size and position of the components within the container.
It's strongly recommended cause for example if you have to resizes components or show in differentes resolutions you delegate this work to layout managers

I don't know the expected behavior of a null layout, but without further requirements you might as well just instantiate with the zero-arg constructor:
new JPanel();

If you didn't set any layout to the panel, when adding components the panel don't know where to put the component, so baisicly the component don't show until you set a specific location for components one by one by component.setBounds(x,y,width,hieght) method.
Note that it's not a good practice to remove the layout manager because of the different platformes, suppose that your program working on Window and MacOS and Linux, you'v better to use the layout managers instead.
Take a look at this post also and see #Andrew Thompson's comment on my answer:
Java GUIs might have to work on a number of platforms, on different
screen resolutions & using different PLAFs. As such they are not
conducive to exact placement of components. For a robust GUI, instead
use layout managers, or combinations of them, along with layout
padding & borders for white space, to organize the components.
After all:
If you have a requirement or an assignment telling you you must use absolute layout, then use it, otherwise avoid it.

It is OK to use containers with no layout manager because you actually CAN set container's layout to NULL. And it's a nice idea to position your components with setBounds(). But in this case, you just have to consider your container. What size it need to be? A layout manager would calculate this for you, and if you don't have one, you have to set the size of your panel by yourself, according to components you have added to it.
As pointed by others here, the case it that the border-layout manager of your frame needs the preferred size of your NORTH panel (actually, the preferred height). And you have to set it, or values will be zeros and the container will become invisible. Note that for the CENTER panel this is not needed as it gets all space possible.
I had a problem like yours before and have written a fast function to resize a container according to bounds of a given component. It will be as large as needed to show this component, so dimension (w,h) and position (x,y) are considered. There's an "auto-resize" version that can be used once, after all components are added.
public static void updatePreferredSize(Container cont, Component comp) {
int w = cont.getPreferredSize().width;
int h = cont.getPreferredSize().height;
int W = comp.getBounds().x + comp.getBounds().width;
int H = comp.getBounds().y + comp.getBounds().height;
if (W>w||H>h) cont.setPreferredSize(new Dimension(W>w?W:w, H>h?H:h));
}
public static void autoPreferredSize(Container cont) {
for (Component comp : cont.getComponents())
updatePreferredSize(cont, comp);
}
You can use updatePreferredSize() after adding every component to a panel, or use autoPreferredSize() once, after all addings.
// [...]
panelN.add(button);
updatePreferredSize(panelN, button);
// [...]
// or...
// [...]
autoPreferredSize(panelN);
// [...]
frame.setVisible(true);
This way, if you do not set you north panel height with a fixed value, with help of these functions you can expect your button will be visible according the position you set it with setBounds().

How to visualize missing Inner Panels?

I am trying to form the following Frame:
The main Frame uses a BorderLayout.
Into at this Frame and I added a Panel which uses BoxLayout - I'll call it P1.
For some reason the Panels I add into P1 are not seen when I run the program.
What's even more confusing is that if P1 uses a GridLayout instead of a BoxLayout, all the Panels I added into P1 are shown.
The EventPanel Class used in the code extends Panel and uses a SpringLayout.
Any ideas how to make P1 work?
Here's the relevant code:
public static void main(String[] args)
{
//Setting Frame
JFrame m_CalendarFrame = new JFrame();
m_CalendarFrame.getContentPane().setLayout(new BorderLayout());
//Setting inner Panel
JPanel P1 = new JPanel();
P1.setLayout(new BoxLayout(P1, BoxLayout.Y_AXIS));
EventPanel Ev = new EventPanel("8:00", "16:00", "Bday", "Go party!");
EventPanel Ev2 = new EventPanel("8:00", "16:00", "Java", "Handing Java Project");
//Adding Two pannels into previous inner Panel
P1.add(Ev);
P1.add(Ev2);
m_CalendarFrame.getContentPane().add(P1, BorderLayout.NORTH);
m_CalendarFrame.setVisible(true);
m_CalendarFrame.setSize(800,800);
}

I'm guess the problem is in your panel that uses the SpringLayout. Start by reading the Swing tutorial on How to Use SpringLayout for working examples and explanations.
If you still need help then post your SSCCE that demonstrates your problem. And start simple. Try using the SpringLayout with one comopnent, then two, then three until you feel more comforatable with it.

I believe it has to do with free space management, but I haven't tested it myself. The BoxLayout uses the preferred size for the components it draws, and the GridLayout distributes the available space evenly among the objects drawn.
What happens if you specify a preferred size for your EventPanel? Based on your code example, you can quickly test this by forcing a preferred size of (800,400) for your EventPanel instances, and see if it does change something when using the BoxLayout.

Java Simple Question about working with JPanels

Just a quick question here. I have a program in which I need to create a number of JPanels, which will then each contain some objects (usually JLabels).
There are a few operations that I have to do each time i create a new JPanel, and I'm wondering if the order in which I do them has any bearing, or if there is a standard practice.
The operations I do are the following:
Declare the JPanel: JPanel panel = new JPanel(...)
Declare the JLabel: JLabel laberl = new JLabel...
Add the JPanel to some other content pane: frame.getContentPane().add(panel)
Set the bounds of the JPanel: panel.setBounds(...)
Add the JLabel to the JPanel: panel.add(label)

In general order isn't important as long as you add all the components to the panel and the panel is added to the content pane "before" you make the frame visible.
The standard practice is to use a layout manager, so there would be no need to set the bounds of the panel you added to the content pane.

The order doesn't matter. However, after creating and adding everything, you need to call revalidate() on the panel (or pack() on its parent window) so that the layout manager (I presume you're using one!) arranges the components as they should be.

Have a method createPanel() that returns the panel with all its children added.
Panel p = createPanel();
p.setBounds(...); // if you must
frame.getContentPane().add(p);
And then
Panel createPanel() {
Panel p = new Panel();
Label l = new Label("Heading");
p.add(l);
return p;
}
The order of constructing and adding items isn't important, except that when you add children, you should add them in the order you want them in the panel.