Chapter 19 - Exercise 7

(3) Find the source code for EnumSet and explain how it works.


以下代码为J2SE的numSet源码。



package java.util;

import sun.misc.SharedSecrets;

/**
 * A specialized {@link Set} implementation for use with enum types.  All of
 * the elements in an enum set must come from a single enum type that is
 * specified, explicitly or implicitly, when the set is created.  Enum sets
 * are represented internally as bit vectors.  This representation is
 * extremely compact and efficient. The space and time performance of this
 * class should be good enough to allow its use as a high-quality, typesafe
 * alternative to traditional <tt>int</tt>-based "bit flags."  Even bulk
 * operations (such as <tt>containsAll</tt> and <tt>retainAll</tt>) should
 * run very quickly if their argument is also an enum set.
 *
 * <p>The iterator returned by the <tt>iterator</tt> method traverses the
 * elements in their <i>natural order</i> (the order in which the enum
 * constants are declared).  The returned iterator is <i>weakly
 * consistent</i>: it will never throw {@link ConcurrentModificationException}
 * and it may or may not show the effects of any modifications to the set that
 * occur while the iteration is in progress.
 *
 * <p>Null elements are not permitted.  Attempts to insert a null element
 * will throw {@link NullPointerException}.  Attempts to test for the
 * presence of a null element or to remove one will, however, function
 * properly.
 *
 * <P>Like most collection implementations, <tt>EnumSet</tt> is not
 * synchronized.  If multiple threads access an enum set concurrently, and at
 * least one of the threads modifies the set, it should be synchronized
 * externally.  This is typically accomplished by synchronizing on some
 * object that naturally encapsulates the enum set.  If no such object exists,
 * the set should be "wrapped" using the {@link Collections#synchronizedSet}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access:
 *
 * <pre>
 * Set&lt;MyEnum&gt; s = Collections.synchronizedSet(EnumSet.noneOf(MyEnum.class));
 * </pre>
 *
 * <p>Implementation note: All basic operations execute in constant time.
 * They are likely (though not guaranteed) to be much faster than their
 * {@link HashSet} counterparts.  Even bulk operations execute in
 * constant time if their argument is also an enum set.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author Josh Bloch
 * @since 1.5
 * @see EnumMap
 * @serial exclude
 */
public abstract class EnumSet<E extends Enum<E>> extends AbstractSet<E>
    implements Cloneable, java.io.Serializable
{
    /**
     * The class of all the elements of this set.
     */
    final Class<E> elementType;

    /**
     * All of the values comprising T.  (Cached for performance.)
     */
    final Enum[] universe;

    private static Enum[] ZERO_LENGTH_ENUM_ARRAY = new Enum[0];

    EnumSet(Class<E>elementType, Enum[] universe) {
        this.elementType = elementType;
        this.universe    = universe;
    }

    /**
     * Creates an empty enum set with the specified element type.
     *
     * @param elementType the class object of the element type for this enum
     *     set
     * @throws NullPointerException if <tt>elementType</tt> is null
     */
    public static <E extends Enum<E>> EnumSet<E> noneOf(Class<E> elementType) {
        Enum[] universe = getUniverse(elementType);
        if (universe == null)
            throw new ClassCastException(elementType + " not an enum");

        if (universe.length <= 64)
            return new RegularEnumSet<>(elementType, universe);
        else
            return new JumboEnumSet<>(elementType, universe);
    }

    /**
     * Creates an enum set containing all of the elements in the specified
     * element type.
     *
     * @param elementType the class object of the element type for this enum
     *     set
     * @throws NullPointerException if <tt>elementType</tt> is null
     */
    public static <E extends Enum<E>> EnumSet<E> allOf(Class<E> elementType) {
        EnumSet<E> result = noneOf(elementType);
        result.addAll();
        return result;
    }

    /**
     * Adds all of the elements from the appropriate enum type to this enum
     * set, which is empty prior to the call.
     */
    abstract void addAll();

