java程式設計思想——第十九章(列舉型別)》
阿新 • • 發佈：2018-12-19
列舉型別##
關鍵字enum可以將一組具名的值的有限集合建立為一種新的型別，而這些具名的值可以作為常規的程式元件使用。
19.1 基本emum特性
Shrubbery.values()：獲取值集合
s.ordinal()：返回次序
s.getDeclaringClass():所屬enum類
s.name()：返回名字
- 將靜態匯入用於emum import static enumerated.Spiciness.*; 可以將emum例項的識別符號帶入當前名稱空間，無需再emum型別修飾emum例項。
19.2 向emum中新增新方法
除了不能繼承外，基本上可以將emum看著常規類。
public enum OzWitch { WEST("Miss Gulch, aka the Wicked Witch of the West"), NORTH("Glinda, the Good Witch of the North"), EAST("Wicked Witch of the East, wearer of the Ruby " + "Slippers, crushed by Dorothy's house"), SOUTH("Good by inference, but missing"); private String description; private OzWitch(String description) { this.description = description; } public String getDescription() { return description; } public static void main(String[] args) { for(OzWitch witch : OzWitch.values()) print(witch + ": " + witch.getDescription()); } }
- 覆蓋enum方法
public enum SpaceShip { SCOUT, CARGO, TRANSPORT, CRUISER, BATTLESHIP, MOTHERSHIP; public String toString() { String id = name(); String lower = id.substring(1).toLowerCase(); return id.charAt(0) + lower; } public static void main(String[] args) { for(SpaceShip s : values()) { System.out.println(s); } } }
19.3 switch語句中的emum
/** * switch語句中的enum * @author Administrator * */ enum Signal{ GREEN,YELLOW,RED } public class TrafficLight { Signal color = Signal.RED; public void change() { switch (color) { case RED: color = Signal.GREEN; break; case GREEN: color = Signal.YELLOW; break; case YELLOW: color = Signal.RED; break; } } @Override public String toString() { return "The Light is "+color; } public static void main(String[] args) { TrafficLight tf = new TrafficLight(); for (int i = 0; i < 7; i++) { print(tf); tf.change(); } } }
19.4 emum 的values()神祕之處
values()是由編譯器新增的static方法。
19.5 實現而非繼承
enum繼承Enum類了，所以只能是實現一個或多個介面。
19.6 隨機選取
public class Enums {
private static Random rand = new Random(47);
public static <T extends Enum<T>> T random(Class<T> ec) {
return random(ec.getEnumConstants());
}
public static <T> T random(T[] values) {
return values[rand.nextInt(values.length)];
}
}
19.7 使用介面組織列舉
19.8 使用EnumSet代替標誌
public class EnumSets {
public static void main(String[] args) {
EnumSet<AlarmPoints> points =
EnumSet.noneOf(AlarmPoints.class); // Empty set
points.add(BATHROOM);
print(points);
points.addAll(EnumSet.of(STAIR1, STAIR2, KITCHEN));
print(points);
points = EnumSet.allOf(AlarmPoints.class);
points.removeAll(EnumSet.of(STAIR1, STAIR2, KITCHEN));
print(points);
points.removeAll(EnumSet.range(OFFICE1, OFFICE4));
print(points);
points = EnumSet.complementOf(points);
print(points);
}
}
19.9 使用EnumMap
EnumMap要求其中的鍵必須來自一個Enum。 命令設計模式：一個只有單一方法的介面，然後從該介面實現具有各自不同行為的子類。
public class EnumMaps {
public static void main(String[] args) {
EnumMap<AlarmPoints,Command> em =
new EnumMap<AlarmPoints,Command>(AlarmPoints.class);
em.put(KITCHEN, new Command() {
public void action() { print("Kitchen fire!"); }
});
em.put(BATHROOM, new Command() {
public void action() { print("Bathroom alert!"); }
});
for(Map.Entry<AlarmPoints,Command> e : em.entrySet()) {
printnb(e.getKey() + ": ");
e.getValue().action();
}
try { // If there's no value for a particular key:
em.get(UTILITY).action();
} catch(Exception e) {
print(e);
