Subclassing Canvas
Hi everyone!
I'm trying to subclass the Canvas class to create a double buffered rectangular area where to draw some image sequence.
This code doesn't work:
public class ImageCanvas extends Canvas {
ImageCanvas () {
this.createBufferStrategy(2);
and I get a "Component must have a valid peer" exception. I tried to add a call to addNotify() before createBufferStrategy(2) but I get a nullPointerException instead.
I'm not a skilled java programmer, so there must be a mistake! Could someone show it to me?
Many many thanks!
Zalexx
I believe that your ImageCanvas must be displayable. See Component.isDisplayable for details.
You may have to supply a Window or a Frame as a parameter to the constructor, or delay setting the buffer strategy until your containment hierarchy is set up.
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[url http://java.sun.com/j2se/1.5.0/docs/api/java/awt/Graphics.html#drawImage(java.awt.Image,%20int,%20int,%20java.awt.image.ImageObserver)]Graphics.drawImage -
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Edited by: Avi4Ora on Mar 31, 2010 3:55 PMHi,
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Overlay Images and Shapes on the same canvas
The example copde below was taken from: http://math.hws.edu/eck/cs124/javanotes3/source/
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The ShapeDraw applet lets the user add small colored shapes to
a drawing area and then drag them around. The shapes are rectangles,
ovals, and roundrects. The user adds a shape to the canvas by
clicking on a button. The shape is added at the upper left corner
of the canvas. The color of the shape is given by the current
setting of a pop-up menu. The user can drag the shapes with the
mouse. Ordinarily, the shapes maintain a given back-to-front order.
However, if the user shift-clicks on a shape, that shape will be
brought to the front.
A menu can be popped up on a shape (by right-clicking or performing
some othe platform-dependent action). This menu allows the user
to change the size and color of a shape. It is also possible to
delete the shape and to bring it to the front.
This file defines the applet class plus several other classes used
by the applet, namely: ShapeCanvas, Shape, RectShape, OvalShape,
and RoundRectShape.
David Eck
July 28, 1998
import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;
import java.util.Vector;
public class ShapeDrawWithMenu extends Applet {
public void init() {
// Set up the applet's GUI. It consists of a canvas, or drawing area,
// plus a row of controls below the canvas. The controls include three
// buttons which are used to add shapes to the canvas and a Choice menu
// that is used to select the color used for a shape when it is created.
// The canvas is set as the "listener" for these controls so that it can
// respond to the user's actions. (The pop-up menu is created by the canvas.)
setBackground(Color.lightGray);
ShapeCanvas canvas = new ShapeCanvas(); // create the canvas
Choice colorChoice = new Choice(); // color choice menu
colorChoice.add("Red");
colorChoice.add("Green");
colorChoice.add("Blue");
colorChoice.add("Cyan");
colorChoice.add("Magenta");
colorChoice.add("Yellow");
colorChoice.add("Black");
colorChoice.add("White");
colorChoice.addItemListener(canvas);
Button rectButton = new Button("Rect"); // buttons for adding shapes
rectButton.addActionListener(canvas);
Button ovalButton = new Button("Oval");
ovalButton.addActionListener(canvas);
Button roundRectButton = new Button("RoundRect");
roundRectButton.addActionListener(canvas);
Panel bottom = new Panel(); // a Panel to hold the control buttons
bottom.setLayout(new GridLayout(1,4,3,3));
bottom.add(rectButton);
bottom.add(ovalButton);
bottom.add(roundRectButton);
bottom.add(colorChoice);
setLayout(new BorderLayout(3,3));
add("Center",canvas); // add canvas and controls to the applet
add("South",bottom);
public Insets getInsets() {
// Says how much space to leave between the edges of the applet and the
// components in the applet.
return new Insets(3,3,3,3);
} // end class ShapeDraw
class ShapeCanvas extends Canvas implements ActionListener, ItemListener,
MouseListener, MouseMotionListener {
// This class represents a canvas that can display colored shapes and
// let the user drag them around. It uses an off-screen images to
// make the dragging look as smooth as possible. A pop-up menu is
// added to the canvas that can be used to performa certain actions
// on the shapes;
Image offScreenCanvas = null; // off-screen image used for double buffering
Graphics offScreenGraphics; // graphics context for drawing to offScreenCanvas
Vector shapes = new Vector(); // holds a list of the shapes that are displayed on the canvas
Color currentColor = Color.red; // current color; when a shape is created, this is its color
ShapeCanvas() {
// Constructor: set background color to white, set up listeners to respond to mouse actions,
// and set up the pop-up menu
setBackground(Color.white);
addMouseListener(this);
addMouseMotionListener(this);
popup = new PopupMenu();
popup.add("Red");
popup.add("Green");
popup.add("Blue");
popup.add("Cyan");
popup.add("Magenta");
popup.add("Yellow");
popup.add("Black");
popup.add("White");
popup.addSeparator();
popup.add("Big");
popup.add("Medium");
popup.add("Small");
popup.addSeparator();
popup.add("Delete");
popup.add("Bring To Front");
add(popup);
popup.addActionListener(this);
} // end construtor
synchronized public void paint(Graphics g) {
// In the paint method, everything is drawn to an off-screen canvas, and then
// that canvas is copied onto the screen.
