Why do I need to catch this exception?

Question

Hello all
This is a question about exception handling. I have to build a diary application that lets you save reminders on particular dates using xml. Just to make it a little tougher, I was not allowed to use the Calendar class. This is the code I wrote:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;
import java.text.*; 
import javax.xml.parsers.*;
import org.xml.sax.*;
import java.io.*;     
import org.w3c.dom.*;
import javax.xml.transform.*;
import javax.xml.transform.dom.*;
import javax.xml.transform.stream.*;                                                                                                                                                                                                                         
import static java.lang.Math.*;
public class CalendarAssignment extends JFrame implements ActionListener
     int MonthLength [] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
     public JButton [] days = new JButton[43];
     private JLabel lbl, reminderlbl;
     private JPanel top, grid, remindercenter, reminderbottom;
     private JFrame reminderframe;
     private JTextField year1, reminderinput, dayno;
     private JComboBox months;
     private Container container;
     private JButton fetch, save, cancel;
     private Document doc;
     private File file;
     private Node node;
     private String year, month, day;
          public static void main( String[] args ) {
               CalendarAssignment c = new CalendarAssignment( );
               c.setSize( 400, 300 );
               c.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
               c.show( );
          public CalendarAssignment( ) {
               int CurrentYear, CurrentMonth;
               String YearString, MonthString, Now;
               Date today;
               container = getContentPane( );
               container.setLayout( new BorderLayout( ) );
               SimpleDateFormat DateFormatter;
               DateFormatter = new SimpleDateFormat( "MM.yyyy" );
               today = new Date( );
               Now = DateFormatter.format( today );
               MonthString = Now.substring( 0,2 );
               YearString = Now.substring( 3,7 );
               CurrentMonth = Integer.valueOf( MonthString );
               CurrentYear = Integer.valueOf( YearString );
               top = new JPanel( );
                    String [] items = { "January", "February", "March", "April", "May", "June",
                                  "July", "August", "September", "October", "November",
                                  "December" };
                    months = new JComboBox( items );
                    months.setEditable( false );
                    months.setSelectedIndex( CurrentMonth - 1 );          
                    year1 = new JTextField( );
                    year1.setText( YearString );
                    year1.setEditable( true );
                    year1.setHorizontalAlignment( year1.CENTER );
                    fetch = new JButton( "Fetch Month" );
                    fetch.addActionListener( this );
                    top.setLayout( new GridLayout( 1, 3, 20, 0 ) );
                    top.add( months );
                    top.add( year1 );
                    top.add( fetch );
               grid = new JPanel( );
                    grid.setLayout( new GridLayout( 7, 7, 0, 0 ) );
                    String [] week = { "Mon", "Tue", "Wed", "Thur", "Fri", "Sat", "Sun" };
                    for ( int a = 0; a < 7; a ++ ) {
                              JLabel lbl = new JLabel( week[a], JLabel.CENTER );
                              grid.add( lbl );
                    for ( int i = 0; i < 42; i ++ ) {
                              days[i] = new JButton( );
                              grid.add( days[i] );
                              days.addActionListener( this );
                    DrawCalendar( CurrentMonth, CurrentYear );
                    container.add( top, BorderLayout.NORTH );
                    container.add( grid, BorderLayout.CENTER );
          private void DrawCalendar( int SelectMonth, int SelectYear ) {
                    int DisplayMonthLength, Buttons;
                    String ButtonID = "";
                    int OffSet = MonthStart( SelectMonth, SelectYear );
                    DisplayMonthLength = MonthLength [SelectMonth - 1];
                    if ( SelectMonth == 2 )
                         DisplayMonthLength += LeapYear( SelectYear );
                    for ( Buttons = 1; Buttons < 43; Buttons ++ ) {
                         if ( ( Buttons <= OffSet ) || ( Buttons > ( DisplayMonthLength + OffSet ) ) ) {
                              ButtonID = "";
                              days[Buttons-1].setEnabled( false );
                         else {
                              ButtonID = Integer.toString( Buttons - OffSet );
                              days[Buttons-1].setEnabled( true );
                    days[Buttons-1].setLabel( ButtonID );
                    grid.add( days[Buttons-1] );
          private int LeapYear( int year ) {
                    int FourHundred, OneHundred, Fourth;
                    FourHundred = year % 400;
                    OneHundred = year % 100;
                    Fourth = year % 4;
                    if( ( ( FourHundred == 0 ) ) || ( ( OneHundred != 0 ) && ( Fourth == 0 ) ) )
                         return ( 1 );
                    else
                         return ( 0 );
          private int MonthStart( int Month, int Year ) {
                    int OffSet, LastMonths, BeforeOrAfter, Years;
