再談在Java中使用枚舉

public void foo(Day);

foo(4);    // compilation error

public static final int SUNDAY    = 0;
public static final int MONDAY    = 1;
public static final int TUESDAY   = 2;
public static final int WEDNESDAY = 3;
public static final int THURSDAY  = 4;
public static final int FRIDAY    = 5;
public static final int SATURDAY  = 6;

for (Day d = SUNDAY;  d <= SATURDAY;  ++d) {
    switch(d) {
        case MONDAY:     ...;
            break;
        case TUESDAY:    ...;
            break;
        case WEDNESDAY:  ...;
            break;
        case THURSDAY:   ...;
            break;
        case FRIDAY:     ...;
            break;
        case SATURDAY:
        case SUNDAY:    ...;
    }
}

public static final int SUNDAY    = 0;
public static final int MONDAY    = 1;
public static final int TUESDAY   = 2;
public static final int WEDNESDAY = 3;
public static final int THURSDAY  = 4;
public static final int FRIDAY    = 5;
public static final int SATURDAY  = 6;

import java.util.Map;
import java.util.HashMap;
import java.util.Iterator;
import java.io.Serializable;
import java.io.InvalidObjectException;
public final class Day implements Comparable, Serializable {
    private static int size    = 0;
    private static int nextOrd = 0;
    private static Map nameMap = new HashMap(10);
    private static Day first   = null;
    private static Day last    = null;
    private final int    ord;
    private final String label;
    private Day prev;
    private Day next;
    public static final Day SUNDAY    = new Day("SUNDAY");
    public static final Day MONDAY    = new Day("MONDAY");
    public static final Day TUESDAY   = new Day("TUESDAY");
    public static final Day WEDNESDAY = new Day("WEDNESDAY");
    public static final Day THURSDAY  = new Day("THURSDAY");
    public static final Day FRIDAY    = new Day("FRIDAY");
    public static final Day SATURDAY  = new Day("SATURDAY");
    /**
     * 用所給的標簽創建一個新的day.
     * (Uses default value for ord.)
     */
    private Day(String label) {
        this(label, nextOrd);
    }
    /**
     * Constructs a new Day with its label and ord value.
     */
    private Day(String label, int ord) {
        this.label = label;
        this.ord   = ord;
        ++size;
        nextOrd = ord + 1;
        nameMap.put(label, this);
        if (first == null)
            first = this;
        if (last != null) {
            this.prev = last;
            last.next = this;
        }
        last = this;
    }
    /**
     * Compares two Day objects based on their ordinal values.
     * Satisfies requirements of interface java.lang.Comparable.
     */
    public int compareTo(Object obj) {
        return ord - ((Day)obj).ord;
    }
    /**
     * Compares two Day objects for equality.  Returns true
     * only if the specified Day is equal to this one.
     */
    public boolean equals(Object obj) {
        return super.equals(obj);
    }
    /**
     * Returns a hash code value for this Day.
     */
    public int hashCode() {
        return super.hashCode();
    }
    /**
     * Resolves deserialized Day objects.
     * @throws InvalidObjectException if deserialization fails.
     */
    private Object readResolve() throws InvalidObjectException {
        Day d = get(label);
        if (d != null)
            return d;
        else {
            String msg = "invalid deserialized object:  label = ";
            throw new InvalidObjectException(msg + label);
        }
    }
    /**
     * Returns Day with the specified label.
     * Returns null if not found.
     */
    public static Day get(String label) {
        return (Day) nameMap.get(label);
    }
    /**
     * Returns the label for this Day.
     */
    public String toString() {
        return label;
    }
    /**
     * Always throws CloneNotSupportedException;  guarantees that
     * Day objects are never cloned.
     *
     * @return (never returns)
     */
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }
    /**
     * Returns an iterator over all Day objects in declared order.
     */
    public static Iterator iterator() {
        // anonymous inner class
        return new Iterator()
        {
            private Day current = first;
            public boolean hasNext() {
                return current != null;
            }
            public Object next() {
                Day d   = current;
                current = current.next();
                return d;
            }
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
    }
    /**
     * Returns the ordinal value of this Day.
     */
    public int ord() {
        return this.ord;
    }
    /**
     * Returns the number of declared Day objects.
     */
    public static int size() {
        return size;
    }
    /**
     * Returns the first declared Day.
     */
    public static Day first() {
        return first;
    }
    /**
     * Returns the last declared Day.
     */
    public static Day last() {
        return last;
    }
    /**
     * Returns the previous Day before this one in declared order.
     * Returns null for the first declared Day.
     */
    public Day prev() {
        return this.prev;
    }
    /**
     * Returns the next Day after this one in declared order.
     * Returns null for the last declared Day.
     */
    public Day next() {
        return this.next;
    }
}