Switching from null layout to GridBagLayout in Java

Question

Below is the runnable code for a calendar application. This application wasn't written by me, and it uses a null layout. I need much more flexibility for my purposes, so I'm trying to use a layout manager instead. However, nothing except the panel border shows up when I try it my way. Also, the original absolute-positioned code is commented out.

import javax.swing.*;
import javax.swing.table.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;

public class CalendarPanel extends JPanel{

    private static final long serialVersionUID = 1L;
    static JLabel lblMonth, lblYear;
    static JButton btnPrev, btnNext;
    static JTable tblCalendar;
    static JComboBox cmbYear;
    static DefaultTableModel mtblCalendar;
    static JScrollPane stblCalendar;
    static int realYear, realMonth, realDay, currentYear, currentMonth;
    private static GridBagLayout gridBag = new GridBagLayout();
    private GridBagConstraints constraints = new GridBagConstraints();

    public CalendarPanel(){
        super(gridBag);

        createControls();
        setBorder();
        registerActionListeners();
        addControls();
        setBounds();
        setDate();
        addHeaders();
        setBackground();
        setTableProperties();
        populateTable();
        refreshCalendar(realMonth, realYear);

    }

    private void createControls(){
        lblMonth = new JLabel ("January");
        lblYear = new JLabel ("Change year:");
        cmbYear = new JComboBox();
        btnPrev = new JButton ("<");
        btnNext = new JButton (">");
        mtblCalendar = new DefaultTableModel(){

            private static final long serialVersionUID = 1L;

        public boolean isCellEditable(int rowIndex, int mColIndex){return false;}};
        tblCalendar = new JTable(mtblCalendar);
        stblCalendar = new JScrollPane(tblCalendar);
    }

    private void setBorder(){
        this.setBorder(BorderFactory.createTitledBorder("Calendar"));
    }

    private void registerActionListeners(){
        btnPrev.addActionListener(new btnPrev_Action());
        btnNext.addActionListener(new btnNext_Action());
        cmbYear.addActionListener(new cmbYear_Action());
    }

    private void addControls(){
        GridBagLayout topGridBag = new GridBagLayout();

        JPanel topPanel = new JPanel(topGridBag);

        topGridBag.setConstraints(btnPrev, constraints);

        topGridBag.setConstraints(lblMonth, constraints);

        topGridBag.setConstraints(lblMonth, constraints);

    /*this.add(lblMonth);
    this.add(lblYear);
    this.add(cmbYear);
    this.add(btnPrev);
    this.add(btnNext);
    this.add(stblCalendar);
        */

    }

    private void setBounds(){
        /*this.setBounds(0, 0, 320, 335);
        lblMonth.setBounds(160-lblMonth.getPreferredSize().width/2, 25, 100, 25);
        lblYear.setBounds(10, 305, 80, 20);
        cmbYear.setBounds(230, 305, 80, 20);
        btnPrev.setBounds(10, 25, 50, 25);
        btnNext.setBounds(260, 25, 50, 25);
        stblCalendar.setBounds(10, 50, 300, 250);*/
    }

    private void setDate(){
        GregorianCalendar cal = new GregorianCalendar(); //Create calendar
        realDay = cal.get(GregorianCalendar.DAY_OF_MONTH); //Get day
        realMonth = cal.get(GregorianCalendar.MONTH); //Get month
        realYear = cal.get(GregorianCalendar.YEAR); //Get year
        currentMonth = realMonth; //Match month and year
        currentYear = realYear;
    }

    private void addHeaders(){
        String[] headers = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};
        for (int i=0; i<7; i++){
            mtblCalendar.addColumn(headers[i]);
        }
    }

    private void setBackground(){
        tblCalendar.getParent().setBackground(tblCalendar.getBackground()); 
    }

    private void setTableProperties(){
        //No resize/reorder
        tblCalendar.getTableHeader().setResizingAllowed(false);
        tblCalendar.getTableHeader().setReorderingAllowed(false);

        //Single cell selection
        tblCalendar.setColumnSelectionAllowed(true);
        tblCalendar.setRowSelectionAllowed(true);
        tblCalendar.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

