Having a problem saving the graphics on a canvas.

Question

I'm having a problem saving graphics on a canvas. It draws on the canvas but when another window comes up the graphics disappear. I put in a method to take the graphics on the canvas and repaint it with the graphics. But this comes up blank. So I don't know why this is happening. It is probably the paint method but I don't know why. If anyone has any ideas could you please respond because I have had this problem for ages now and it's driving me mad and I have to get this finished or I'm going to be a lot of trouble. I'll show you the code for the class I am concerned with. It is long but most of it can be disregarded. The only parts which are relevent are the paint method and where the drawline e.t.c are called which is in the mouse release
package com.project.CSSE4;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.ImageFilter;
import java.awt.image.CropImageFilter;
import java.awt.image.FilteredImageSource;
import java.io.*;
import java.util.*;
import java.net.*;
import javax.swing.*;
import java.sql.*;
public class CanvasOnly extends JPanel
       implements MouseListener, MouseMotionListener
   public static final int line = 1;
   public static final int freehand = 2;
   public static final int text = 3;
   public static final int mode_paint = 0;
   public static final int mode_xor = 1;
   public Color drawColor;
   public int drawThickness = 1;
   public int drawType = 0;
   public boolean fill = false;
   private boolean dragging = false;
   public int oldx = 0;
   public int oldy = 0;
   private int rectangleWidth;
   private int rectangleHeight;
   private tempLine draftLine;
   private tempText draftText;
   public Canvas pad;
   public static final Font defaultFont =
     new Font("Helvetica", Font.BOLD, 14);
   protected boolean floatingText = false;
   boolean showingPicture;
   protected Image offScreen;
   public JTextArea coor;
   public JButton writeIn;
   Connection connection;
   String codeLine;
   int x = 0;
   int y = 0;
   public CanvasOnly()
       try
            Class.forName("com.mysql.jdbc.Driver").newInstance();
       }catch( Exception e)
             System.err.println("Unable to find and load driver");
             System.exit(1);
        pad = new Canvas();
        pad.setBackground(Color.white);
        pad.setVisible(true);
        pad.setSize(400, 400);
       coor = new JTextArea(15, 15);
       writeIn = new JButton("load TExt");
       writeIn.addActionListener(new ActionListener()
               public void actionPerformed(ActionEvent event1)
                 try
               Statement statement = connection.createStatement();
               ResultSet rs = statement.executeQuery("SELECT * FROM submit WHERE file_name = 'cmd.java'");
               rs.next();
               String one1 = rs.getString("student_id");
               //System.out.println("one1 :" + one1);
               String two1 = rs.getString("file_name");
               //System.out.println("two1 : " + two1);
                InputStream textStream = rs.getAsciiStream("file");
                BufferedReader textReader = new BufferedReader(
                                 new InputStreamReader(textStream));
                codeLine = textReader.readLine();
                x = 0;
                y = -12;
                while(codeLine != null)
                    y = y + 12;
                    //fileText.append( line + "\n");
                    //canvasPad.drawTheString(line, x, y);
                    drawText(Color.black, x, y, codeLine, mode_paint);
                    codeLine = textReader.readLine();
                 textReader.close();
                pad.setSize(400, y);
                Timestamp three1 = rs.getTimestamp("ts");
                //System.out.println(three1);
                textReader.close();
                rs.close();
          }catch (SQLException e)
            System.err.println(e);
          catch(IOException ioX)
            System.err.println(ioX);
        //setSize(300,300);
        drawColor = Color.black;
        pad.addMouseListener(this);
     pad.addMouseMotionListener(this);
     offScreen = null;
   public Image getContents()
     // Returns the contents of the canvas as an Image.  Only returns
     // the portion which is actually showing on the screen
     // If the thing showing on the canvas is a picture, just send back
     // the picture
     if (showingPicture)
         return (offScreen);
     ImageFilter filter =
         new CropImageFilter(0, 0, getWidth(), getHeight());
     Image newImage =
         createImage(new FilteredImageSource(offScreen.getSource(),
                              filter));
     return(newImage);
    public void setImage(Image theImage)
     // Fit it to the canvas
     offScreen = theImage;
     repaint();
     showingPicture = true;
    synchronized public void paint(Graphics g)
     int width = 0;
     int height = 0;
     //The images are stored on an off screen buffer.
