Threads

1
Threads

• java supports multiple threads of execution
• it appears as if the threads are executing
  concurrently
• a class must extend Thread or implement Runnable
• Runnable declares the method run()

2
Threads

• a thread is controlled by the run() method
• a thread starts when its start() method is called
• a thread stops when its stop() method is called
  or when run() returns
• separate threads are useful in animation

3
animation

• the basic idea is to draw an image, wait a while
  and then draw the image again, modifying it
  slightly
• the main thread of an applet can draw the image
  in its paint() method
• the other thread can control when the image is
  changed and what it is changed to.

4
simple animation

• write an applet that draws a car moving across
  the screen
• in paint() draw the car at position x,y
• in run() update the values of x,y and call
  repaint()
• in init() set the initial values of x and y
• in start() start the thread
• in stop() stop the thread

5
reduce flickering

• override update()
• clip
• double buffering

6
update

• repaint() calls update()
• update() clears the screen and calls paint()
• if you are repainting the whole screen, override
  update such that it does not clear the screen
  before calling paint

7
clip

• clipping restricts the area to be changed
• clipRect(x,y,width,height) makes the clip area
  the intersection of the old clip area and this
  one
• setClip(x,y,width,height) set the new clip area

8
double buffering

• draw the image to an off-screen buffer
• when done move it to the screen area

9
Image myImage Graphics myGraphics public void
init()myImage createImage(getSize)().width,
getSize().height) myGraphicsmyImage.getGraphi
cs() public void update(Graphics
g) paint(myGraphics) g.drawImage(myImage,0,0,t
his)