        //Set row/column count
        tblCalendar.setRowHeight(38);
        mtblCalendar.setColumnCount(7);
        mtblCalendar.setRowCount(6);
    }

    private void populateTable(){
        for (int i=realYear-100; i<=realYear+100; i++){
            cmbYear.addItem(String.valueOf(i));
        }
    }

    private static void refreshCalendar(int month, int year){
        //Variables
        String[] months =  {"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"};
        int nod, som; //Number Of Days, Start Of Month

        //Allow/disallow buttons
        btnPrev.setEnabled(true);
        btnNext.setEnabled(true);
        if (month == 0 && year <= realYear-10){btnPrev.setEnabled(false);} //Too early
        if (month == 11 && year >= realYear+100){btnNext.setEnabled(false);} //Too late
        lblMonth.setText(months[month]); //Refresh the month label (at the top)
        lblMonth.setBounds(160-lblMonth.getPreferredSize().width/2, 25, 180, 25); //Re-align label with calendar
        cmbYear.setSelectedItem(String.valueOf(year)); //Select the correct year in the combo box

        //Clear table
        for (int i=0; i<6; i++){
            for (int j=0; j<7; j++){
                mtblCalendar.setValueAt(null, i, j);
            }
        }

        //Get first day of month and number of days
        GregorianCalendar cal = new GregorianCalendar(year, month, 1);
        nod = cal.getActualMaximum(GregorianCalendar.DAY_OF_MONTH);
        som = cal.get(GregorianCalendar.DAY_OF_WEEK);

        //Draw calendar
        for (int i=1; i<=nod; i++){
            int row = new Integer((i+som-2)/7);
            int column  =  (i+som-2)%7;
            mtblCalendar.setValueAt(i, row, column);
        }

        //Apply renderers
        tblCalendar.setDefaultRenderer(tblCalendar.getColumnClass(0), new tblCalendarRenderer());
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        JPanel calendarPanel = new CalendarPanel();
        frame.add(calendarPanel);

        frame.setSize(400, 400);
        frame.setLocationRelativeTo(null);
        //frame.setResizable(false);
        frame.setVisible(true);
    }

    static class tblCalendarRenderer extends DefaultTableCellRenderer{
        private static final long serialVersionUID = 1L;

        public Component getTableCellRendererComponent (JTable table, Object value, boolean selected, boolean focused, int row, int column){
            super.getTableCellRendererComponent(table, value, selected, focused, row, column);
            if (column == 0 || column == 6){ //Week-end
                setBackground(new Color(255, 220, 220));
            }
            else{ //Week
                setBackground(new Color(255, 255, 255));
            }
            if (value != null){
                if (Integer.parseInt(value.toString()) == realDay && currentMonth == realMonth && currentYear == realYear){ //Today
                    setBackground(new Color(220, 220, 255));
                }
            }
            setBorder(null);
            setForeground(Color.black);
            return this;  
        }
    }

    static class btnPrev_Action implements ActionListener{
        public void actionPerformed (ActionEvent e){
            if (currentMonth == 0){ //Back one year
                currentMonth = 11;
                currentYear -= 1;
            }
            else{ //Back one month
                currentMonth -= 1;
            }
            refreshCalendar(currentMonth, currentYear);
        }
    }
    static class btnNext_Action implements ActionListener{
        public void actionPerformed (ActionEvent e){
            if (currentMonth == 11){ //Forward one year
                currentMonth = 0;
                currentYear += 1;
            }
            else{ //Forward one month
                currentMonth += 1;
            }
            refreshCalendar(currentMonth, currentYear);
        }
    }
    static class cmbYear_Action implements ActionListener{
        public void actionPerformed (ActionEvent e){
            if (cmbYear.getSelectedItem() != null){
                String b = cmbYear.getSelectedItem().toString();
                currentYear = Integer.parseInt(b);
                refreshCalendar(currentMonth, currentYear);
            }
        }
    }
}

Answer 1

The short answer is: You forgot to add your components to the container. I'm assuming you thought that the setConstraint() methods both added the component with the specified constraints. Actually, you must first add the component and then set their constraints.