makeOffScreenCanvas();
g.drawImage(offScreenCanvas,0,0,this);
public void update(Graphics g) {
// Update method is called when canvas is to be redrawn.
// Just call the paint method.
paint(g);
void makeOffScreenCanvas() {
// Erase the off-screen canvas and redraw all the shapes in the list.
// (First, if canvas has not yet been created, then create it.)
if (offScreenCanvas == null) {
offScreenCanvas = createImage(getSize().width,getSize().height);
offScreenGraphics = offScreenCanvas.getGraphics();
offScreenGraphics.setColor(getBackground());
offScreenGraphics.fillRect(0,0,getSize().width,getSize().height);
int top = shapes.size();
for (int i = 0; i < top; i++) {
Shape s = (Shape)shapes.elementAt(i);
s.draw(offScreenGraphics);
public void itemStateChanged(ItemEvent evt) {
// This is called to respond to item events. Such events
// can only be sent by the color choice menu,
// so respond by setting the current color according to
// the selected item in that menu.
Choice colorChoice = (Choice)evt.getItemSelectable();
switch (colorChoice.getSelectedIndex()) {
case 0: currentColor = Color.red; break;
case 1: currentColor = Color.green; break;
case 2: currentColor = Color.blue; break;
case 3: currentColor = Color.cyan; break;
case 4: currentColor = Color.magenta; break;
case 5: currentColor = Color.yellow; break;
case 6: currentColor = Color.black; break;
case 7: currentColor = Color.white; break;
public void actionPerformed(ActionEvent evt) {
// Called to respond to action events. The three shape-adding
// buttons have been set up to send action events to this canvas.
// Respond by adding the appropriate shape to the canvas. This
// also be a command from a pop-up menu.
String command = evt.getActionCommand();
if (command.equals("Rect"))
addShape(new RectShape());
else if (command.equals("Oval"))
addShape(new OvalShape());
else if (command.equals("RoundRect"))
addShape(new RoundRectShape());
else
doPopupMenuCommand(command);
synchronized void addShape(Shape shape) {
// Add the shape to the canvas, and set its size/position and color.
// The shape is added at the top-left corner, with size 50-by-30.
// Then redraw the canvas to show the newly added shape.
shape.setColor(currentColor);
shape.reshape(3,3,50,30);
shapes.addElement(shape);
repaint();
// ------------ This rest of the class implements dragging and the pop-up menu ---------------------
PopupMenu popup;
Shape selectedShape = null; // This is null unless a menu has been popped up on this shape.
Shape draggedShape = null; // This is null unless a shape has been selected for dragging.
int prevDragX; // During dragging, these record the x and y coordinates of the
int prevDragY; // previous position of the mouse.
Shape clickedShape(int x, int y) {
// Find the frontmost shape at coordinates (x,y); return null if there is none.
for ( int i = shapes.size() - 1; i >= 0; i-- ) { // check shapes from front to back
Shape s = (Shape)shapes.elementAt(i);
if (s.containsPoint(x,y))
return s;
return null;
void doPopupMenuCommand(String command) {
// Handle a command from the pop-up menu.
if (selectedShape == null) // should be impossible
return;
if (command.equals("Red"))
selectedShape.setColor(Color.red);
else if (command.equals("Green"))
selectedShape.setColor(Color.green);
else if (command.equals("Blue"))
selectedShape.setColor(Color.blue);
else if (command.equals("Cyan"))
selectedShape.setColor(Color.cyan);
else if (command.equals("Magenta"))
selectedShape.setColor(Color.magenta);
else if (command.equals("Yellow"))
selectedShape.setColor(Color.yellow);
else if (command.equals("Black"))
selectedShape.setColor(Color.black);
else if (command.equals("White"))
selectedShape.setColor(Color.white);
else if (command.equals("Big"))
selectedShape.resize(75,45);
else if (command.equals("Medium"))
selectedShape.resize(50,30);
else if (command.equals("Small"))
selectedShape.resize(25,15);
else if (command.equals("Delete"))
shapes.removeElement(selectedShape);
else if (command.equals("Bring To Front")) {
shapes.removeElement(selectedShape);
shapes.addElement(selectedShape);
repaint();
synchronized public void mousePressed(MouseEvent evt) {
// User has pressed the mouse. Find the shape that the user has clicked on, if
// any. If there is a shape at the position when the mouse was clicked, then
// start dragging it. If the user was holding down the shift key, then bring
// the dragged shape to the front, in front of all the other shapes.
