JAVA-Enum 枚举

【参考】枚举类名建议带上 Enum 后缀，枚举成员名称需要全大写，单词间用下划线隔开。
说明：枚举其实就是特殊的类，域成员均为常量，且构造方法被默认强制是私有。
正例：枚举名字为 ProcessStatusEnum 的成员名称：SUCCESS / UNKNOWN_REASON。

创建一个枚举类

public enum DayEnum {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY
}

使用

public class EnumTest {
    DayEnum day;

    public EnumTest(DayEnum day) {
        this.day = day;
    }

    public void tellItLikeItIs() {
        switch (day) {
            case MONDAY:
                System.out.println("Mondays are bad.");
                break;
            case FRIDAY:
                System.out.println("Fridays are better.");
                break;
            case SATURDAY: case SUNDAY:
                System.out.println("Weekends are best.");
                break;
            default:
                System.out.println("Midweek days are so-so.");
                break;
        }
    }

    public static void main(String[] args) {
        EnumTest firstDay = new EnumTest(DayEnum.MONDAY);
        firstDay.tellItLikeItIs();
        EnumTest thirdDay = new EnumTest(DayEnum.WEDNESDAY);
        thirdDay.tellItLikeItIs();
        EnumTest fifthDay = new EnumTest(DayEnum.FRIDAY);
        fifthDay.tellItLikeItIs();

        EnumTest sixthDay = new EnumTest(DayEnum.SATURDAY);
        sixthDay.tellItLikeItIs();
        EnumTest seventhDay = new EnumTest(DayEnum.SUNDAY);
        seventhDay.tellItLikeItIs();
    }
}

带构造器枚举类

当存在字段和方法时，枚举常量列表必须以分号结尾，且定义枚举常量在字段和方法之前

枚举类型的构造函数必须是私有(没有修饰符默认为 private)，不能自己手动调用​​枚举类的构造函数，JVM 会自动创建在枚举类中定义的常量。所以看似一样的枚举值一定不相等，因为这可能不是同一个对象实例

/**
 * 行星上一个物体的重力和重量
 */
public enum Planet {
    MERCURY(3.303e+23, 2.4397e6),
    VENUS(4.869e+24, 6.0518e6),
    EARTH(5.976e+24, 6.37814e6),
    MARS(6.421e+23, 3.3972e6),
    JUPITER(1.9e+27, 7.1492e7),
    SATURN(5.688e+26, 6.0268e7),
    URANUS(8.686e+25, 2.5559e7),
    NEPTUNE(1.024e+26, 2.4746e7);

    // 以公斤为单位的
    private final double mass;
    // 以米为单位的
    private final double radius;

    Planet(double mass, double radius) {
        this.mass = mass;
        this.radius = radius;
    }

    private double mass() {
        return mass;
    }

    private double radius() {
        return radius;
    }

    // 万有引力常数 (m3 kg-1 s-2)
    public static final double G = 6.67300E-11;

    double surfaceGravity() {
        return G * mass / (radius * radius);
    }

    double surfaceWeight(double otherMass) {
        return otherMass * surfaceGravity();
    }
}

使用

public static void main(String[] args) {
    double earthWeight = Double.parseDouble("175");
    double mass = earthWeight / EARTH.surfaceGravity();
    for (Planet p : Planet.values()) {
        System.out.printf("你在 %s 上的体重是 %f%n", p, p.surfaceWeight(mass));
    }
}

Enum  相关方法

public static void main(String[] args) {
    // 获取所有枚举常量
    DayEnum[] values = DayEnum.values();
    for (DayEnum value : values) {
        System.out.print(value);
        // 枚举常量在枚举类中的序号，零开始
        System.out.println("\t" + value.ordinal());
    }

    // 获取指定枚举常量，若不存在会报异常
    System.out.println(DayEnum.valueOf("SATURDAY"));
    System.out.println(DayEnum.valueOf(DayEnum.class, "SATURDAY"));