    /**
     * Creates an enum set with the same element type as the specified enum
     * set, initially containing the same elements (if any).
     *
     * @param s the enum set from which to initialize this enum set
     * @throws NullPointerException if <tt>s</tt> is null
     */
    public static <E extends Enum<E>> EnumSet<E> copyOf(EnumSet<E> s) {
        return s.clone();
    }

    /**
     * Creates an enum set initialized from the specified collection.  If
     * the specified collection is an <tt>EnumSet</tt> instance, this static
     * factory method behaves identically to {@link #copyOf(EnumSet)}.
     * Otherwise, the specified collection must contain at least one element
     * (in order to determine the new enum set's element type).
     *
     * @param c the collection from which to initialize this enum set
     * @throws IllegalArgumentException if <tt>c</tt> is not an
     *     <tt>EnumSet</tt> instance and contains no elements
     * @throws NullPointerException if <tt>c</tt> is null
     */
    public static <E extends Enum<E>> EnumSet<E> copyOf(Collection<E> c) {
        if (c instanceof EnumSet) {
            return ((EnumSet<E>)c).clone();
        } else {
            if (c.isEmpty())
                throw new IllegalArgumentException("Collection is empty");
            Iterator<E> i = c.iterator();
            E first = i.next();
            EnumSet<E> result = EnumSet.of(first);
            while (i.hasNext())
                result.add(i.next());
            return result;
        }
    }

    /**
     * Creates an enum set with the same element type as the specified enum
     * set, initially containing all the elements of this type that are
     * <i>not</i> contained in the specified set.
     *
     * @param s the enum set from whose complement to initialize this enum set
     * @throws NullPointerException if <tt>s</tt> is null
     */
    public static <E extends Enum<E>> EnumSet<E> complementOf(EnumSet<E> s) {
        EnumSet<E> result = copyOf(s);
        result.complement();
        return result;
    }

    /**
     * Creates an enum set initially containing the specified element.
     *
     * Overloadings of this method exist to initialize an enum set with
     * one through five elements.  A sixth overloading is provided that
     * uses the varargs feature.  This overloading may be used to create
     * an enum set initially containing an arbitrary number of elements, but
     * is likely to run slower than the overloadings that do not use varargs.
     *
     * @param e the element that this set is to contain initially
     * @throws NullPointerException if <tt>e</tt> is null
     * @return an enum set initially containing the specified element
     */
    public static <E extends Enum<E>> EnumSet<E> of(E e) {
        EnumSet<E> result = noneOf(e.getDeclaringClass());
        result.add(e);
        return result;
    }

    /**
     * Creates an enum set initially containing the specified elements.
     *
     * Overloadings of this method exist to initialize an enum set with
     * one through five elements.  A sixth overloading is provided that
     * uses the varargs feature.  This overloading may be used to create
     * an enum set initially containing an arbitrary number of elements, but
     * is likely to run slower than the overloadings that do not use varargs.
     *
     * @param e1 an element that this set is to contain initially
     * @param e2 another element that this set is to contain initially
     * @throws NullPointerException if any parameters are null
     * @return an enum set initially containing the specified elements
     */
    public static <E extends Enum<E>> EnumSet<E> of(E e1, E e2) {
        EnumSet<E> result = noneOf(e1.getDeclaringClass());
        result.add(e1);
        result.add(e2);
        return result;
    }

    /**
     * Creates an enum set initially containing the specified elements.
     *
     * Overloadings of this method exist to initialize an enum set with
     * one through five elements.  A sixth overloading is provided that
     * uses the varargs feature.  This overloading may be used to create
     * an enum set initially containing an arbitrary number of elements, but
     * is likely to run slower than the overloadings that do not use varargs.
     *
     * @param e1 an element that this set is to contain initially
     * @param e2 another element that this set is to contain initially
     * @param e3 another element that this set is to contain initially
     * @throws NullPointerException if any parameters are null
     * @return an enum set initially containing the specified elements
     */
    public static <E extends Enum<E>> EnumSet<E> of(E e1, E e2, E e3) {
        EnumSet<E> result = noneOf(e1.getDeclaringClass());
        result.add(e1);
        result.add(e2);
        result.add(e3);
        return result;
    }

