阅读EnumSet抽象类源码

EnumSet

EnumSet是Java枚举类型的泛型容器，Java既然有了SortedSet、TreeSet、HashSet等容器，为何还要多一个EnumSet<T>呢？答案肯定是EnumSet有一定的特性，举个例子，EnumSet的速度很快。其他特性就不一一列举了，毕竟本文的内容不是介绍EnumSet的特性。

专门为枚举类设计的集合类，所有元素必须是枚举类型

EnumSet的集合元素是有序的，内部以位向量的形成存储，因此占用内存小，效率高

不允许加入null元素

源码

package java.util;

import sun.misc.SharedSecrets;

public abstract class EnumSet<E extends Enum<E>> extends AbstractSet<E>
  implements Cloneable, java.io.Serializable
{
  /**
   * 元素类型
   */
  final Class<E> elementType;

  /**
   * 通过数组存储元素
   */
  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;
  }

  /**
   * 创造一个空的 enum set 并制定其元素类型
   * @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);
  }

  /**
   * 创建一个包含所有在指定元素类型的元素的枚举set
   *
   * @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();

  /**
   * 创建一个枚举设置相同的元素类型与指定枚举set
   *
   * @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();
  }

  /**
   * 创建一个枚举集从指定集合初始化，最初包含相同的元素
   * @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;
    }
  }

  /**
   * 创建一个枚举集合，其元素与 s 相同
   * @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;
  }

  /**
   * 1 个元素枚举集合
   *
   * @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;
  }

  /**
   * 2 个元素枚举集合
   *
   * @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;
  }

  /**
   * 3 个元素枚举集合
   *
   * @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;
  }

  /**
   * 4 个元素枚举集合
   * @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;
  }

  /**
   * 5 个元素枚举集合
   *
   * @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;
  }

  /**
   * n 个元素枚举集合
   *
   * @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;
  }

  /**
   * 区间内元素的 枚举集合
   *
   * @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");
  }
}

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