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After stretch, the last panel is not stretched, but second last did in panel

Following Java's official tutorial:(https://docs.oracle.com/javase/tutorial/uiswing/components/table.html)
However, there is no source code for this, picture:
So I did it myself. My stupid idea is that there are 4 areas in here. So BorderLayout does not work well for me. Because I already tried BorderLayout.PAGE_END. It's not working. So I make one panel consolidate all 3 panels except the first scroll pane. It's inefficient, but works.
But now the issue is when I drag(stretch) the frame, the last text-area not stretched, but the second last selection option is stretched.
How to make the last text area stretch when I resize the frame?
Below is my code:
JRadioButton mulintselRadioButton = new JRadioButton("Multiple Interval Selection");
JRadioButton singleselRadioButton = new JRadioButton("Single Selection");
JRadioButton singleIIntSelRadioButton = new JRadioButton("Single Interval Selection");
JCheckBox rowSelection = new JCheckBox("Row Selection");
JCheckBox columnSelection = new JCheckBox("Column Selection");
JCheckBox cellSelection = new JCheckBox("cell Selection");
ButtonGroup G = new ButtonGroup();
ButtonGroup GButton = new ButtonGroup();
GButton.add(rowSelection);
GButton.add(columnSelection);
GButton.add(cellSelection);
G.add(mulintselRadioButton);
G.add(singleIIntSelRadioButton);
G.add(singleselRadioButton);
JTable table = new JTable(data, columnNames);
TableColumn column = null;
table.setPreferredScrollableViewportSize(new Dimension(400,70));
table.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
table.setFillsViewportHeight(true);
JScrollPane scrollPane = new JScrollPane(table);
JPanel selectionMode = new JPanel(new GridLayout(4,1));
JPanel selectionOption = new JPanel(new GridLayout(4,1));
JPanel textAreaPanel = new JPanel(new GridLayout(1,1));
JPanel consolidatedPanel = new JPanel(new BorderLayout());
JLabel selcttionModeTitle = new JLabel("Selection Mode");
JLabel selcttionOptionTitle = new JLabel("Selection Options");
JTextArea textArea = new JTextArea("thsisaskjhkjk shial");
textAreaPanel.add(textArea);
selectionMode.add(selcttionModeTitle);
selectionMode.add(singleIIntSelRadioButton);
selectionMode.add(singleselRadioButton);
selectionMode.add(mulintselRadioButton);
selectionOption.add(selcttionOptionTitle);
selectionOption.add(rowSelection);
selectionOption.add(columnSelection);
selectionOption.add(cellSelection);
textArea.setText("hsknd hkcjshksdjl sldh RadioButton mulintselRadioButton = new JRadioButton(\"Multiple Interval Selection\");\n" +
" JRadioButton singleselRadioButton = new JRadioButton(\"Single Selection\");\n" +
" JRadioButton singleIIntSelRadioButton = new JRadioButton(\"Single Interval Selection\");\n" +
" JCheckBox rowSelection = new JCheckBox ");
add(scrollPane,BorderLayout.NORTH);
consolidatedPanel.add(selectionMode,BorderLayout.NORTH);
consolidatedPanel.add(selectionOption,BorderLayout.CENTER);
consolidatedPanel.add(textAreaPanel,BorderLayout.SOUTH);
add(consolidatedPanel);

Following the Java's official tutorial (the link of which you have posted in your question), one can read that it says that you can consult the example index... If you follow this link you can find a table which gives you the link to the file you are searching for. Specifically the code you are looking for is in the following link:
https://docs.oracle.com/javase/tutorial/uiswing/examples/components/TableSelectionDemoProject/src/components/TableSelectionDemo.java
As for your question, you can use a BoxLayout to achieve what you are asking, exactly as the TableSelectionDemo does. The result can strech the table and the text area, but not the middle section.