    /**
     * Creates an enum set initially containing the specified elements.
     *
     * Overloadings of this method exist to initialize an enum set with
     * one through five elements.  A sixth overloading is provided that
     * uses the varargs feature.  This overloading may be used to create
     * an enum set initially containing an arbitrary number of elements, but
     * is likely to run slower than the overloadings that do not use varargs.
     *
     * @param e1 an element that this set is to contain initially
     * @param e2 another element that this set is to contain initially
     * @param e3 another element that this set is to contain initially
     * @param e4 another element that this set is to contain initially
     * @throws NullPointerException if any parameters are null
     * @return an enum set initially containing the specified elements
     */
    public static <E extends Enum<E>> EnumSet<E> of(E e1, E e2, E e3, E e4) {
        EnumSet<E> result = noneOf(e1.getDeclaringClass());
        result.add(e1);
        result.add(e2);
        result.add(e3);
        result.add(e4);
        return result;
    }

    /**
     * Creates an enum set initially containing the specified elements.
     *
     * Overloadings of this method exist to initialize an enum set with
     * one through five elements.  A sixth overloading is provided that
     * uses the varargs feature.  This overloading may be used to create
     * an enum set initially containing an arbitrary number of elements, but
     * is likely to run slower than the overloadings that do not use varargs.
     *
     * @param e1 an element that this set is to contain initially
     * @param e2 another element that this set is to contain initially
     * @param e3 another element that this set is to contain initially
     * @param e4 another element that this set is to contain initially
     * @param e5 another element that this set is to contain initially
     * @throws NullPointerException if any parameters are null
     * @return an enum set initially containing the specified elements
     */
    public static <E extends Enum<E>> EnumSet<E> of(E e1, E e2, E e3, E e4,
                                                    E e5)
    {
        EnumSet<E> result = noneOf(e1.getDeclaringClass());
        result.add(e1);
        result.add(e2);
        result.add(e3);
        result.add(e4);
        result.add(e5);
        return result;
    }

    /**
     * Creates an enum set initially containing the specified elements.
     * This factory, whose parameter list uses the varargs feature, may
     * be used to create an enum set initially containing an arbitrary
     * number of elements, but it is likely to run slower than the overloadings
     * that do not use varargs.
     *
     * @param first an element that the set is to contain initially
     * @param rest the remaining elements the set is to contain initially
     * @throws NullPointerException if any of the specified elements are null,
     *     or if <tt>rest</tt> is null
     * @return an enum set initially containing the specified elements
     */
    @SafeVarargs
    public static <E extends Enum<E>> EnumSet<E> of(E first, E... rest) {
        EnumSet<E> result = noneOf(first.getDeclaringClass());
        result.add(first);
        for (E e : rest)
            result.add(e);
        return result;
    }

    /**
     * Creates an enum set initially containing all of the elements in the
     * range defined by the two specified endpoints.  The returned set will
     * contain the endpoints themselves, which may be identical but must not
     * be out of order.
     *
     * @param from the first element in the range
     * @param to the last element in the range
     * @throws NullPointerException if {@code from} or {@code to} are null
     * @throws IllegalArgumentException if {@code from.compareTo(to) > 0}
     * @return an enum set initially containing all of the elements in the
     *         range defined by the two specified endpoints
     */
    public static <E extends Enum<E>> EnumSet<E> range(E from, E to) {
        if (from.compareTo(to) > 0)
            throw new IllegalArgumentException(from + " > " + to);
        EnumSet<E> result = noneOf(from.getDeclaringClass());
        result.addRange(from, to);
        return result;
    }

    /**
     * Adds the specified range to this enum set, which is empty prior
     * to the call.
     */
    abstract void addRange(E from, E to);

    /**
     * Returns a copy of this set.
     *
     * @return a copy of this set
     */
    public EnumSet<E> clone() {
        try {
            return (EnumSet<E>) super.clone();
        } catch(CloneNotSupportedException e) {
            throw new AssertionError(e);
        }
    }

    /**
     * Complements the contents of this enum set.
     */
    abstract void complement();

