《java编程思想》学习笔记19 第19章 枚举类型

1，关键字enum可以把一组具名的值的有限集合创建为一种新的类型，而这些具名的值可以作为常规的程序组件使用。

 

2，基本enum特性

 

创建enum时，编译器会为你生成一个相关的类，这个类继承自java.lang.Enum。调用enum的values（）方法，可以

 

遍历enum实例。

 

enum Shrubbery { GROUND, CRAWLING, HANGING }

public class EnumClass {
  public static void main(String[] args) {
    for(Shrubbery s : Shrubbery.values()) {
      print(s + " ordinal: " + s.ordinal());
      printnb(s.compareTo(Shrubbery.CRAWLING) + " ");
      printnb(s.equals(Shrubbery.CRAWLING) + " ");
      print(s == Shrubbery.CRAWLING);
      print(s.getDeclaringClass());
      print(s.name());
      print("----------------------");
    }
    // Produce an enum value from a string name:
    for(String s : "HANGING CRAWLING GROUND".split(" ")) {
      Shrubbery shrub = Enum.valueOf(Shrubbery.class, s);
      print(shrub);
    }
  }
} /* Output:

 

3，将静态导入enum

 

package enumerated;
import static enumerated.Spiciness.*;

public class Burrito {
  Spiciness degree;
  public Burrito(Spiciness degree) { this.degree = degree;}
  public String toString() { return "Burrito is "+ degree;}
  public static void main(String[] args) {
    System.out.println(new Burrito(NOT));
    System.out.println(new Burrito(MEDIUM));
    System.out.println(new Burrito(HOT));
  }
} /* Output:

 

4，向enum中添加新方法

 

除了不能继承自一个enum之外，我们基本上可以把enum看作一个常规的类，也就是说，我们可以向enum中添加方法，

 

enum甚至可以有main（）方法。

 

public enum OzWitch {
  // Instances must be defined first, before methods:
  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;
  // Constructor must be package or private access:
  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());
  }
} /* Output:

 

如果你打算定义自己的方法，那么必须在enum实例序列的最后添加一个分号。同时，java要求你必须先定义enum实例。

 

我们只能在enum定义的内部使用其构造器创建enum实例。一旦enum的定义结束，编译器就不允许我们再使用其构造器

 

来创建任何实例了。

 

 

5，覆盖enum的方法

 

覆盖enum的toString（）方法与覆盖一般的类的方法没有区别：

 

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);
    }
  }
} /

 

6，switch语句中的enum

 

enum Signal { GREEN, YELLOW, RED, }

public class TrafficLight {
  Signal color = Signal.RED;
  public void change() {
    switch(color) {
      // Note that you don't have to say Signal.RED
      // in the case statement:
      case RED:    color = Signal.GREEN;
                   break;
      case GREEN:  color = Signal.YELLOW;
                   break;
      case YELLOW: color = Signal.RED;
                   break;
    }
  }
  public String toString() {
    return "The traffic light is " + color;
  }
  public static void main(String[] args) {
    TrafficLight t = new TrafficLight();
    for(int i = 0; i < 7; i++) {
      print(t);
      t.change();
    }
  }
} /* Output:
The traffic light is RED
The traffic light is GREEN
The traffic light is YELLOW
The traffic light is RED
The traffic light is GREEN
The traffic light is YELLOW
The traffic light is RED

 

7，values（）是由编译器添加的static方法

 

enum Explore { HERE, THERE }

public class Reflection {
  public static Set<String> analyze(Class<?> enumClass) {
    print("----- Analyzing " + enumClass + " -----");
    print("Interfaces:");
    for(Type t : enumClass.getGenericInterfaces())
      print(t);
    print("Base: " + enumClass.getSuperclass());
    print("Methods: ");
    Set<String> methods = new TreeSet<String>();
    for(Method m : enumClass.getMethods())
      methods.add(m.getName());
    print(methods);
    return methods;
  }
  public static void main(String[] args) {
    Set<String> exploreMethods = analyze(Explore.class);
    Set<String> enumMethods = analyze(Enum.class);
    print("Explore.containsAll(Enum)? " +
      exploreMethods.containsAll(enumMethods));
    printnb("Explore.removeAll(Enum): ");
    exploreMethods.removeAll(enumMethods);
    print(exploreMethods);
    // Decompile the code for the enum:
    OSExecute.command("javap bin/Explore");
  }
} /* Output:

 

8，使用接口组织枚举

 

