ParticleCanvas in Java

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1112 ParticleCanvas
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ParticleCanvas is a simple subclass of javaawtCanvas that provides a drawing area for all of the Particles Its main responsibility is to invoke draw for all existing particles whenever its paint method is called
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However, the ParticleCanvas itself does not create or manage the particles It needs either to be told about them or to ask about them Here, we choose the former The instance variable particles holds the array of existing Particle objects This field is set when necessary by the applet, but is used in the paint method We can again apply our default rules, which in this case lead to the creation of little synchronized get and set methods (also known as accessor and assignment methods) for particles, otherwise avoiding direct access of the particles variable itself To simplify and to enforce proper usage, the particles field is never allowed to be null It is instead initialized to an empty array:
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class ParticleCanvas extends Canvas { private Particle[] particles = new Particle[0]; ParticleCanvas(int size) { setSize(new Dimension(size, size)); } // intended to be called by applet protected synchronized void setParticles(Particle[] ps) { if (ps == null) throw new IllegalArgumentException("Cannot set null"); particles = ps; }
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protected synchronized Particle[] getParticles() { return particles; } public void paint(Graphics g) { // override Canvaspaint Particle[] ps = getParticles(); for (int i = 0; i < pslength; ++i) ps[i]draw(g); } }
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1113 ParticleApplet The Particle and ParticleCanvas classes could be used as the basis of several different programs But in ParticleApplet all we want to do is set each of a collection of particles in autonomous "continuous" motion and update the display accordingly to show where they are To comply with standard applet conventions, these activities should begin when Appletstart is externally invoked (normally from within a web browser), and should end when Appletstop is invoked (We could also add buttons allowing users to start and stop the particle animation themselves) There are several ways to implement all this Among the simplest is to associate an independent loop with each particle and to run each looping action in a different thread Actions to be performed within new threads must be defined in classes implementing javalangRunnable This interface lists only the single method run, taking no arguments, returning no results, and throwing no checked exceptions:
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public interface javalangRunnable { void run(); }
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An interface encapsulates a coherent set of services and attributes (broadly, a role) without assigning this functionality to any particular object or code Interfaces are more abstract than classes since they say nothing at all about representations or code All they do is describe the signatures (names, arguments, result types, and exceptions) of public operations, without even pinning down the classes of the objects that can perform them The classes that can support Runnable typically have nothing in common except that they contain a run method Each instance of the Thread class maintains the control state necessary to execute and manage the call sequence comprising its action The most commonly used constructor in class Thread accepts a Runnable object as an argument, which arranges to invoke the Runnable's run method when the thread is started While any class can implement Runnable, it often turns out to be both convenient and helpful to define a Runnable as an anonymous inner class The ParticleApplet class uses threads in this way to put particles into motion, and cancels them when the applet is finished This is done by overriding the standard Applet methods start
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and stop (which have the same names as, but are unrelated to, methods Threadstart and Threadstop)
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The above interaction diagram shows the main message sequences during execution of the applet In addition to the threads explicitly created, this applet interacts with the AWT event thread, described in more detail in 414 The producer-consumer relationship extending from the omitted right hand side of the interaction diagram takes the approximate form:
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public class ParticleApplet extends Applet { protected Thread[] threads = null; // null when not running
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protected final ParticleCanvas canvas = new ParticleCanvas(100); public void init() { add(canvas); } protected Thread makeThread(final Particle p) { // utility Runnable runloop = new Runnable() { public void run() { try { for(;;) { pmove(); canvasrepaint(); Threadsleep(100); // 100msec is arbitrary } } catch (InterruptedException e) { return; } } }; return new Thread(runloop); } public synchronized void start() { int n = 10; // just for demo if (threads == null) { // bypass if already started Particle[] particles = new Particle[n]; for (int i = 0; i < n; ++i) particles[i] = new Particle(50, 50); canvassetParticles(particles); threads = new Thread[n]; for (int i = 0; i < n; ++i) { threads[i] = makeThread(particles[i]); threads[i]start(); } } } public synchronized void stop() { if (threads != null) { // bypass if already stopped for (int i = 0; i < threadslength; ++i) threads[i]interrupt(); threads = null; } } }
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The action in makeThread defines a "forever" loop (which some people prefer to write equivalently as "while (true)") that is broken only when the current thread is interrupted During each iteration, the particle moves, tells the canvas to repaint so the move will be displayed, and then does nothing for a while, to slow things down to a humanviewable rate Threadsleep pauses the current thread It is later resumed by a system timer One reason that inner classes are convenient and useful is that they capture all appropriate context variables here p and canvas without the need to create a separate class with fields that record these values This convenience comes at the price of one minor awkwardness: All captured method arguments and local variables must be declared as final, as a guarantee that the values can indeed be captured unambiguously Otherwise, for example, if p were reassigned after constructing the Runnable inside method makeThread, then it would be ambiguous whether to use the original or the assigned value when executing the Runnable The call to canvasrepaint does not directly invoke canvaspaint The repaint method instead places an UpdateEvent on a javaawtEventQueue (This may be internally optimized and further manipulated to eliminate duplicate events) A javaawtEventDispatchThread asynchronously takes this event from the queue and dispatches it by (ultimately) invoking canvaspaint This thread and possibly other system-created threads may exist even in nominally single-threaded programs The activity represented by a constructed Thread object does not begin until invocation of the Threadstart method As discussed in 312, there are several ways to cause a thread's activity to stop The simplest is just to have the run method terminate normally But in infinitely looping methods, the best option is to use Threadinterrupt An interrupted thread will automatically abort (via an InterruptedException) from the methods Objectwait, Threadjoin, and Threadsleep Callers can then catch this exception and take any appropriate action to shut down Here, the catch in runloop just causes the run method to exit, which in turn causes the thread to terminate The start and stop methods are synchronized to preclude concurrent starts or stops Locking works out OK here even though these methods need to perform many operations (including calls to other objects) to achieve the required started-to-stopped or stopped-to-started state transitions Nullness of variable threads is used as a convenient state indicator
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