        //offScreen is the image declared at top
     if (offScreen != null)
              //intislise the widt and heigth depending on if showingpicture is true
          if (!showingPicture)
                   width = offScreen.getWidth(this);
                   height = offScreen.getHeight(this);
               //width = getWidth(this);
               //height = getHeight(this);  //offScreen
          else
               //width = pad.getSize().width;
               //height = getSize().height;
               width = pad.getWidth();
               //width = getSize().width;
               height = pad.getHeight();
               //height = getSize().height;
                //Draws as much of the specified image as has already
                //been scaled to fit inside the specified rectangle
                //The "this" is An asynchronous update interface for receiving
                //notifications about Image information as the Image is constructed
//This is causing problems
          g.drawImage(offScreen, 0, 0, width, height, pad);
          g.dispose();
     //clear the canvas with this method
    synchronized public void clear()
     // The maximum size of the usable drawing canvas, for now, will be
     // 1024x768
     offScreen = createImage(1024, 768);
     //Creates an off-screen drawable image to be used for double buffering
     pad.setBackground(Color.white);
     //Set the showingPicture to false for paint method
     showingPicture = false;
     repaint();
     synchronized public void drawLine(Color color, int startx, int starty,
                          int endx, int endy, int thickness,
                          int mode)
     int dx, dy;
     Graphics g1 = pad.getGraphics();
     Graphics g2;
     //if image is not intialised to null
     //the getGraphics is used for freehand drawing
     //Image.getGraphics() is often used for double buffering by rendering
        //into an offscreen buffer.
     if (offScreen != null)
         g2 = offScreen.getGraphics();
     else
         g2 = g1;
        //mode is put into the method and XOR is final and equal to 1
     if (mode == this.mode_xor)
              //calls the setXOR mode for g1 and g2
              //Sets the paint mode of this graphics context to alternate
                //between this graphics context's current color and the
                //new specified color.
          g1.setXORMode(Color.white);//This will
          g2.setXORMode(Color.white);
     else
              //Sets this graphics context's current color to the
                //specified color
          g1.setColor(color);
          g2.setColor(color);
     if (endx > startx)
         dx = (endx - startx);
     else
         dx = (startx - endx);
     if (endy > starty)
         dy = (endy - starty);
     else
         dy = (starty - endy);
     if (dx >= dy)
          starty -= (thickness / 2);
          endy -= (thickness / 2);
     else
          startx -= (thickness / 2);
          endx -= (thickness / 2);
     for (int count = 0; count < thickness; count ++)
          g1.drawLine(startx, starty, endx, endy);
          g2.drawLine(startx, starty, endx, endy);
          if (dx >= dy)
              { starty++; endy++; }
          else
              { startx++; endx++; }
        //Disposes of this graphics context and releases any system
        //resources that it is using.
     g1.dispose();
     g2.dispose();
     //This method is not causing trouble
     synchronized public void drawText(Color color, int x, int y,
            String text, int mode)
     Graphics g1 = pad.getGraphics();
     Graphics g2;
     if (offScreen != null)
         g2 = offScreen.getGraphics();
     else
         g2 = g1;
     if (mode == this.mode_xor)
          g1.setXORMode(Color.white);
          g2.setXORMode(Color.white);
     else
          g1.setColor(color);
          g2.setColor(color);
     g1.setFont(new Font("Times Roman", Font.PLAIN, 10));
     g2.setFont(new Font("Times Roman", Font.PLAIN, 10));
     g1.drawString(text, x, y);
     g2.drawString(text, x, y);
     g1.dispose();
     g2.dispose();
      //connect to database
  public void connectToDB()
    try
       connection = DriverManager.getConnection(
       "jdbc:mysql://localhost/submissions?user=root&password=football");
                 //may have to load in a username and password
                                                 //code "?user=spider&password=spider"
    }catch(SQLException connectException)
       System.out.println("Unable to connect to db");
       System.exit(1);
  //use this method to instatiate connectToDB method
  public void init()
       connectToDB();
    protected void floatText(String text)
     draftText = new tempText(this.drawColor,
                        this.oldx,
                        this.oldy,
                                    text);
     this.floatingText = true;
    //nothing happens when the mouse is clicked
    public void mouseClicked(MouseEvent e)
    //When the mouse cursor enters the canvas make it the two
    //straigth lines type
    public void mouseEntered(MouseEvent e)
     pad.setCursor(new Cursor(Cursor.CROSSHAIR_CURSOR));
    //When mouse exits canvas set to default type
    public void mouseExited(MouseEvent E)
     pad.setCursor(new Cursor(Cursor.DEFAULT_CURSOR));
    //used for creating the shapes and positioning thetext
    public void mousePressed(MouseEvent e)
         // Save the coordinates of the mouse being pressed
     oldx = e.getX();
     oldy = e.getY();
     // If we are doing lines, rectangles, or ovals, we will show
     // draft lines to suggest the final shape of the object
     //Draw type is a publc int which can be changed
     if (drawType == this.line)
        draftLine = new tempLine(drawColor, oldx, oldy, oldx,
                             oldy, drawThickness);
        // Set the draw mode to XOR and draw it.