int x = evt.getX(); // x-coordinate of point where mouse was clicked
int y = evt.getY(); // y-coordinate of point
if (evt.isPopupTrigger()) { // If this is a pop-up menu event that
selectedShape = clickedShape(x,y); // occurred over a shape, record which shape
if (selectedShape != null) // it is and show the menu.
popup.show(this,x,y);
else {
draggedShape = clickedShape(x,y);
if (draggedShape != null) {
prevDragX = x;
prevDragY = y;
if (evt.isShiftDown()) { // Bring the shape to the front by moving it to
shapes.removeElement(draggedShape); // the end of the list of shapes.
shapes.addElement(draggedShape);
repaint(); // repaint canvas to show shape in front of other shapes
synchronized public void mouseDragged(MouseEvent evt) {
// User has moved the mouse. Move the dragged shape by the same amount.
if (draggedShape != null) {
int x = evt.getX();
int y = evt.getY();
draggedShape.moveBy(x - prevDragX, y - prevDragY);
prevDragX = x;
prevDragY = y;
repaint(); // redraw canvas to show shape in new position
synchronized public void mouseReleased(MouseEvent evt) {
// User has released the mouse. Move the dragged shape, then set
// shapeBeingDragged to null to indicate that dragging is over.
// If the shape lies completely outside the canvas, remove it
// from the list of shapes (since there is no way to ever move
// it back onscreen).
int x = evt.getX();
int y = evt.getY();
if (draggedShape != null) {
draggedShape.moveBy(x - prevDragX, y - prevDragY);
if ( draggedShape.left >= getSize().width || draggedShape.top >= getSize().height ||
draggedShape.left + draggedShape.width < 0 ||
draggedShape.top + draggedShape.height < 0 ) { // shape is off-screen
shapes.removeElement(draggedShape); // remove shape from list of shapes
draggedShape = null;
repaint();
else if (evt.isPopupTrigger()) { // If this is a pop-up menu event that
selectedShape = clickedShape(x,y); // occurred over a shape, record the
if (selectedShape != null) // shape and show the menu.
popup.show(this,x,y);
public void mouseEntered(MouseEvent evt) { } // Other methods required for MouseListener and
public void mouseExited(MouseEvent evt) { } // MouseMotionListener interfaces.
public void mouseMoved(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
} // end class ShapeCanvas
abstract class Shape {
// A class representing shapes that can be displayed on a ShapeCanvas.
// The subclasses of this class represent particular types of shapes.
// When a shape is first constucted, it has height and width zero
// and a default color of white.
int left, top; // Position of top left corner of rectangle that bounds this shape.
int width, height; // Size of the bounding rectangle.
Color color = Color.white; // Color of this shape.
void reshape(int left, int top, int width, int height) {
// Set the position and size of this shape.
this.left = left;
this.top = top;
this.width = width;
this.height = height;
void resize(int width,int height) {
// Set the size without changing the position
this.width = width;
this.height = height;
void moveTo(int x, int y) {
// Move upper left corner to the point (x,y)
this.left = x;
this.top = y;
void moveBy(int dx, int dy) {
// Move the shape by dx pixels horizontally and dy pixels veritcally
// (by changing the position of the top-left corner of the shape).
left += dx;
top += dy;
void setColor(Color color) {
// Set the color of this shape
this.color = color;
boolean containsPoint(int x, int y) {
// Check whether the shape contains the point (x,y).
// By default, this just checks whether (x,y) is inside the
// rectangle that bounds the shape. This method should be
// overridden by a subclass if the default behaviour is not
// appropriate for the subclass.
if (x >= left && x < left+width && y >= top && y < top+height)
return true;
else
return false;
abstract void draw(Graphics g);
// Draw the shape in the graphics context g.
// This must be overriden in any concrete subclass.
