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How do I put a JPanel in center of a JFrame irrespective the size of JFrame (of course its size is greater than jpanel)? [duplicate]

I'm using the NetBeans GUI builder to handle my layout (I'm terrible with LayoutManagers) and am trying to place a simple JLabel so that it is always centered (horizontally) inside its parent JPanel. Ideally, this would maintain true even if the JPanel was resized, but if that's a crazy amount of coding than it is sufficient to just be centered when the JPanel is first created.
I'm bad enough trying to handle layouts myself, but since the NetBeans GUI Builder autogenerates immutable code, it's been impossible for me to figure out how to do this centering, and I haven't been able to find anything online to help me.
Thanks to anybody who can steer me in the right direction!

Here are four ways to center a component:
import java.awt.*;
import javax.swing.*;
import javax.swing.border.*;
class CenterComponent {
public static JLabel getLabel(String text) {
return getLabel(text, SwingConstants.LEFT);
}
public static JLabel getLabel(String text, int alignment) {
JLabel l = new JLabel(text, alignment);
l.setBorder(new LineBorder(Color.RED, 2));
return l;
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
JPanel p = new JPanel(new GridLayout(2,2,4,4));
p.setBackground(Color.black);
p.setBorder(new EmptyBorder(4,4,4,4));
JPanel border = new JPanel(new BorderLayout());
border.add(getLabel(
"Border", SwingConstants.CENTER), BorderLayout.CENTER);
p.add(border);
JPanel gridbag = new JPanel(new GridBagLayout());
gridbag.add(getLabel("GridBag"));
p.add(gridbag);
JPanel grid = new JPanel(new GridLayout());
grid.add(getLabel("Grid", SwingConstants.CENTER));
p.add(grid);
// from #0verbose
JPanel box = new JPanel();
box.setLayout(new BoxLayout(box, BoxLayout.X_AXIS ));
box.add(Box.createHorizontalGlue());
box.add(getLabel("Box"));
box.add(Box.createHorizontalGlue());
p.add(box);
JFrame f = new JFrame("Streeeetch me..");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setContentPane(p);
f.pack();
f.setLocationByPlatform(true);
f.setVisible(true);
}
});
}
}

By using Borderlayout, you can put any of JComponents to the CENTER area. For an example, see an answer to Stack Overflow question Get rid of the gap between JPanels. This should work.

Even with BoxLayout you can achieve that:
JPanel listPane = new JPanel();
listPane.setLayout(new BoxLayout(listPane, BoxLayout.X_AXIS ));
JLabel label = new JLabel();
listPane.add(Box.createHorizontalGlue());
listPane.add(label);
listPane.add(Box.createHorizontalGlue());
mKorbel's solution is perfect for your goal. Anyway I always like to suggest BoxLayout because it's very flexible.

Mara: "thanks for your response, however the NetBeans GUI Build uses GroupLayout and this is not overridable."
Not true! Right click anywhere inside JFrame (or any other GUI container) in NetBeans GUI builder and select "Set Layout". By default is selected "Free Design", which is Group layout, but you can select any other layout including Border layout as advised by mKorbel.

There's many ways to do this, depending on the layout manager(s) you use. I suggest you read the Laying Out Components Within a Container tutorial.
I believe the following will work, regardless of layout manager:
JLabel.setHorizontalAlignment(SwingConstants.CENTER)

How can I add ScrollBar to JTextArea in Java Swing?

Anyone help me how to add a scroll bar to a JTextArea with Swing in Java?
The JTextArea just disappear when I add the scroll bar on it.
Hope somebody get me add a vertical scrollbar on it.
Additional explanation will be very thankful
public class Practice extends JFrame {
JFrame frame = new JFrame("AAA");
JTextArea textarea = new JTextArea();
JScrollPane scroll = new JScrollPane(textarea);
JPanel panelForScroll = new JPanel(null);
public Practice(){
frame.setLayout(null);
frame.setBounds(100,100,400,710);
frame.setResizable(false);
frame.setVisible(true);
textarea.setEditable(false);
textarea.setFont(new Font("arian", Font.BOLD, 16));
textarea.setBounds(20, 280, 340, 70);
panelForScroll.add(scroll);
frame.add(panelForScroll); //can't find text area....
}
public static void main(String[] args) {
new Practice();
}
}

