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);
    }
}