} // end of class Shape
class RectShape extends Shape {
// This class represents rectangle shapes.
void draw(Graphics g) {
g.setColor(color);
g.fillRect(left,top,width,height);
g.setColor(Color.black);
g.drawRect(left,top,width,height);
class OvalShape extends Shape {
// This class represents oval shapes.
void draw(Graphics g) {
g.setColor(color);
g.fillOval(left,top,width,height);
g.setColor(Color.black);
g.drawOval(left,top,width,height);
boolean containsPoint(int x, int y) {
// Check whether (x,y) is inside this oval, using the
// mathematical equation of an ellipse.
double rx = width/2.0; // horizontal radius of ellipse
double ry = height/2.0; // vertical radius of ellipse
double cx = left + rx; // x-coord of center of ellipse
double cy = top + ry; // y-coord of center of ellipse
if ( (ry*(x-cx))*(ry*(x-cx)) + (rx*(y-cy))*(rx*(y-cy)) <= rx*rx*ry*ry )
return true;
else
return false;
class RoundRectShape extends Shape {
// This class represents rectangle shapes with rounded corners.
// (Note that it uses the inherited version of the
// containsPoint(x,y) method, even though that is not perfectly
// accurate when (x,y) is near one of the corners.)
void draw(Graphics g) {
g.setColor(color);
g.fillRoundRect(left,top,width,height,width/3,height/3);
g.setColor(Color.black);
g.drawRoundRect(left,top,width,height,width/3,height/3);
}Manveer-Singh
Please don't post in old threads that are long dead. When you have a question, please start a topic of your own. Feel free to provide a link to an old thread if relevant.
I'm locking this 3 year old thread now.
db
edit You have earlier been advised not to post in old dead threads.
[http://forums.sun.com/thread.jspa?threadID=354443]
Continuing to ignore this advice will render your account liable to be blocked.
Edited by: Darryl.Burke -
Problem with IFCMP60, subclassing, property classes and Object Libraries
I'm looking to write a little batch script that recompiles all my forms. I've got one off the net - Mine is a little more complicated but I'm basing it on
FOR %%F in (*.fmb) do call ifcmp60 %%F userid=uname/pwd@dbase batch=YES Window_State=Minimize
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When I run my batch script some forms compile okay but some don't. I've figured out that the ones that don't are the ones like the above - and looking at the .err produced, they fail on the subclassed fields.
e.g.
Compilation error on procedure PROCESS_MULTI_QUERY:
PL/SQL ERROR 49 at line 139, column 33
bad bind variable 'multi_query.input_file'
there is a block multi_query with a field input_file and this field uses a property_class subclassed from the object library. So ifcmp60 doesn't realise that ":MULTI_QUERY.INPUT_FILE" refers to the block item. However it is just fine with the other items on the canvas that are not subclassed.
Anyone else encountered this? Can I fix it without breaking all the subclassing?False alarm: A colleague had recompiled the OLB under 10g, so Forms 6i didn't recognise it.
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Cant add() a Canvas into a JPane
Hello all!
i having problem to include canvas within a JPanel.
i subclass the Canvas with an image , i test it and its works :
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import java.util.*;
import java.net.*;
public class testImage extends JFrame{
public testImage(){
imageViewer m = new imageViewer();
m.initImage("http://127.0.0.1/image/samifox.gif");
getContentPane().add(m);
public static void main(String[] args) {
JFrame frame = new testImage();
frame.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
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frame.setSize(300,300);
frame.setVisible(true);
class imageViewer extends Canvas{
Image image;
URL url;
Toolkit tool;
public void initImage(String path){
try{
url = new URL(path);
}catch(Exception e){
System.out.println(e.toString());
image = Toolkit.getDefaultToolkit().getImage(url);
setImage(image);
void setImage(Image image){
this.image = image ;
repaint() ;
public void paint(Graphics g){
super.paint(g) ;
try{
g.drawImage(image,0,0,this) ;
//g.drawString("ssss",25,25,this);
} catch (NullPointerException e) {
System.out.println(e.toString());
}but i cant include this canvas subclass into a JPane i tried :
//xxx
import javax.swing.*;
import javax.swing.text.*;
import java.awt.*; //for layout managers
import java.awt.event.*; //for action and window events
import com.borland.jbcl.layout.*;
public class testPanel extends JFrame{
myJPane mypanel;
public testPanel(){
mypanel=new myJPane();
getContentPane().add(mypanel);
public static void main(String[] args) {
JFrame frame = new testPanel();
frame.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
//frame.pack();
frame.setSize(300,300);
frame.setVisible(true);
class myJPane extends JPanel{
imageViewer imgViewer;
public myJPane(){
imgViewer = new imageViewer();
imgViewer.initImage("http://127.0.0.1/image/samifox.gif");
add(imgViewer);
i add and remove from JPanel lots of component but when i had the use canvas it seem like i dont understand someting that very basic...
any kind of help will be appriciated
Shayhi Pual!
