15 枚举

枚举类型确保JVM中仅存在一个常量实例。

使用枚举类实现单例模式，非常优越。

public enum PizzaDeliverySystemConfiguration {
    INSTANCE;
    PizzaDeliverySystemConfiguration() {
        // Initialization configuration which involves
        // overriding defaults like delivery strategy
    }
 
    private PizzaDeliveryStrategy deliveryStrategy = PizzaDeliveryStrategy.NORMAL;
 
    public static PizzaDeliverySystemConfiguration getInstance() {
        return INSTANCE;
    }
 
    public PizzaDeliveryStrategy getDeliveryStrategy() {
        return deliveryStrategy;
    }
}
PizzaDeliveryStrategy deliveryStrategy = PizzaDeliverySystemConfiguration.getInstance().getDeliveryStrategy();

使用枚举类实现策略模式也非常优越。

public enum PizzaDeliveryStrategy {
    EXPRESS {
        @Override
        public void deliver(Pizza pz) {
            System.out.println("Pizza will be delivered in express mode");
        }
    },
    NORMAL {
        @Override
        public void deliver(Pizza pz) {
            System.out.println("Pizza will be delivered in normal mode");
        }
    };
 
    public abstract void deliver(Pizza pz);
}
//给pizza类添加下面的方法
public void deliver() {
    if (isDeliverable()) {
        PizzaDeliverySystemConfiguration.getInstance().getDeliveryStrategy()
          .deliver(this);
        this.setStatus(PizzaStatus.DELIVERED);
    }
}