    // 比较枚举常量，与 == 方式相同，返回布尔值
    System.out.println(DayEnum.FRIDAY.equals(DayEnum.FRIDAY));
    // 比较枚举常量，采用 ordinal 序号方式，返回数字
    System.out.println(DayEnum.FRIDAY.compareTo(DayEnum.FRIDAY));
}

反编译 DayEnum

所有枚举类都隐式继承了抽象类 java.lang.Enum，用 javap 工具可以看到。所以枚举类不能继承其他类，JAVA 中为单继承，多实现

枚举常量会隐式被修饰成公共的静态常量，即枚举常量只会有一个实例，比较时可以使用 ==，枚举类的 equals 方法是 final 修饰

public final class javaenum.DayEnum extends java.lang.Enum<javaenum.DayEnum> {
  public static final javaenum.DayEnum SUNDAY;

  public static final javaenum.DayEnum MONDAY;

  public static final javaenum.DayEnum TUESDAY;

  public static final javaenum.DayEnum WEDNESDAY;

  public static final javaenum.DayEnum THURSDAY;

  public static final javaenum.DayEnum FRIDAY;

  public static final javaenum.DayEnum SATURDAY;

  public static javaenum.DayEnum[] values();
    Code:
       0: getstatic     #1                  // Field $VALUES:[Ljavaenum/DayEnum;
       3: invokevirtual #2                  // Method "[Ljavaenum/DayEnum;".clone:()Ljava/lang/Object;
       6: checkcast     #3                  // class "[Ljavaenum/DayEnum;"
       9: areturn

  public static javaenum.DayEnum valueOf(java.lang.String);
    Code:
       0: ldc           #4                  // class javaenum/DayEnum
       2: aload_0
       3: invokestatic  #5                  // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
       6: checkcast     #4                  // class javaenum/DayEnum
       9: areturn

  static {};
    Code:
       0: new           #4                  // class javaenum/DayEnum
       3: dup
       4: ldc           #7                  // String SUNDAY
       6: iconst_0
       7: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      10: putstatic     #9                  // Field SUNDAY:Ljavaenum/DayEnum;
      13: new           #4                  // class javaenum/DayEnum
      16: dup
      17: ldc           #10                 // String MONDAY
      19: iconst_1
      20: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      23: putstatic     #11                 // Field MONDAY:Ljavaenum/DayEnum;
      26: new           #4                  // class javaenum/DayEnum
      29: dup
      30: ldc           #12                 // String TUESDAY
      32: iconst_2
      33: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      36: putstatic     #13                 // Field TUESDAY:Ljavaenum/DayEnum;
      39: new           #4                  // class javaenum/DayEnum
      42: dup
      43: ldc           #14                 // String WEDNESDAY
      45: iconst_3
      46: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      49: putstatic     #15                 // Field WEDNESDAY:Ljavaenum/DayEnum;
      52: new           #4                  // class javaenum/DayEnum
      55: dup
      56: ldc           #16                 // String THURSDAY
      58: iconst_4
      59: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      62: putstatic     #17                 // Field THURSDAY:Ljavaenum/DayEnum;
      65: new           #4                  // class javaenum/DayEnum
      68: dup
      69: ldc           #18                 // String FRIDAY
      71: iconst_5
      72: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      75: putstatic     #19                 // Field FRIDAY:Ljavaenum/DayEnum;
      78: new           #4                  // class javaenum/DayEnum
      81: dup
      82: ldc           #20                 // String SATURDAY
      84: bipush        6
      86: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      89: putstatic     #21                 // Field SATURDAY:Ljavaenum/DayEnum;
      92: bipush        7
      94: anewarray     #4                  // class javaenum/DayEnum
      97: dup
      98: iconst_0
      99: getstatic     #9                  // Field SUNDAY:Ljavaenum/DayEnum;
     102: aastore
     103: dup
     104: iconst_1
     105: getstatic     #11                 // Field MONDAY:Ljavaenum/DayEnum;
     108: aastore
     109: dup
     110: iconst_2
     111: getstatic     #13                 // Field TUESDAY:Ljavaenum/DayEnum;
     114: aastore
     115: dup
     116: iconst_3
     117: getstatic     #15                 // Field WEDNESDAY:Ljavaenum/DayEnum;
     120: aastore
     121: dup
     122: iconst_4
     123: getstatic     #17                 // Field THURSDAY:Ljavaenum/DayEnum;
     126: aastore
     127: dup
     128: iconst_5
     129: getstatic     #19                 // Field FRIDAY:Ljavaenum/DayEnum;
     132: aastore
     133: dup
     134: bipush        6
     136: getstatic     #21                 // Field SATURDAY:Ljavaenum/DayEnum;
     139: aastore
     140: putstatic     #1                  // Field $VALUES:[Ljavaenum/DayEnum;
     143: return
}