Are you getting error messages? no , i dont get any compiler or run-time error
Or does the program run and not do what you expect? yes , exectly , you see i extend Canvas and i draw in it an Image. now i have extended JPane , i need to add the extended canvas to JPane, like i said i dont recieve any error but the program doesnt show the canvas on the JPane like i except it.
the Canvas does show in a JFrame but when i try too add the Canvas to JPanel , i faild.
i hope its more understandable now!
thank you
Shay -
Oracle 10G Forms and Reports Developer Issue (CANVAS Property)!!!
Okay guys. I have a form that uses a canvas named as CANVAS4 and another canvas (horizontal toolbar) named HEADER that is the subclass of HEADER and a library called TDAS.
What happening is, my CANVAS4 color is r240g240b255 and when i try to set the color of HEADER as r240g240b255, it sets the color but don't display this color on screen.
How i come to know that it sets the color?
I've used get_canvas_property() to get the background color property of HEADER.
I almost spent four to five hours but all in vain.
I want you guys to help me as soon as possible. I'll be very thankful to you.
And please do it soon.
Regards...CANVAS 4 set to color r240g240b255 and it shows while HEADER when sets to r240g240b255, doesn't set it.
Both canvases has default color gray12 -
Hi
I am trying to add a canvas to an applet. I am successful in doing so. There is a JMenuBar added to the applet. when I click on the file menu The contents are being displayed behind the canvas and as a result I am neither able to see the buttons nor select them. Could some one help me with this.
Note: Once you compile the code you wouldn't be able to see the contents right away. Click on left top,immidiately below the applet menu. This will repaint the contentpane. This is some thing I am working on.
applet class
package aple;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Shape;
import java.awt.geom.AffineTransform;
import java.util.ArrayList;
import javax.swing.JApplet;
public class NewJApplet extends JApplet{
public Graphics2D g2;
AffineTransform atf = new AffineTransform();
sketch sk = new sketch();
@Override
public void init() {
getContentPane().add(sk);
Gui gui = new Gui();
// menubar
this.setJMenuBar(gui.Gui());
@Override
gui class
package aple;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
public class Gui {
JMenuBar menuBar;
JMenu file,edit,insert,view,draw,circle;
JMenuItem nu,close,saveAs,open,centreandradius,line,rectangle,point,arc;
public JMenuBar Gui(){
//Menubar
menuBar = new JMenuBar();
//Menus
file = new JMenu("File");
menuBar.add(file);
edit = new JMenu("Edit");
menuBar.add(edit);
view = new JMenu("View");
menuBar.add(view);
insert = new JMenu("Insert");
draw = new JMenu("Draw");
circle = new JMenu("Circle");
draw.add(circle);
insert.add(draw);
menuBar.add(insert);
//MenuItems
nu = new JMenuItem("New");
file.add(nu);
open = new JMenuItem("Open");
file.add(open);
saveAs = new JMenuItem("SaveAs");
file.add(saveAs);
close = new JMenuItem("Close");
file.add(close);
line = new JMenuItem("Line");
draw.add(line);
centreandradius = new JMenuItem("Centre and Radius");
circle.add(centreandradius);
rectangle = new JMenuItem("Rectangle");
draw.add(rectangle);
point = new JMenuItem("Point");
draw.add(point);
arc = new JMenuItem("arc");
draw.add("Arc");
return(menuBar);
sketch class
package aple;
import java.awt.Canvas;
import java.awt.Color;
import java.awt.Graphics;
public class sketch extends Canvas{
public void sketch(){
this.setBackground(Color.green);
@Override
public void paint(Graphics g){
// g.fillRect(50,50, 50, 50);
}When you were using JPanel, your "setBackground" didn't work because you were overriding its paint(Graphics) method without calling "super.paint(g);". If you don't do this, the panel won't paint its background.
You should also override "protected void paintComponent(Graphics g)" in any JComponent, such as JPanel, instead of just "paint(Graphics)". (And don't forget to call super.paintComponent(g) FIRST in your subclass's overridden method, if you want the background painted):
class MyJPanelSubclass extends JPanel {
protected void paintComponent(Graphics g) {
super.paintComponent(g); // Paints background and any other stuff normally painted.
// Do your custom painting here.
}
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