                    int AllDays = 0;
                    int YearDays = 365;
                    int YearMonths = 12;
                    BeforeOrAfter = Year - 2006;
                    Years = abs( BeforeOrAfter );
                    if( BeforeOrAfter != 0 )
                         BeforeOrAfter = BeforeOrAfter / Years;
                    switch( BeforeOrAfter ) {
                         case 1:
                              for( int a = 2006; a < Year; a ++ ) {
                                   AllDays += YearDays + LeapYear( a );
                              AllDays += LastMonthsCalc( Month, Year );
                         break;
                         case -1:
                              for( int a = 2005; a > Year; a -- ) {
                                   AllDays += YearDays + LeapYear( a );
                              for( LastMonths = YearMonths; LastMonths >= Month; LastMonths -- ) {
                                   AllDays += MonthLength[LastMonths - 1];
                                   if( LastMonths == 2 )
                                        AllDays += LeapYear( Year );
                         break;
                         default:
                              if( Month > 1 )
                                   AllDays += ( LastMonthsCalc( Month, Year ) );
                    OffSet = AllDays % 7;
                    if( BeforeOrAfter ==( -1 ) )
                         return( 6 - OffSet );
                    else if( OffSet > 0 )
                         return( OffSet - 1 );
                    else
                         return( 6 );
          private int LastMonthsCalc( int Month, int Year ) {
                    int Counter;
                    int days = 0;
                    for( Counter = 1; Counter < Month; Counter ++ ) {
                         days += MonthLength[Counter - 1];
                         if( Counter == 2 )
                              days += LeapYear( Year );
                    return( days );
          public void CreateReminder( String buttonID, String yearID, String monthID ) {
                    reminderframe = new JFrame( );
                    reminderlbl = new JLabel( );
                    reminderframe.setLayout( new GridLayout( 2, 1, 0, 0 ) );
                    remindercenter = new JPanel( );
                         reminderlbl = new JLabel( "Please type in reminder to be saved for " + buttonID + " " + monthID + " " + yearID + ":", JLabel.CENTER );
                         reminderinput = new JTextField( 30 );
                         reminderinput.setHorizontalAlignment( reminderinput.CENTER );
                         reminderinput.setEditable( true );
                         remindercenter.setLayout( new GridLayout( 2, 1, 0, 0 ) );
                         remindercenter.add( reminderlbl );
                         remindercenter.add( reminderinput );
                    reminderbottom = new JPanel( );
                         save = new JButton( "Save" );
                         save.addActionListener( this );
                         cancel = new JButton( "Cancel" );
                         cancel.addActionListener( this );
                         dayno = new JTextField( buttonID );
                         dayno.setEditable( false );
                         dayno.setEnabled( false );
                         dayno.show( false );
                         reminderbottom.setLayout( new FlowLayout( ) );
                         reminderbottom.add( save );
                         reminderbottom.add( cancel );
                         reminderbottom.add( dayno );
                    reminderframe.setSize( 500, 75 );
                    reminderframe.add( remindercenter );
                    reminderframe.add( reminderbottom );
                    reminderframe.pack( );
                    reminderframe.show( );
          public void SaveReminder( String dayID, String yearID, String monthID ) throws Exception {
                    file = new File( "Diary.xml" );
                    doc = DocumentBuilderFactory.newInstance( ).newDocumentBuilder( ).parse( file.toURL( ).toString( ) );
                    String year = yearID;
                    String month = monthID;
                    String day = dayID;
                    //System.out.println( year );
                    //System.out.println( month );
                    //System.out.println( day );
                    CreateEntry( doc.getDocumentElement( ) );
                    writeXmlFile( );
          public boolean CreateEntry( Node node ) {
                    Node searchNode;
                    searchNode = getYear( node );
                    if( searchNode == null ) {
                         Element newNode = doc.createElement( "Year" );
                         searchNode = node.appendChild( newNode );
                         newNode.setAttribute( "Id", year );
                    node = searchNode;
                    searchNode = getMonth( node );
                    if( searchNode == null ) {
                         Element newNode = doc.createElement( "Month" );
                         searchNode = node.appendChild( newNode );
                         newNode.setAttribute( "Id", month );
                    node = searchNode;
                    searchNode = getDay( node );
                    if( searchNode == null ) {
                         Element newNode = doc.createElement( "Day" );
                         searchNode = node.appendChild( newNode );
                         newNode.setAttribute( "Id", day );
                    node = searchNode;
                    String entry = reminderinput.getText( );
