Chris Johnson

• Part One: Introduction
  
• Part Two: AWT Components
  
• Part Three: Text and Labels
  
• Part Four: Setting Things Out...
  
• Part Five: Selection Components
  
  
• Part Six: Graphics...
  
  • Drawing
  • Scrollbars
  
  
• Part Seven: Additional information
  
  
• Back to Main Index

public void paint (Graphics g){
	g.drawstring("Hello World", 80, 120);
}

Here are sections of the code that draw various filled and an unfilled rectangles:

	rectangle = new Canvas(); // first create a canvas to draw into

	rectangle.drawRoundRect(new_x, new_y, width, height, 30, 30);

	rectangle.draw3DRect(new_x, new_y, width-1, height-1, true); // true here if raised 
        rectangle.fill3DRect(new_x+3, new_y+3, width-7, height-7, false); 

A lot more could be said under this section. For instance, there are various methods for setting the color of graphical objects. These are illustrated by the Color Palette applet at the end of this part of the course. Similarly, more could be said about the use of the graphics class to support animation techniques. This is the subject of a course in its own right. For more information about this then check out the latest version of Mary Campione and Kathy Walrath's Java Tutorial at java.sun.com.

	Scrollbar redScroll = new Scrollbar(Scrollbar.VERTICAL, 0, 128, 0, 128);

The Applet responds to scrolling events by saving the scrollbar's new value in a variable that is accessible to the Component that it presents. The repaint() method is then invoked on the Component.

	public boolean handleEvent(Event evt) {
                      switch (evt.id) {
                        case Event.SCROLL_LINE_UP:
                        case Event.SCROLL_LINE_DOWN:
                        case Event.SCROLL_PAGE_UP:
                        case Event.SCROLL_PAGE_DOWN:
                        case Event.SCROLL_ABSOLUTE:
                          if (evt.target == vert) {
                              canvas.ty = ((Integer)evt.arg).intValue();
                              canvas.repaint();
                          }
                          if (evt.target == horz) {
                              canvas.tx = ((Integer)evt.arg).intValue();
                              canvas.repaint();       }
                      }
                      return super.handleEvent(evt);
                  }

public void paint(Graphics g) {
        g.translate(-tx, -ty);
        g.drawImage(image, 0, 0, getBackground(), this);
}

• NewGridBagLayout.java
  This is a generic class that can be used to set the constraints associated with the components in a GridBagLayout. It includes methods such as setPositionSizeAdd that simply provides a single call rather than repe ated calls to add a component to a Layout Manager and set both its size and position. It was used in the Calculator case study in a previous part of this course.
  
  
• ColorCanvas.java
  This relatively simple class implements a canvas that is initially coloured black. As the user selects a point on the scroll canvas, see below, this color canvas is repainted to show the colour of the pixel that is under the users' selection.
  
  
• ScrollCanvas.java
  This class implements a scrollable area that is painted with colours that reflect the RGB values provided by three scroll bars. Two scroll bars (Red and Green) are attatched to the side of the canvas whilst the third (Blue) is placed below this main area. An event handle (mouseDown) is called if the user selects a Pixel in the scroll canvas - this provides the value that is used to update the ColorCanvas with the selected RGB values.
  
  
• ColorChooser.java
  The main class that initialises the display format shown below (using NewGridBagLayout). It also includes the event handler that reacts to user interaction with the scroll bar. In this case, the scroll canvas must be updated to reflect the new range of colours in the panel.