Aligning JLabel to Left or Right inside BoxLayout with Y_AXIS Constraint of JPanel

I have a JPanel with Constraint's of Y_Axis so that whenever I add a new Component it will automatically be Added on a new Line.But the Problem is that the Label inside is not Aligned to Left or Right. It is displayed at some distance above the JTable. How can JLabel be displayed at desired Alginment.
JPanel panel = new JPanel();
panel.setLayout(new BoxLayout(panel,BoxLayout.Y_AXIS));
Then I added a JLabel inside panel.
JLabel labelSemester = new JLabel("Semester 1: ",SwingConstants.LEFT);
panel.add(labelSemester);
After label, I added a new JTable inside panel,
// Column Names for the Table
Object[] col_names = {"ID", "Name", "CH", "Marks", "Grade"};
// row data for the table
Object[][] table_rows = {{"CS123","Introduction to Computing",3,80,"A-"}};// One row only
JTable table = new JTable(table_rows, col_names);
panel.add(new JScrollPane(table));
Then I added a JFrame and added the Panel to show in the frame
JFrame frame = new JFrame();
// frame Title
frame.setTitle("DMC");
frame.setSize(400,400);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
// adding panel inside frame
frame.add(panel);
// displaying frame
frame.show()
Note:
I have added code for auto Adjustment of column width of JTable.
Output can be seen from attached Image

All components added to the BoxLayout need the same alignmentX, otherwise you can get some weird layouts:
//JLabel labelSemester = new JLabel("Semester 1: ",SwingConstants.LEFT);
JLabel labelSemester = new JLabel("Semester 1: ");
label.semester.setAlignmentX(JLabel.LEFT_ALIGNMENT);
panel.add(labelSemester);
...
JTable table = new JTable(table_rows, col_names);
//panel.add(new JScrollPane(table));
JScrollPane scrollPane = new JScrollPane( table );
scrollPane.setAlignmentX(JScrollPane.LEFT_ALIGNMENT);
panel.add( scrollPane );
Read the section from the Swing BoxLayout tutorial on Fixing Alignment Problems for more information. Keep a link to the tutorial handy for all Swing basics.

BoxLayout adding left margin

I've got a JPanel that has a BoxLayout (Page axis), and I want to lay out two components, one on top of the other.
My problem is the margin to the left of the large lipsum box, how can I get rid of this? If I don't add the top components, there is no margin.
Here's my code, the second image is created by not adding headerPanel:
JLabel commandLabel = new JLabel(command);
JLabel paramLabel = new JLabel(params);
JLabel descLabel = new JLabel("<html><body style='width: 200px;'>" + description + "</body></html>");
Font baseFont = commandLabel.getFont(), commandFont, paramFont, descFont;
commandFont = baseFont.deriveFont(Font.BOLD);
paramFont = baseFont.deriveFont(Font.ITALIC);
descFont = baseFont.deriveFont(Font.PLAIN);
commandLabel.setFont(commandFont);
paramLabel.setFont(paramFont);
descLabel.setFont(descFont);
descLabel.setAlignmentX(LEFT_ALIGNMENT);
descLabel.setBorder(BorderFactory.createStrokeBorder(new BasicStroke()));
JPanel headerPanel = new JPanel(new FlowLayout(FlowLayout.LEADING));
headerPanel.add(commandLabel);
headerPanel.add(paramLabel);
this.add(headerPanel);
this.add(descLabel);
This class extends JPanel, and is added to a JFrame, which is simply pack()'d

Though I couldn't tell where the observed behaviour comes from, the expected display could be achieved by using an intermediate JPanel to contain your label, rather than adding the JLabel directly :
JLabel commandLabel = new JLabel(command);
JLabel paramLabel = new JLabel(params);
JLabel descLabel = new JLabel("<html><body style='width: 200px;'>" + description + "</body></html>");
Font baseFont = commandLabel.getFont(), commandFont, paramFont, descFont;
commandFont = baseFont.deriveFont(Font.BOLD);
paramFont = baseFont.deriveFont(Font.ITALIC);
descFont = baseFont.deriveFont(Font.PLAIN);
commandLabel.setFont(commandFont);
paramLabel.setFont(paramFont);
descLabel.setFont(descFont);
descLabel.setAlignmentX(LEFT_ALIGNMENT);
descLabel.setBorder(BorderFactory.createStrokeBorder(new BasicStroke()));
JPanel headerPanel = new JPanel(new FlowLayout(FlowLayout.LEADING));
JPanel descPanel = new JPanel(new FlowLayout(FlowLayout.LEADING));// added
headerPanel.add(commandLabel);
headerPanel.add(paramLabel);
descPanel.add(descLabel);// added
this.add(headerPanel);
this.add(descPanel);// modified

My problem is the margin to the left of the large lipsum box, how can I get rid of this?
You need to make the alignments of your components consistent. That is the alignment "X" property of all the components should be left aligned.
I'm guessing the JLabel is center aligned so you need to use:
descLabel.setAlignmentX(JLabel.LEFT_ALIGNMENT);
See Fixing Alignment Problems section from the Swing tutorial on How to Use BoxLayout for more information and examples.