                    Node textNode = doc.createTextNode( entry );
                    node.appendChild( textNode );
                    return true;
          private Node getYear( Node node ) {
                    node = node.getFirstChild( );
                    while( node != null ) {
                         if(node.getNodeName( ).equals( "Year" ) && String.valueOf( node.getAttributes( ).item( 0 ).getNodeValue( ) ) == year )
                              return node;
                              node = node.getNextSibling( );                    
                    return null;
          private Node getMonth( Node node ) {
                    node = node.getFirstChild( );
                    while( node != null ) {
                         if( node.getNodeName( ).equals( "Month" ) && String.valueOf( node.getAttributes( ).item( 0 ).getNodeValue( ) ) == month )
                              return node;
                              node = node.getNextSibling( );                    
                    return null;
          private Node getDay( Node node ) {
                    node = node.getFirstChild( );
                    while( node != null ) {
                         if( node.getNodeName( ).equals( "Day" ) && String.valueOf( node.getAttributes( ).item( 0 ).getNodeValue( ) ) == day )
                              return node;
                              node = node.getNextSibling( );                    
                    return null;
          private void writeXmlFile( ) throws Exception {
               Source source = new DOMSource( doc );
                    Result result = new StreamResult( file );
               Transformer xformer = TransformerFactory.newInstance( ).newTransformer( );
               xformer.setOutputProperty( OutputKeys.INDENT,"yes" );
               xformer.setOutputProperty( OutputKeys.DOCTYPE_SYSTEM,"Diary.dtd" );
               xformer.transform( source, result );
          public void actionPerformed( ActionEvent e ) {
                    String IDButton = e.getActionCommand( );
                    String IDYear = year1.getText( );
                    Object IDMonthObj = months.getSelectedItem( );
                    if( e.getSource( ) == fetch ) {
                         String YearText = year1.getText( );
                         int YearNumber = Integer.valueOf( YearText );
                         int MonthsIndex = months.getSelectedIndex( ) + 1;
                         DrawCalendar( MonthsIndex, YearNumber );
                    else if( e.getSource( ) == cancel ) {
                         reminderframe.hide( );
                    else if( e.getSource( ) == save ) {
                         String IDDay = dayno.getText( );
                         String IDMonth = String.valueOf( IDMonthObj );
                         SaveReminder( IDDay, IDYear, IDMonth );
                    else {
                         String IDMonth = String.valueOf( IDMonthObj );
                         Toolkit.getDefaultToolkit( ).beep( );
                         int n = JOptionPane.showConfirmDialog( null, "Set reminder on this date?", "Question", JOptionPane.YES_NO_OPTION );
                         if( n == JOptionPane.YES_OPTION ) {
                              CreateReminder( IDButton, IDYear, IDMonth );
If you compile it, you will realise that I get an error about exception handling. My lecturer gave me an example code of how to do the same thing without using a GUI:
import javax.xml.parsers.*;
import org.xml.sax.*;
import java.io.*;
import java.util.*;
import org.w3c.dom.*;
import javax.xml.transform.*;
import javax.xml.transform.dom.*;
import javax.xml.transform.stream.*;
public class CallDOM
     Document doc;
     File file;
     Scanner input;
     int year, month, day;
     String currentYear, currentMonth;
     public static void main(String args[]) throws Exception
          CallDOM cd=new CallDOM();          
     CallDOM() throws Exception
          file=new File("Diary.xml");
          //create DOM from file
          doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(file.toURL().toString());
          input=new Scanner(System.in);
          System.out.println("1) Create entry");
          System.out.println("2) Read entry");
          System.out.println("3) Show Diary");
          System.out.println("4) Quit");
          int choice=input.nextInt();
          while(choice!=4)
               switch(choice)
                    case 1: GetDate();CreateEntry(doc.getDocumentElement());break;
                    case 2: GetDate();ReadEntry(doc.getDocumentElement());break;
                    case 3: ShowDiary(doc.getDocumentElement());
               System.out.println("1) Create entry");
               System.out.println("2) Read entry");
               System.out.println("3) Show Diary");
               System.out.println("4) Quit");
               choice=input.nextInt();
          writeXmlFile();
     public void GetDate()
          System.out.println("Enter date (dd mm yyyy)");
          day=input.nextInt();
          month=input.nextInt();
          year=input.nextInt();
     public boolean ReadEntry(Node node)
          node = getYear(node);
          if(node==null) return false;
          node = getMonth(node);
          if(node==null) return false;
          node = getDay(node);
          if(node==null) return false;
          node = node.getFirstChild();
          while(node!=null)
               System.out.println(node.getNodeValue().trim());     
               node=node.getNextSibling();
          return true;               
     public boolean CreateEntry(Node node)
          Node searchNode;
          searchNode = getYear(node);