        drawLine(draftLine.color, draftLine.startx,
        draftLine.starty, draftLine.endx,
        draftLine.endy, drawThickness, this.mode_xor);
    //mouse listener for when the mouse button is released
    public void mouseReleased(MouseEvent e)
         if (drawType == this.line)
          // Erase the draft line
          drawLine(draftLine.color, draftLine.startx, draftLine.starty,
                draftLine.endx, draftLine.endy, drawThickness,
                this.mode_xor);
          // Add the real line to the canvas
          //When the imput changes to "mode_paint" it is drawen
          //on the canvas
          drawLine(drawColor, oldx, oldy, e.getX(), e.getY(),
                drawThickness, this.mode_paint);
          dragging = false;
     else if (drawType == this.text)
          if (floatingText)
               // The user wants to place the text (s)he created.
               // Erase the old draft text
               drawText(drawColor, draftText.x, draftText.y,
                                        draftText.text, this.mode_xor);
                   String str = Integer.toString(e.getX());
                   String str1  = Integer.toString(e.getY());
                   coor.append(str + " " + str1 + "\n");
               // Set the new coordinates
               draftText.x = e.getX();
               draftText.y = e.getY();
               // Draw the permanent text
               drawText(drawColor, draftText.x, draftText.y,
                     draftText.text, this.mode_paint);
               floatingText = false;
     public void mouseDragged(MouseEvent e)
        if (drawType == this.freehand)
          drawLine(drawColor, oldx, oldy, e.getX(), e.getY(),
                drawThickness, this.mode_paint);
          oldx = e.getX();
          oldy = e.getY();
     else
         dragging = true;
     if (drawType == this.line)
        // Erase the old draft line
        drawLine(draftLine.color, draftLine.startx, draftLine.starty,
          draftLine.endx, draftLine.endy, drawThickness,
          this.mode_xor);
        // Draw the new draft line
        draftLine.endx = e.getX();
        draftLine.endy = e.getY();
        drawLine(draftLine.color, draftLine.startx, draftLine.starty,
          draftLine.endx, draftLine.endy, drawThickness,
          this.mode_xor);
     public void mouseMoved(MouseEvent e)
         if (floatingText)
          // When the user has entered some text to place on the
          // canvas, it remains sticky with the cursor until another
          // click is entered to place it.
          // Erase the old draft text
          drawText(drawColor, draftText.x, draftText.y,
                            draftText.text, this.mode_xor);
          // Set the new coordinates
          draftText.x = e.getX();
          draftText.y = e.getY();
          // Draw the new floating text
          drawText(drawColor, draftText.x, draftText.y,
                    draftText.text, this.mode_xor);
     //declare  a class for the line shown before the is as wanted
    class tempLine
     public Color color;
     public int startx;
     public int starty;
     public int endx;
     public int endy;
     public int thickness;
     public tempLine(Color mycolor, int mystartx, int mystarty,
                  int myendx, int myendy, int mythickness)
         color = mycolor;
         startx = mystartx;
         starty = mystarty;
         endx = myendx;
         endy = myendy;
         thickness = mythickness;
    class tempText
     public Color color;
     public int x;
     public int y;
     public String text;
     public tempText(Color mycolor, int myx, int myy, String mytext)
         color = mycolor;
         x = myx;
         y = myy;
         text = mytext;
}

Answer 1

http://www.java2s.com/ExampleCode/2D-Graphics/DemonstratingUseoftheImageIOLibrary.htm