Java BoxLayout use full width

I am creating a messaging program that has multiple chats. On the side of the chat window there is a JPanel containing a List Array of all the buttons to switch chats.I am having trouble getting all the buttons in the side panel to be the same width, no matter what they contain. But whatever I try doesn't seem to work and I am looking for some help. Please can you explain what the code does and how it can be used so I can learn it for next time. Sorry if the comments aren't the best it's a work in progress as I wait till code works before adding detailed comments otherwise I am constantly changing them. :(
This is what it looks like without any chats:
This is with multiple chats and you can see the width variation
Here is the code that is run when a new user is added:
public void newUser() {
JPanel dialogue = new JPanel();
dialogue.setLayout(new BoxLayout(dialogue, BoxLayout.Y_AXIS));
//Creating/adding dialogue components
JLabel Enter_ip = new JLabel("Enter the ip address");
JTextField Get_ip = new JTextField("");
dialogue.add(Enter_ip);
dialogue.add(Get_ip);
dialogue.add(Box.createHorizontalStrut(15));
JLabel Enter_name = new JLabel("Enter the user's name");
JTextField Get_name = new JTextField("");
dialogue.add(Enter_name);
dialogue.add(Get_name);
//Creating the dialogue box
JOptionPane.showConfirmDialog(null, dialogue, "New User", JOptionPane.OK_CANCEL_OPTION);
//Getting data from dialogue box
String ip = Get_ip.getText();
String name = Get_name.getText();
//Try connecting to other user here
//Adding user message data
int size = Users_Messages_Data.size();
Users_Messages_Data.add(new ArrayList());//New user
Users_Messages_Data.get(size).add(new ArrayList());//Messages
Users_Messages_Data.get(size).add(new ArrayList());//Details
Users_Messages_Data.get(size).get(1).add(name);
Users_Messages_Data.get(size).get(1).add(ip);
Users_Messages_Data.get(size).get(1).add("port number");
//adds new UserButton
int temp = users.size();
users.add(new JButton(Users_Messages_Data.get(size).get(1).get(0)));
users.get(temp).addActionListener(this);
users.get(temp).setSize(new Dimension(500, 500));
SelectUser.add(users.get(temp), gbc);
Messaging.revalidate();
pack();
}
and here is the initialisation method:
public void MessagingGUI() {
//Creates JFrame and pane
Messaging = new JFrame();
Container pane = getContentPane();
JLabel info = new JLabel("29/07/2016 15:36");
//Creates user chats panel
SelectUser = new JPanel(new GridBagLayout());
gbc = new GridBagConstraints();
gbc.weightx = 1;
gbc.fill = GridBagConstraints.HORIZONTAL;
gbc.gridwidth = GridBagConstraints.REMAINDER;
SelectUser.setLayout(new BoxLayout(SelectUser, BoxLayout.Y_AXIS));
SelectUser.setSize(new Dimension(500, 500));
//Adds different chats
users = new ArrayList<JButton>();
int x;
for (x = 0; x < Users_Messages_Data.size(); x++) {
users.add(new JButton(Users_Messages_Data.get(x).get(1).get(0)));
users.get(x).addActionListener(this);
SelectUser.add(users.get(x), gbc);
}
JButton newUser = new JButton("+");
newUser.addActionListener(this);
SelectUser.add(newUser);
JScrollPane UserScroll = new JScrollPane(SelectUser,
JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
//Creates messages feed
JPanel Messages = new JPanel();
Messages.setLayout(new BoxLayout(Messages, BoxLayout.Y_AXIS));
AllMessages = new JTextArea(10, 30);
AllMessages.setBackground(Color.WHITE);
AllMessages.setEditable(false);
AllMessages.setBorder(BorderFactory.createLineBorder(Color.BLUE, 1));
JScrollPane MessageScroll = new JScrollPane(AllMessages,
JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
//Creates user text entry box
UserText = new JTextArea(5, 30);
//UserText.setLineWrap(true);
//UserText.setWrapStyleWord(true);
UserText.setBorder(BorderFactory.createLineBorder(Color.CYAN, 1));
UserText.setText("Enter Message. Press enter to send");
UserText.setFocusable(true);
UserText.addKeyListener(this);
UserText.setPreferredSize(new Dimension(5, 20));
//Adds all components to pane
Messages.add(info);
Messages.add(MessageScroll);
Messages.add(UserText);
pane.add(UserScroll, BorderLayout.WEST);
pane.add(Messages, BorderLayout.CENTER);
//JFrame setup
Messaging.setTitle("Messaging");
Messaging.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Messaging.setContentPane(pane);
Messaging.setVisible(true);
Messaging.setSize(400, 350);
Load_User(current_user);
}