          if(searchNode==null)
               Element newNode = doc.createElement("Year");
               searchNode = node.appendChild(newNode);
               newNode.setAttribute("Id",Integer.toString(year));
          node = searchNode;
          searchNode = getMonth(node);
          if(searchNode==null)
               Element newNode = doc.createElement("Month");
               searchNode = node.appendChild(newNode);
               newNode.setAttribute("Id",Integer.toString(month));
          node = searchNode;
          searchNode = getDay(node);
          if(searchNode==null)
               Element newNode = doc.createElement("Day");
               searchNode = node.appendChild(newNode);
               newNode.setAttribute("Id",Integer.toString(day));
          node = searchNode;
          System.out.println("Enter Text");
          String entry=input.next();
          entry+=input.nextLine();
          Node textNode = doc.createTextNode(entry);
          node.appendChild(textNode);
          return true;               
     public void ShowDiary(Node node)
          Stack<Node> stack=new Stack<Node>();
          Node child;
          stack.push(node);
          while(!stack.empty())
               node = stack.pop();
               if(ProcessNode(node))
                    child = node.getLastChild();
                    while(child!=null)
                         stack.push(child);
                         child = child.getPreviousSibling();
     private boolean ProcessNode(Node node)
          if(node.getNodeName().equals("Year"))
               currentYear=node.getAttributes().item(0).getNodeValue();
          if(node.getNodeName().equals("Month"))
               currentMonth=node.getAttributes().item(0).getNodeValue();
          if(node.getNodeName().equals("Day"))
               System.out.print(node.getAttributes().item(0).getNodeValue()+"/"+
                                        currentMonth+"/"+currentYear+": ");
               node=node.getFirstChild();
               while(node!=null)
                    System.out.println(node.getNodeValue().trim());
                    node=node.getNextSibling();
               return false;
          return true;               
     private Node getYear(Node node)
          node=node.getFirstChild();
          while(node!=null)
               if(node.getNodeName().equals("Year")
                    && Integer.valueOf(node.getAttributes().item(0).getNodeValue())==year)
                         return node;
               node = node.getNextSibling();                    
          return null;
     private Node getMonth(Node node)
          node=node.getFirstChild();
          while(node!=null)
               if(node.getNodeName().equals("Month")
                    && Integer.valueOf(node.getAttributes().item(0).getNodeValue())==month)
                    return node;
               node = node.getNextSibling();                    
          return null;
     private Node getDay(Node node)
          node=node.getFirstChild();
          while(node!=null)
               if(node.getNodeName().equals("Day")
                    && Integer.valueOf(node.getAttributes().item(0).getNodeValue())==day)
                    return node;
               node = node.getNextSibling();                    
          return null;
     private void writeXmlFile() throws Exception
        Source source = new DOMSource(doc);
        Result result = new StreamResult(file);
        Transformer xformer = TransformerFactory.newInstance().newTransformer();
        xformer.setOutputProperty(OutputKeys.INDENT,"yes");
        xformer.setOutputProperty(OutputKeys.DOCTYPE_SYSTEM,"Diary.dtd");
        xformer.transform(source, result);
}My question is, why does the exceptions in the example code not need to be caught? And why do I have to catch the exceptions in my code? Several exceptions are thrown in the example code, but there is no catch statement.
Thanks for any advice!!

Answer 1

Since your question didn't come with any relevant details, and since you have huge steaming piles of irrelevant code, all I can give is a general answer to "Why do I need to catch this exception?"
[url http://java.sun.com/docs/books/tutorial/essential/exceptions/index.html]Exception tutorial at http://java.sun.com/docs/books/tutorial/essential/exceptions/index.html
Here's a quick overview of exceptions:
The base class for all exceptions is Throwable. Java provides Exception and Error that extend Throwable. RuntimeException (and many others) extend Exception.
RuntimeException and its descendants, and Error and its descendants, are called unchecked exceptions. Everything else is a checked exception.
If your method, or any method it calls, can throw a checked exception, then your method must either catch that exception, or declare that your method throws that exception. This way, when I call your method, I know at compile time what can possibly go wrong and I can decide whether to handle it or just bubble it up to my caller. Catching a given exception also catches all that exception's descendants. Declaring that you throw a given exception means that you might throw that exception or any of its descendants.
Unchecked exceptions (RuntimeException, Error, and their descendants) are not subject to those restrictions. Any method can throw any unchecked exception at any time without declaring it. This is because unchecked exceptions are either the sign of a coding error (RuntimeException), which is totally preventable and should be fixed rather than handled by the code that encounters it, or a problem in the VM, which in general can not be predicted or handled.