    /**
     * Throws an exception if e is not of the correct type for this enum set.
     */
    final void typeCheck(E e) {
        Class eClass = e.getClass();
        if (eClass != elementType && eClass.getSuperclass() != elementType)
            throw new ClassCastException(eClass + " != " + elementType);
    }

    /**
     * Returns all of the values comprising E.
     * The result is uncloned, cached, and shared by all callers.
     */
    private static <E extends Enum<E>> E[] getUniverse(Class<E> elementType) {
        return SharedSecrets.getJavaLangAccess()
                                        .getEnumConstantsShared(elementType);
    }

    /**
     * This class is used to serialize all EnumSet instances, regardless of
     * implementation type.  It captures their "logical contents" and they
     * are reconstructed using public static factories.  This is necessary
     * to ensure that the existence of a particular implementation type is
     * an implementation detail.
     *
     * @serial include
     */
    private static class SerializationProxy <E extends Enum<E>>
        implements java.io.Serializable
    {
        /**
         * The element type of this enum set.
         *
         * @serial
         */
        private final Class<E> elementType;

        /**
         * The elements contained in this enum set.
         *
         * @serial
         */
        private final Enum[] elements;

        SerializationProxy(EnumSet<E> set) {
            elementType = set.elementType;
            elements = set.toArray(ZERO_LENGTH_ENUM_ARRAY);
        }

        private Object readResolve() {
            EnumSet<E> result = EnumSet.noneOf(elementType);
            for (Enum e : elements)
                result.add((E)e);
            return result;
        }

        private static final long serialVersionUID = 362491234563181265L;
    }

    Object writeReplace() {
        return new SerializationProxy<>(this);
    }

    // readObject method for the serialization proxy pattern
    // See Effective Java, Second Ed., Item 78.
    private void readObject(java.io.ObjectInputStream stream)
        throws java.io.InvalidObjectException {
        throw new java.io.InvalidObjectException("Proxy required");
    }
}


首先,EnumSet是抽象类。和大多数类库自带Set一样,它继承自AbstractSet,获得了AbstractSet和AbstractMap的部分功能。

然后,至少我见到了有两个类是继承EnumSet的:一个RegularEnumSet,一个JumboEnumSet。但之所以我们在代码中很少见到直接调用这两个类的构造函数,是因为EnumSet的构造函数承担了根据枚举规模选择调用它们其中的哪一个构造函数。通过noneOf()函数中的如下部分代码,基本可以判断:RegularEnumSet用在枚举规模小于64的情况下。而JamboEnumSet用在大于64的情况下。要做这样的拆分,估计还是出于效率的考虑。毕竟可扩展大小的开销不小。



    public static <E extends Enum<E>> EnumSet<E> noneOf(Class<E> elementType) {
        Enum[] universe = getUniverse(elementType);
        if (universe == null)
            throw new ClassCastException(elementType + " not an enum");

        if (universe.length <= 64)
            return new RegularEnumSet<>(elementType, universe);
        else
            return new JumboEnumSet<>(elementType, universe);
    }


在RegularEnumSet中维护的主力数据结构确实如书中所说是一个long型:



private long elements = 0L;


然后看它的add()函数:明显是把long型64 bit中,某个枚举实例的次序属性(ordinal)所对应的位置设为1。用1 bit表示这个枚举实例的存在。所以也确实像书里说的,long型中的每一位,映射到一个具体的枚举实例。



    public boolean add(E e) {
        typeCheck(e);

        long oldElements = elements;
        elements |= (1L << ((Enum)e).ordinal());
        return elements != oldElements;
    }


至于JumboEnumSet,其中维护的主力容器是一个long型数组。看看它的构造器:



class JumboEnumSet<E extends Enum<E>> extends EnumSet<E> {
	private static final long serialVersionUID = 334349849919042784L;
	
	private long elements[];

    private int size = 0;

    JumboEnumSet(Class<E>elementType, Enum[] universe) {
        super(elementType, universe);
        elements = new long[(universe.length + 63) >>> 6];
    }
	//... ...


相当于枚举规模对64取模获得的数组长度。


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