在一个接口的内部，创建实现该接口的枚举，以此将元素进行分组，可以达到将枚举元素分类组织的目的。

 

public interface Food {
  enum Appetizer implements Food {
    SALAD, SOUP, SPRING_ROLLS;
  }
  enum MainCourse implements Food {
    LASAGNE, BURRITO, PAD_THAI,
    LENTILS, HUMMOUS, VINDALOO;
  }
  enum Dessert implements Food {
    TIRAMISU, GELATO, BLACK_FOREST_CAKE,
    FRUIT, CREME_CARAMEL;
  }
  enum Coffee implements Food {
    BLACK_COFFEE, DECAF_COFFEE, ESPRESSO,
    LATTE, CAPPUCCINO, TEA, HERB_TEA;
  }
} ///:~

 

public class TypeOfFood {
  public static void main(String[] args) {
    Food food = Appetizer.SALAD;
    food = MainCourse.LASAGNE;
    food = Dessert.GELATO;
    food = Coffee.CAPPUCCINO;
  }
} ///:~

 

9，使用EnumSet代替标志

 

public enum AlarmPoints {
  STAIR1, STAIR2, LOBBY, OFFICE1, OFFICE2, OFFICE3,
  OFFICE4, BATHROOM, UTILITY, KITCHEN
} ///:~

 

import java.util.*;
import static enumerated.AlarmPoints.*;
import static net.mindview.util.Print.*;

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);
  }
} /* Output:

10，使用EnumMap

 

EnumMap是一种特殊的Map，他要求其中的key必须来自一个enum。

 

interface Command { void action(); }

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);
    }
  }
} /* Output:

 

11，常量相关的方法

 

java的enum有一个非常有趣的特性，它允许程序员为enum实例编写方法，从而为每个enum实例赋予各自不同的行为。

 

要实现常量相关的方法，你需要为enum定义一个或多个abstract方法，然后为每个enum实例实现该方法。

 

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());
  }
} /* (Execute to see output) *///:~

 

12，多路分发

 

package enumerated;
public enum Outcome { WIN, LOSE, DRAW } ///:~

 

import java.util.*;
import static enumerated.Outcome.*;

interface Item {
  Outcome compete(Item it);
  Outcome eval(Paper p);
  Outcome eval(Scissors s);
  Outcome eval(Rock r);
}

class Paper implements Item {
  public Outcome compete(Item it) { return it.eval(this); }
  public Outcome eval(Paper p) { return DRAW; }
  public Outcome eval(Scissors s) { return WIN; }
  public Outcome eval(Rock r) { return LOSE; }
  public String toString() { return "Paper"; }
} 

class Scissors implements Item {
  public Outcome compete(Item it) { return it.eval(this); }
  public Outcome eval(Paper p) { return LOSE; }
  public Outcome eval(Scissors s) { return DRAW; }
  public Outcome eval(Rock r) { return WIN; }
  public String toString() { return "Scissors"; }
}

class Rock implements Item {
  public Outcome compete(Item it) { return it.eval(this); }
  public Outcome eval(Paper p) { return WIN; }
  public Outcome eval(Scissors s) { return LOSE; }
  public Outcome eval(Rock r) { return DRAW; }
  public String toString() { return "Rock"; }
} 

public class RoShamBo1 {
  static final int SIZE = 20;
  private static Random rand = new Random(47);
  public static Item newItem() {
    switch(rand.nextInt(3)) {
      default:
      case 0: return new Scissors();
      case 1: return new Paper();
      case 2: return new Rock();
    }
  }
  public static void match(Item a, Item b) {
    System.out.println(
      a + " vs. " + b + ": " +  a.compete(b));
  }
  public static void main(String[] args) {
    for(int i = 0; i < SIZE; i++)
      match(newItem(), newItem());
  }
} /* Output: 

 

使用enum分发：

 

import static enumerated.Outcome.*;

public enum RoShamBo2 implements Competitor<RoShamBo2> {
  PAPER(DRAW, LOSE, WIN),
  SCISSORS(WIN, DRAW, LOSE),
  ROCK(LOSE, WIN, DRAW);
  private Outcome vPAPER, vSCISSORS, vROCK;
  RoShamBo2(Outcome paper,Outcome scissors,Outcome rock) {
    this.vPAPER = paper;
    this.vSCISSORS = scissors;
    this.vROCK = rock;
  } 
  public Outcome compete(RoShamBo2 it) {
    switch(it) {
      default:
      case PAPER: return vPAPER;
      case SCISSORS: return vSCISSORS;
      case ROCK: return vROCK;
    }
  }
  public static void main(String[] args) {
    RoShamBo.play(RoShamBo2.class, 20);
  }
} /* Output:

java1.0对术语 enumeration的选择是一个清晰性的反例。应该用iterater。