Enum 源码

/**
 * 使用枚举类型作为 Collection 或 Map 中 Key 的类型时
 * 可用 {@linkplain java.util.EnumSet set} 和 {@linkplain java.util.EnumMap map} 实现，专业且高效
 */
public abstract class Enum<E extends Enum<E>> implements Comparable<E>, Serializable {
    // 枚举常量的名称，获取时使用{@link #toString}方法而不是访问此字段
    private final String name;

    // 此方法主要用于在特殊情况下使用
    public final String name() {
        return name;
    }

    // 此枚举常数的序数（其位置在枚举声明中，分配初始常量序数为零）
    // 专门设计供复杂基于枚举的数据结构使用，例如 {@link java.util.EnumSet} 和 {@link java.util.EnumMap}
    private final int ordinal;

    public final int ordinal() {
        return ordinal;
    }

    /**
     * 唯一构造函数。程序员无法调用，由编译器调用
     * @param name    枚举常量的名称，用于声明它的标识符
     * @param ordinal 枚举常量的序数（声明它在枚举中的位置，初始常量的序数为零）
     */
    protected Enum(String name, int ordinal) {
        this.name = name;
        this.ordinal = ordinal;
    }

    // 返回声明中包含的此枚举常量的名称，可以覆盖该方法
    public String toString() {
        return name;
    }

    public final boolean equals(Object other) {
        return this == other;
    }

    public final int hashCode() {
        return super.hashCode();
    }

    // 保证了枚举类永远不会被克隆
    protected final Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }

    // 返回负整数，零或正整数
    // 枚举常量只能与同一枚举类型的其他枚举常量相比较。此方法实现是根据 ordinal 值比较
    public final int compareTo(E o) {
        Enum<?> other = (Enum<?>) o;
        Enum<E> self = this;
        if (self.getClass() != other.getClass() && self.getDeclaringClass() != other.getDeclaringClass())
            throw new ClassCastException();
        return self.ordinal - other.ordinal;
    }

    // 返回与此枚举常量的枚举类型对应的 Class 对象
    @SuppressWarnings("unchecked")
    public final Class<E> getDeclaringClass() {
        Class<?> clazz = getClass();
        Class<?> zuper = clazz.getSuperclass();
        return (zuper == Enum.class) ? (Class<E>) clazz : (Class<E>) zuper;
    }

    /**
     * @param enumType 枚举类型的 Class 对象，用于从中返回常量
     * @param name     要返回的常量的名称
     * @param <T>      要返回常量的其枚举类型
     * @return 具有指定名称的指定枚举类型的枚举常量
     */
    public static <T extends Enum<T>> T valueOf(Class<T> enumType, String name) {
        T result = enumType.enumConstantDirectory().get(name);
        if (result != null)
            return result;
        if (name == null)
            throw new NullPointerException("Name is null");
        throw new IllegalArgumentException("No enum constant " + enumType.getCanonicalName() + "." + name);
    }

    // 枚举类不能有finalize方法
    protected final void finalize() {}

    // 默认防止反序列化
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        throw new InvalidObjectException("can't deserialize enum");
    }

    private void readObjectNoData() throws ObjectStreamException {
        throw new InvalidObjectException("can't deserialize enum");
    }
}

---

https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html

https://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.9

https://www.ibm.com/developerworks/cn/java/j-lo-enum/index.html

https://blog.csdn.net/javazejian/article/details/71333103