枚举类常见用法，A Guide to Java Enums

Java 5 引入枚举类， 详情可参考 官方文档.

啥是枚举类

public enum PizzaStatus {ORDERED,READY, DELIVERED; 
}

Custom Enum Methods

@Data
public class Pizza {private PizzaStatus status;public enum PizzaStatus {ORDERED,READY,DELIVERED;}public boolean isDeliverable() {if (getStatus() == PizzaStatus.READY) {return true;}return false;}
}

Comparing Enum Types Using “==” Operator

the “==” operator provides compile-time and run-time safety.
run-time safety

if(testPz.getStatus().equals(Pizza.PizzaStatus.DELIVERED));  ← null 会报 NullPointerExceptionif(testPz.getStatus() == Pizza.PizzaStatus.DELIVERED);  ← Either value can be null and we won't get a NullPointerException

compile-time safety

PizzaStatus.DELIVERED.equals(PizzaStatus2.ORDERED)    ← 编译不会报错，会返回falsePizzaStatus.DELIVERED == PizzaStatus2.ORDERED    ← 编译会报错

Using Enum Types in Switch Statements

public int getDeliveryTimeInDays() {switch (status) {case ORDERED: return 5;case READY: return 2;case DELIVERED: return 0;}return 0;
}

Fields, Methods and Constructors in Enums

@Data
public class Pizza {public enum PizzaStatus {ORDERED(5) {@Overridepublic boolean isOrdered() {return true;}}, READY(2) {@Overridepublic boolean isReady() {return true;}}, DELIVERED(0) {@Overridepublic boolean isDelivered() {return true;}};private int timeToDelivery;PizzaStatus(int timeToDelivery) {this.timeToDelivery = timeToDelivery;}// 下单public boolean isOrdered() {return false;}// 准备public boolean isReady() {return false;}// 交付public boolean isDelivered() {return false;}public int getTimeToDelivery() {return timeToDelivery;}}private PizzaStatus status;public boolean isDeliverable() {return this.status.isReady();}public void printTimeToDeliver() {System.out.println("Time to delivery is " + this.getStatus().getTimeToDelivery());}
}

    public static void main(String[] args) {Pizza testPz = new Pizza();testPz.setStatus(Pizza.PizzaStatus.READY); // truetestPz.printTimeToDeliver(); // Time to delivery is 2}

EnumSet

与HashSet相比更高效

@Data
public class Pizza {private static EnumSet<PizzaStatus> undeliveredPizzaStatuses = EnumSet.of(PizzaStatus.ORDERED, PizzaStatus.READY);private PizzaStatus status;public enum PizzaStatus {ORDERED(5) {@Overridepublic boolean isOrdered() {return true;}}, READY(2) {@Overridepublic boolean isReady() {return true;}}, DELIVERED(0) {@Overridepublic boolean isDelivered() {return true;}};private int timeToDelivery;PizzaStatus(int timeToDelivery) {this.timeToDelivery = timeToDelivery;}// 下单public boolean isOrdered() {return false;}// 准备public boolean isReady() {return false;}// 交付public boolean isDelivered() {return false;}public int getTimeToDelivery() {return timeToDelivery;}}public boolean isDeliverable() {return this.status.isReady();}public void printTimeToDeliver() {System.out.println("Time to delivery is " + this.getStatus().getTimeToDelivery() + " days");}public static List<Pizza> getAllUndeliveredPizzas(List<Pizza> input) {return input.stream().filter((s) -> undeliveredPizzaStatuses.contains(s.getStatus())).collect(Collectors.toList());}
}

public static void main(String[] args) {List<Pizza> pzList = new ArrayList<>();Pizza pz1 = new Pizza();pz1.setStatus(Pizza.PizzaStatus.DELIVERED);Pizza pz2 = new Pizza();pz2.setStatus(Pizza.PizzaStatus.ORDERED);Pizza pz3 = new Pizza();pz3.setStatus(Pizza.PizzaStatus.ORDERED);Pizza pz4 = new Pizza();pz4.setStatus(Pizza.PizzaStatus.READY);pzList.add(pz1);pzList.add(pz2);pzList.add(pz3);pzList.add(pz4);List<Pizza> undeliveredPzs = Pizza.getAllUndeliveredPizzas(pzList);System.out.println(undeliveredPzs.size()); // 3
}

