Java多例设计模式

class TrafficLight{
    private String lightType;
    private static final TrafficLight GREENLIGHT = new TrafficLight("GreenLight");
    private static final TrafficLight REDLIGHT = new TrafficLight("RedLight");
    private static final TrafficLight YELLOWLIGHT = new TrafficLight("YellowLight");
    
    private TrafficLight(String lightType){
        this.lightType = lightType;
    }
    
    public static TrafficLight getInstance(String ch){
        switch(ch){
        case "GreenLight":
            return GREENLIGHT;
        case "RedLight":
            return REDLIGHT;
        case "YellowLight":
            return YELLOWLIGHT;
        default:
            return null;
        }
    }
    
    public String toString(){
        return this.lightType;
    }
}

public class TrafficLightDemo {
    public static void main(String[] args) {
        TrafficLight GreenLight = TrafficLight.getInstance("GreenLight");
        TrafficLight RedLight = TrafficLight.getInstance("RedLight");
        TrafficLight YellowLight = TrafficLight.getInstance("YellowLight");
        System.out.println(GreenLight);
        System.out.println(RedLight);
        System.out.println(YellowLight);
    }
}