There are several errors in your code:
You're using a null layout, this is discouraged as it produces more problems than solutions, specially when you try to use JScrollPanes, since they take the preferredSize of the Component to decide whether to add the scroll bars or not. See Why is it frowned upon to use a null layout in Swing? for more information about this. To fix this, remove this line:
frame.setLayout(null);
And instead use a layout manager or combinations of them along with borders for extra spacing between components.
While null layouts might seem like the best, easiest and faster way to design complex GUIs for Swing newbies, the more you progress in it, the more problems related to the use of them you'll find (as it's the case)
You're extending your class from JFrame and you're creating an instance of JFrame in it too, please use one or the other. When you extend JFrame you're saying your class is a JFrame and thus it cannot be placed inside another Container because JFrame is a rigid container. I recommend to forget the extends JFrame part, since anyway you're not using the JFrame that is generated by this action and stay with the object you created. See https://stackoverflow.com/questions/41252329/java-swing-using-extends-jframe-vs-calling-it-inside-of-class for a more detailed answer about this problem.
You're making your GUI visible before you have added all the elements, this could cause your GUI to not display all the elements until you hover over them, this line:
frame.setVisible(true);
Should be one of the last lines in your program
You're not placing your program on the Event Dispatch Thread (EDT) which makes your application to not be thread safe, you can fix it by writing this on your main method.
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
//Place your constructor here
}
});
You're setting bounds for the textArea but not for the scrollPane, but you should really not be setting the bounds manually (see point #1 again).
Now, you can make a simple GUI with a JTextArea with a JScrollPane as follows:
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.SwingUtilities;
public class ScrollPaneToTextArea {
private JTextArea textArea;
private JFrame frame;
private JScrollPane scroll;
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
new ScrollPaneToTextArea().createAndShowGui();
}
});
}
public void createAndShowGui() {
frame = new JFrame("ScrollPane to TextArea");
textArea = new JTextArea(10, 20); //Rows and cols to be displayed
scroll = new JScrollPane(textArea);
// scroll = new JScrollPane(textArea, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
frame.add(scroll); //We add the scroll, since the scroll already contains the textArea
frame.pack();
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}
Which produces this output and the scroll bars are added when needed (i.e. when text goes further than the rows it can handle in the view)
If you want the vertical scroll bars to appear always you can uncomment the line:
scroll = new JScrollPane(textArea, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
Which will produce the following outputs:
You can read more about JScrollPane in the docs and JTextArea also in their own docs.

JPanel panelForScroll = new JPanel(null);
This sets the NULL Layout to this JPanel. This would require more configuration (just as you did for the frame object).
Just remove the null (also from frame.setLayout(null)!)

You have to use Jtextpane to get the scroll bar on textarea.
JTextArea ta = new JTextArea();
JScrollPane sp = new JScrollPane(ta);
JFrame f = new JFrame();
f.getContentPane().add(sp);

you are setting the panel's layout to null,then you didn't specify the scroll bar bounds. Since you only have one component in your panel which is the scroll bar I recommend using FlowLayout

Java GUI FullScreen window with smaller window inside

I am working on a Java desktop application. It uses MySQL database to store all data etc. I use swing for the GUI.
The GUI of this application is layed out as follows:
Main Window taking the entire screen size (with image in the
background)
Internal Window 800 x 600 centered within the Main
Window (that holds current content that can be switched between
using menu and/or event within the application.
LoginPanel.java:
import javax.swing.*;
import java.awt.*;
public class LoginPanel {
private JPanel loginPanel;
public void loginForm()
{
JButton loginSubmit = new JButton("Login");
loginPanel = new JPanel();
loginPanel.add(loginSubmit);
loginPanel.setSize(800, 600);
}
public JComponent getGUI()
{
return loginPanel;
}
public static void main(String[] args)
{
}
}
Main.java:
import javax.swing.*;
import java.awt.*;
public class Main {
public static void main(String[] args)
{
JFrame mainFrame;
mainFrame = new JFrame();
mainFrame.setLayout(new BorderLayout());
mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
mainFrame.setTitle("Caledonian Library System");
LoginPanel loginObj = new LoginPanel();
mainFrame.add(loginObj.getGUI());
mainFrame.pack();
mainFrame.setVisible(true);
}
}
Should I maybe use box layout? any suggestions?

