Calendar Implementation :Swing GUI Based java program

A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time , typically days ,weeks,months and years . A date is the designation of a single, specific day within such a system. Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycle of thee sun or the moon. Many civilizations and societies have devised a calendar, usually derived from other calendars on which they model their systems, suited to their particular needs.

A calendar is also a physical device (often paper). This is the most common usage of the word. Other similar types of calendars can include computerized systems, which can be set to remind the user of upcoming events and appointments.

A calendar can also mean a list of planned events, such as a court calendar.

The English word calendar is derived from the Latin word kalendae, which was the Latin name of the first day of every month.

There is a built in calendar class in java found in the java.util package , under the name calendar.A Calendar object can produce all the calendar field values needed to implement the date-time formatting for a particular language and calendar style (for example, Japanese-Gregorian, Japanese-Traditional). Calendar defines the range of values returned by certain calendar fields, as well as their meaning. For example, the first month of the calendar system has value MONTH == JANUARY for all calendars. 

One thing to notice in the calendar class is the difference between roll() and add() . 

• Calendar.roll()
  Changes a specific unit and leaves 'larger' (in terms of time-month is 'larger' than day) units unchanged. The API example is that given a date of August 31, 1999, rolling by (Calendar.MONTH, 8) yields April 30, 1999. That is, the DAY was changed to meet April's maximum, but the 'larger' unit, YEAR, was unchanged.
• Calendar.add()
  Will cause the next 'larger' unit to change, if necessary. That is, given a date of August 31, 1999, add(Calendar.MONTH, 8) yields April 30, 2000. add() also forces a recalculation of milliseconds and all fields.

Calendar provides internationalization support.

Calendar is a abstract class, and you cannot use the constructor to create an instance. Instead, you use the static method Calendar.getInstance() to instantiate an implementation sub-class.

* Calendar.getInstance(): return a Calendar instance based on the current time in the default time zone with 

   the default locale.

* Calendar.getInstance(Timezone zone)

* Calendar.getInstance(Locale aLocale)

* Calendar.getInstance(TimeZone zone, Locale aLocale)

Looking into the source code reveals that: getInstance() returns a GregorianCalendar instance for all locales, (except BuddhistCalendar for Thai ("th_TH") and JapaneseImperialCalendar for Japanese ("ja_JP")).

The most important method in Calendar  is get ( int calendarField ), which produces an int . The calendarField are defined as static constant .

Demo here :

Code :

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

public class CalendarProgram{
    static JLabel lblMonth, lblYear;
    static JButton btnPrev, btnNext;
    static JTable tblCalendar;
    static JComboBox cmbYear;
    static JFrame frmMain;
    static Container pane;
    static DefaultTableModel mtblCalendar; //Table model
    static JScrollPane stblCalendar; //The scrollpane
    static JPanel pnlCalendar;
    static int realYear, realMonth, realDay, currentYear, currentMonth;
    
    public static void main (String args[]){
        //Look and feel
        try {UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());}
        catch (ClassNotFoundException e) {}
        catch (InstantiationException e) {}
        catch (IllegalAccessException e) {}
        catch (UnsupportedLookAndFeelException e) {}
        
        //Prepare frame
        frmMain = new JFrame ("Gestionnaire de clients"); //Create frame
        frmMain.setSize(330, 375); //Set size to 400x400 pixels
        pane = frmMain.getContentPane(); //Get content pane
        pane.setLayout(null); //Apply null layout
        frmMain.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //Close when X is clicked
        
        //Create controls
        lblMonth = new JLabel ("January");
        lblYear = new JLabel ("Change year:");
        cmbYear = new JComboBox();
        btnPrev = new JButton ("<<");
        btnNext = new JButton (">>");
        mtblCalendar = new DefaultTableModel(){public boolean isCellEditable(int rowIndex, int mColIndex){return false;}};
        tblCalendar = new JTable(mtblCalendar);
        stblCalendar = new JScrollPane(tblCalendar);
        pnlCalendar = new JPanel(null);
        
        //Set border
        pnlCalendar.setBorder(BorderFactory.createTitledBorder("Calendar"));
        
        //Register action listeners
        btnPrev.addActionListener(new btnPrev_Action());
        btnNext.addActionListener(new btnNext_Action());
        cmbYear.addActionListener(new cmbYear_Action());
        
        //Add controls to pane
        pane.add(pnlCalendar);
        pnlCalendar.add(lblMonth);
        pnlCalendar.add(lblYear);
        pnlCalendar.add(cmbYear);
        pnlCalendar.add(btnPrev);
        pnlCalendar.add(btnNext);
        pnlCalendar.add(stblCalendar);
        
        //Set bounds
        pnlCalendar.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);
        
        //Make frame visible
        frmMain.setResizable(false);
        frmMain.setVisible(true);
        
        //Get real month/year
        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;
        
        //Add headers
        String[] headers = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"}; //All headers
        for (int i=0; i<7; i++){
            mtblCalendar.addColumn(headers[i]);
        }
        
        tblCalendar.getParent().setBackground(tblCalendar.getBackground()); //Set background
        
        //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);
        
        //Populate table
        for (int i=realYear-100; i<=realYear+100; i++){
            cmbYear.addItem(String.valueOf(i));
        }
        
        //Refresh calendar
        refreshCalendar (realMonth, realYear); //Refresh calendar
    }
    
    public 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());
    }
    
    static class tblCalendarRenderer extends DefaultTableCellRenderer{
        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){ //Foward one year
                currentMonth = 0;
                currentYear += 1;
            }
            else{ //Foward 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);
            }
        }
    }
}