EnumMap

EnumMap与对应的HashMap相比，它是一个高效而紧凑的实现，内部表示为数组。

public static EnumMap<PizzaStatus, List<Pizza>> groupPizzaByStatus(List<Pizza> pizzaList) {EnumMap<PizzaStatus, List<Pizza>> pzByStatus = new EnumMap<PizzaStatus, List<Pizza>>(PizzaStatus.class);for (Pizza pz : pizzaList) {PizzaStatus status = pz.getStatus();if (pzByStatus.containsKey(status)) {pzByStatus.get(status).add(pz);} else {List<Pizza> newPzList = new ArrayList<Pizza>();newPzList.add(pz);pzByStatus.put(status, newPzList);}}return pzByStatus;
}

public static EnumMap<PizzaStatus, List<Pizza>> groupPizzaByStatus2(List<Pizza> pzList) {EnumMap<PizzaStatus, List<Pizza>> map = pzList.stream().filter(a -> a != null && a.getStatus() != null).collect(Collectors.groupingBy(Pizza::getStatus, () -> new EnumMap<>(PizzaStatus.class), Collectors.toList()));return map;
}

public static Map<PizzaStatus, List<Pizza>> groupPizzaByStatus3(List<Pizza> pizzaList) {Map<PizzaStatus, List<Pizza>> collect = pizzaList.stream().filter(a -> a != null && a.getStatus() != null).collect(Collectors.groupingBy(Pizza::getStatus));return collect;
}

public static void main(String[] args) {List<Pizza> pzList = new ArrayList<>();Pizza pz1 = new Pizza();pz1.setStatus(Pizza.PizzaStatus.DELIVERED);Pizza pz2 = new Pizza();pz2.setStatus(Pizza.PizzaStatus.ORDERED);Pizza pz3 = new Pizza();pz3.setStatus(Pizza.PizzaStatus.ORDERED);Pizza pz4 = new Pizza();pz4.setStatus(Pizza.PizzaStatus.READY);pzList.add(pz1);pzList.add(pz2);pzList.add(pz3);pzList.add(pz4);EnumMap<Pizza.PizzaStatus, List<Pizza>> map = Pizza.groupPizzaByStatus(pzList);System.out.println(map);//{ORDERED=[Pizza(status=ORDERED), Pizza(status=ORDERED)], READY=[Pizza(status=READY)], DELIVERED=[Pizza(status=DELIVERED)]}EnumMap<Pizza.PizzaStatus, List<Pizza>> map2 = Pizza.groupPizzaByStatus2(pzList);System.out.println(map2);// {ORDERED=[Pizza(status=ORDERED), Pizza(status=ORDERED)], READY=[Pizza(status=READY)], DELIVERED=[Pizza(status=DELIVERED)]}Map<Pizza.PizzaStatus, List<Pizza>> map3 = Pizza.groupPizzaByStatus3(pzList);System.out.println(map3);// {READY=[Pizza(status=READY)], ORDERED=[Pizza(status=ORDERED), Pizza(status=ORDERED)], DELIVERED=[Pizza(status=DELIVERED)]}
}

Strategy Pattern

public enum PizzaDeliveryStrategy {EXPRESS {@Overridepublic void deliver(Pizza pz) {System.out.println("Pizza will be delivered in express mode");}}, NORMAL {@Overridepublic void deliver(Pizza pz) {System.out.println("Pizza will be delivered in normal mode");}};public abstract void deliver(Pizza pz);
}

Singleton Pattern

public enum PizzaDeliverySystemConfiguration {INSTANCE;PizzaDeliverySystemConfiguration() {// Initialization configuration which involves overriding defaults like delivery strategy// 初始化配置，包括覆盖默认值，如交付策略}private PizzaDeliveryStrategy deliveryStrategy = PizzaDeliveryStrategy.NORMAL;public static PizzaDeliverySystemConfiguration getInstance() {return INSTANCE;}public PizzaDeliveryStrategy getDeliveryStrategy() {return deliveryStrategy;}
}

Pizza 类里追加如下方法

public void deliver() {if (isDeliverable()) {PizzaDeliverySystemConfiguration.getInstance().getDeliveryStrategy().deliver(this);this.setStatus(PizzaStatus.DELIVERED);}
}

public static void main(String[] args) {Pizza pz = new Pizza();pz.setStatus(Pizza.PizzaStatus.READY);pz.deliver(); // Pizza will be delivered in normal mode System.out.println(pz.getStatus() == Pizza.PizzaStatus.DELIVERED); // true
}

Java 8 and Enums

public static List<Pizza> getAllUndeliveredPizzas(List<Pizza> input) {return input.stream().filter((s) -> undeliveredPizzaStatuses.contains(s.getStatus())).collect(Collectors.toList());
}

public static EnumMap<PizzaStatus, List<Pizza>> groupPizzaByStatus2(List<Pizza> pzList) {EnumMap<PizzaStatus, List<Pizza>> map = pzList.stream().filter(a -> a != null && a.getStatus() != null).collect(Collectors.groupingBy(Pizza::getStatus, () -> new EnumMap<>(PizzaStatus.class), Collectors.toList()));return map;
}

public static Map<PizzaStatus, List<Pizza>> groupPizzaByStatus3(List<Pizza> pizzaList) {Map<PizzaStatus, List<Pizza>> collect = pizzaList.stream().filter(a -> a != null && a.getStatus() != null).collect(Collectors.groupingBy(Pizza::getStatus));return collect;
}

JSON Representation of Enum

-----------------------学习笔记摘自：A Guide to Java Enums

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