Ok, I have just run a test program, and have achieved the result you are looking for. I have used a GridBagLayout which defaults to centre into the Container it is added to. It will not show up with the borders or other buttons built into a JFrame (though you can add a border if you wish later).
JFrame mainframe = new JFrame();
JPanel mainPanel = new JPanel();
GridBagLayout gridLayout = new GridBagLayout();
mainPanel.setLayout(gridLayout);
//GridBagConstraints allow you to set various features of the way the components appear
//in the grid. You can set this up as you wish, but defaults are fine for this example
GridBagConstraints gridConstraints = new GridBagConstraints();
//Just using FlowLayout as a test for now
JPanel centerPanel = new JPanel(new FlowLayout());
centerPanel.add(new JLabel("Hello"));
centerPanel.add(new JLabel("Centered"));
mainPanel.add(centerPanel, gridConstraints);
mainFrame.add(mainPanel);
If you found that the space around the side of your centered panel wasn't being used, and you wanted it to be use, you could try nesting mainPanel inside another panel that is using a BorderLayout, making sure that it is in BorderLayout.CENTER.
In the example I didn't bother changing GridBagConstraints from the default, as it was ok for this demonstration. However you can edit it as you wish, and then apply to each component you add to the GridBagLayout, making sure to include the GridBagConstraints object in each mainPanel.add(). Check the GridBagLayout tutorials for some good information.
Of course, if you would like more components in the centre other than the main window, you can then simply add them to the mainPanel (making sure to change the position in the GridLayout). There are going to be numerous ways of achieving what you want, but it really depends on what you feel looks good. The Layout Managers will do all of the resizing work for you.

JFrame not displaying button or background color

My JFrame is not displaying the button or background color that is set in the constructor. I am only getting a blank box when I start the program. Not sure what is wrong with the code.
//imports
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;;
public class StartingTheCode{
JButton CalculateButton;
JTextField Ans;
JPanel p;
JFrame f;
public static void main (String[] args){
new StartingTheCode();
}
//constructor
StartingTheCode(){
f = new JFrame("test");
f.setVisible(true);
f.setSize(600,600);
f.setLocationRelativeTo(null);
f.setResizable(false);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
p = new JPanel();
p.setBackground(Color.BLUE); // not displaying blue background
CalculateButton = new JButton("+"); // should display button
CalculateButton.setSize(30,30);
CalculateButton.setLocation(5,5);
}
}

You're not adding your button or your JPanel to anything, and so no JFrame is magically going to display them.
You should add your JButton to your JPanel via its add(...) method, and then add the JPanel to the JFrame via its add(...) method, and do so before setting the JFrame visible.
Most importantly, you should read the Swing tutorials, since I speak from experience when saying you'll get no-where just guessing at this stuff. This is all well explained there.
As an aside, avoid setting the sizes of any components and instead read the tutorial section on use of the layout managers as it will allow you to simplify and empower your code greatly.

You need to add your calculateButton to the JPanel with p.add(calculateButton) and add the panel to the frame with f.add(p)

Setting JPanel layout

(Say) I've created a JPanel with three buttons. I want to place the buttons as follows (I've done this using netbeans GUI editor. But I need to write the whole GUI manually).
Can some one show me a way to achieve this.
(In words, I need to place some buttons right aligned, some other left aligned.)

I guess you want the Configure button to be as far to the left as possible, and the ok and cancel grouped together to the right. If so, I would suggest using a BorderLayout and place the Configure button in WEST, and a flow-layout for Ok, Cancel and place that panel in the EAST.
Another option would be to use GridBagLayout and make use of the GridBagConstrant.anchor attribute.
Since you're taking the time to avoid the NetBeans GUI editor, here's a nice example for you :-)
Code below:
import java.awt.BorderLayout;
import javax.swing.*;
public class FrameTestBase {
public static void main(String args[]) {
// Will be left-aligned.
JPanel configurePanel = new JPanel();
configurePanel.add(new JButton("Configure"));
// Will be right-aligned.
JPanel okCancelPanel = new JPanel();
okCancelPanel.add(new JButton("Ok"));
okCancelPanel.add(new JButton("Cancel"));
// The full panel.
JPanel buttonPanel = new JPanel(new BorderLayout());
buttonPanel.add(configurePanel, BorderLayout.WEST);
buttonPanel.add(okCancelPanel, BorderLayout.EAST);
// Show it.
JFrame t = new JFrame("Button Layout Demo");
t.setContentPane(buttonPanel);
t.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
t.setSize(400, 65);
t.setVisible(true);
}
}