}
}
}
19.10 常量相關方法
enum可以為enum例項編寫方法。實現不同enum的不同行為。 具體做法：需要為enum定義一個或多個抽象方法，然後為每個enum實現該抽象方法。
public enum ConstantSpecificMethod {
DATE_TIME {
String getInfo() {
return
DateFormat.getDateInstance().format(new Date());
}
},
CLASSPATH {
String getInfo() {
return System.getenv("CLASSPATH");
}
},
VERSION {
String getInfo() {
return System.getProperty("java.version");
}
};
abstract String getInfo();
public static void main(String[] args) {
for(ConstantSpecificMethod csm : values())
System.out.println(csm.getInfo());
}
}
- 使用emun責任鏈 責任鏈設計模式：以多種不同的方式來解決一個問題，然後將他們連結在一起。
class Mail {
// The NO's lower the probability of random selection:
enum GeneralDelivery {YES,NO1,NO2,NO3,NO4,NO5}
enum Scannability {UNSCANNABLE,YES1,YES2,YES3,YES4}
enum Readability {ILLEGIBLE,YES1,YES2,YES3,YES4}
enum Address {INCORRECT,OK1,OK2,OK3,OK4,OK5,OK6}
enum ReturnAddress {MISSING,OK1,OK2,OK3,OK4,OK5}
GeneralDelivery generalDelivery;
Scannability scannability;
Readability readability;
Address address;
ReturnAddress returnAddress;
static long counter = 0;
long id = counter++;
public String toString() { return "Mail " + id; }
public String details() {
return toString() +
", General Delivery: " + generalDelivery +
", Address Scanability: " + scannability +
", Address Readability: " + readability +
", Address Address: " + address +
", Return address: " + returnAddress;
}
// Generate test Mail:
public static Mail randomMail() {
Mail m = new Mail();
m.generalDelivery= Enums.random(GeneralDelivery.class);
m.scannability = Enums.random(Scannability.class);
m.readability = Enums.random(Readability.class);
m.address = Enums.random(Address.class);
m.returnAddress = Enums.random(ReturnAddress.class);
return m;
}
public static Iterable<Mail> generator(final int count) {
return new Iterable<Mail>() {
int n = count;
public Iterator<Mail> iterator() {
return new Iterator<Mail>() {
public boolean hasNext() { return n-- > 0; }
public Mail next() { return randomMail(); }
public void remove() { // Not implemented
throw new UnsupportedOperationException();
}
};
}
};
}
}
public class PostOffice {
enum MailHandler {
GENERAL_DELIVERY {
boolean handle(Mail m) {
switch(m.generalDelivery) {
case YES:
print("Using general delivery for " + m);
return true;
default: return false;
}
}
},
MACHINE_SCAN {
boolean handle(Mail m) {
switch(m.scannability) {
case UNSCANNABLE: return false;
default:
switch(m.address) {
case INCORRECT: return false;
default:
print("Delivering "+ m + " automatically");
return true;
}
}
}
},
VISUAL_INSPECTION {
boolean handle(Mail m) {
switch(m.readability) {
case ILLEGIBLE: return false;
default:
switch(m.address) {
case INCORRECT: return false;
default:
print("Delivering " + m + " normally");
return true;
}
}
}
},
RETURN_TO_SENDER {
boolean handle(Mail m) {
switch(m.returnAddress) {
case MISSING: return false;
default:
print("Returning " + m + " to sender");
return true;
}
}
};
abstract boolean handle(Mail m);
}
static void handle(Mail m) {
for(MailHandler handler : MailHandler.values())
if(handler.handle(m))
return;
print(m + " is a dead letter");
}
public static void main(String[] args) {
for(Mail mail : Mail.generator(10)) {
print(mail.details());
handle(mail);
print("*****");
}
}
}
- 使用emun狀態機