First of all, variable names should NOT start with an upper case character. Some of your variable names are correct, other are not. Be consistent and follow Java conventions!!!
.I am having trouble getting all the buttons in the side panel to be the same width,
Don't use a BoxLayout. It does not automatically resize on the opposite axis of the layout.
Instead you can use:
a GridLayout to make all the buttons the same size. The GridLayout will also fill the area vertically which is not what you want so you will need to nest panels. So create a parent panel using a BorderLayout. Add your panel using the GridLayout with the buttons to the BorderLayout.PAGE_START of this parent panel. Then add the BorderLayout panel to the scroll pane.
a GridBagLayout. You will need to use the "fill" contstraint to have the component fill the width of the cell.
Read the Swing tutorial on Layout Managers for more information and examples.
Or maybe a different approach is to use a JList to display the users.

JScrollPane in Java

I wrote a little code to see how the scroll Pane functions but my code never worked.
here's the code,
public Fenetre(){
this.setTitle("Data Simulator");
this.setSize(300, 300);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setLocationRelativeTo(null);
String hello = "hello";
int number = 69;
JPanel content = new JPanel();
content.setBackground(Color.LIGHT_GRAY);
//Box imad = Box.createHorizontalBox();
JTextArea textArea = new JTextArea(10, 10);
JLabel imad = new JLabel();
imad.setText(hello + " your favorite number is " + number + "\nRight?");
JScrollPane scrollPane = new JScrollPane();
setPreferredSize(new Dimension(450, 110));
scrollPane.setHorizontalScrollBarPolicy(HORIZONTAL_SCROLLBAR_ALWAYS);
scrollPane.setVerticalScrollBarPolicy(VERTICAL_SCROLLBAR_ALWAYS);
scrollPane.setEnabled(true);
scrollPane.setWheelScrollingEnabled(true);
scrollPane.setViewportView(textArea);
scrollPane.setViewportView(imad);
add(scrollPane, BorderLayout.CENTER);
//---------------------------------------------
//On ajoute le conteneur
scrollPane.add(textArea);
scrollPane.add(imad);
content.add(textArea);
content.add(imad);
content.add(scrollPane);
this.setContentPane(content);
this.setVisible(true);
this.setResizable(false);
}
When I run it, I get a little window with the textArea and next to the text area a very little white square, which is the scrollpane i suppose because when I remove it from the code, this square disappears. When I write in the text area and exceed the window's dimension, I can't scroll vertically using the mouse wheel, and not horizontally at all. I saw many examples on internet and I can't understand why my code doesn't work??
Any help explaining how scrollpane works?

scrollPane.setViewportView(textArea);
scrollPane.setViewportView(imad);
Only one component can be added to the viewport of the scroll pane, so the label replaces the text area.
content.add(textArea);
content.add(imad);
A component can only have a single parent. The above code removes the label from the scrollpane, so nothing is now in the scrollpane.
Try something like:
JScrollPane = new JScrollPane( textArea );
JPanel content = new JPanel( new BorderLayout() );
content.add(scrollPane, BorderLayout.CENTER);
content.add(imad, BorderLayout.PAGE_END);
setContentPane( content );
For a better solution, start with the working example found in the Swing tutorial on How to Use Text Areas and then modify the code. This way you will start with a better structured program that follows Swing standards.