Um semáforo tem um temporizador que emite impulsos a intervalos regulares. Estes impulsos chegam ao controlador do semáforo sob a forma de chamadas ao método tick. O semáforo tem ainda vários botões para controlo de situações especiais. Assim, o botão de pânico que faz com que o controlador mude o semáforo para a cor vermelha, qualquer que seja a anterior (método panic). O botão off faz com que o semáforo mude imediatamente para amarelo intermitente (método off). Esta situação é interrompida através dos botões on (volta ao comportamento normal; método on) ou de pânico. O botão on é a única forma de fazer semáforo sair da situação de pânico e recuperar o funcionamento normal. Além do código de controlo, o semáforo tem ainda um método (status) que permite saber a cor actual do semáforo. O semáforo começa intermitente e quando recupera das situações de intermitência e de pânico fica vermelho.
Implemente o semáforo e a sua máquina de estados.
<java5> /**
* @author David Martins de Matos */
public class TrafficLight {
/** * The traffic light's state. */ private State _state;
/** * Abstract state class. * * This class is internal so that it has access to the traffic light's * internal state. Actual states are subclasses which must use this class' * protected interface. */ public abstract class State {
/** * Tick behavior. */ public abstract void tick();
/** * Panic behavior. */ public abstract void panic();
/** * "On" behavior. */ public void on() { // ignore }
/** * "Off" behavior. */ public abstract void off();
/** * @return traffic light status. */ public abstract String status();
/** * Define the traffic light's new state. * * @param newState * the new state. */ protected void setState(State newState) { _state = newState; }
/** * This method is needed so that new states can be created. * * @return the traffic light. */ protected TrafficLight getLight() { return TrafficLight.this; } }
/** * Initialize traffic light. Starts blinking. */ public TrafficLight() { _state = new Blinking(this); }
/** * Process on button press. */ @SuppressWarnings("nls") public void on() { System.out.print("[" + status() + "]"); _state.on(); System.out.println(" --(on)-> [" + status() + "]"); }
/** * Process off button press. */ @SuppressWarnings("nls") public void off() { System.out.print("[" + status() + "]"); _state.off(); System.out.println(" --(off)-> [" + status() + "]"); }
/** * Process panic button press. */ @SuppressWarnings("nls") public void panic() { System.out.print("[" + status() + "] --(panic)-> "); _state.panic(); System.out.println("[" + status() + "]"); }
/** * Process tick: switch light color. */ @SuppressWarnings("nls") public void tick() { System.out.print("[" + status() + "] --(tick)-> "); _state.tick(); System.out.println("[" + status() + "]"); }
/** * @return traffic light status. */ public String status() { return _state.status(); }
} </java5>
Abstract class for representing normal behavior. See also the next section (color classes).
<java5> /**
* Ticking states are those corresponding to the usual three colors. */
public abstract class Ticking extends TrafficLight.State {
/** * @param light */ Ticking(TrafficLight light) { light.super(); }
/** * All colors switch to blinking when "off" is pressed. * * @see TrafficLight.State#off() */ @Override public void off() { setState(new Blinking(getLight())); }
/** * All colors switch to panic when the button is pressed. * * @see TrafficLight.State#panic() */ @Override public void panic() { setState(new Panic(getLight())); }
} </java5>
Red light.
<java5> /**
* The red light. */
public class Red extends Ticking {
/** * @param light */ Red(TrafficLight light) { super(light); }
/** * @see TrafficLight.State#tick() */ @Override public void tick() { setState(new Green(getLight())); }
/** * @see TrafficLight.State#status() */ @Override @SuppressWarnings("nls") public String status() { return "Red"; } } </java5>
Yellow light.
<java5> /**
* The yellow light. */
public class Yellow extends Ticking {
/** * @param light */ Yellow(TrafficLight light) { super(light); }
/** * @see TrafficLight.State#tick() */ @Override public void tick() { setState(new Red(getLight())); }
/** * @see TrafficLight.State#status() */ @Override @SuppressWarnings("nls") public String status() { return "Yellow"; } } </java5>
Green light.
<java5> /**
* The green light. */
public class Green extends Ticking {
/** * @param light */ Green(TrafficLight light) { super(light); }
/** * @see TrafficLight.State#tick() */ @Override public void tick() { setState(new Yellow(getLight())); }
/** * @see TrafficLight.State#status() */ @Override @SuppressWarnings("nls") public String status() { return "Green"; } } </java5>
<java5> /**
* Panic state. */
public class Panic extends TrafficLight.State {
/** * @param light */ Panic(TrafficLight light) { light.super(); }
/** * Return to "ticking" (normal) mode. * * @see TrafficLight.State#on() */ @Override public void on() { setState(new Red(getLight())); }
/** * @see TrafficLight.State#off() */ @Override public void off() { // do nothing: already off }
/** * @see TrafficLight.State#panic() */ @Override public void panic() { // do nothing: already in panic mode }
/** * @see TrafficLight.State#tick() */ @Override public void tick() { // do nothing: ignore ticks in panic mode }
/** * @see TrafficLight.State#status() */ @Override @SuppressWarnings("nls") public String status() { return "Panic"; } } </java5>
<java5> /**
* Blinking light (off). */
public class Blinking extends TrafficLight.State {
/** * @param light */ Blinking(TrafficLight light) { light.super(); }
/** * Return to "ticking" (normal) mode. * * @see TrafficLight.State#on() */ @Override public void on() { setState(new Red(getLight())); }
/** * @see TrafficLight.State#off() */ @Override public void off() { // do nothing: already off }
/** * @see TrafficLight.State#panic() */ @Override public void panic() { setState(new Panic(getLight())); }
/** * @see TrafficLight.State#tick() */ @Override public void tick() { // do nothing: ignore ticks }
/** * @see TrafficLight.State#status() */ @Override @SuppressWarnings("nls") public String status() { return "Blinking"; } } </java5>
Simple application to illustrate the traffic light's state changes.
<java5> /**
* Simple application to illustrate changes of light states. */
public class App { /** * @param args */ public static void main(String[] args) { TrafficLight light = new TrafficLight(); System.out.println("Light status: " + light.status()); light.off(); light.panic(); light.on(); light.tick(); light.tick(); light.tick(); light.tick(); light.panic(); light.off(); light.on(); light.off(); } } </java5>
java App
Result:
Light status: Blinking [Blinking] --(off)-> [Blinking] [Blinking] --(panic)-> [Panic] [Panic] --(on)-> [Red] [Red] --(tick)-> [Green] [Green] --(tick)-> [Yellow] [Yellow] --(tick)-> [Red] [Red] --(tick)-> [Green] [Green] --(panic)-> [Panic] [Panic] --(off)-> [Panic] [Panic] --(on)-> [Red] [Red] --(off)-> [Blinking]