LinkedHashSet 源代码

1，在LinkedHashSet源代码中可以发现，只是简单的实现了构造方法，其实具体实现在HashSet中，如果是LinkedHashSet，则底层是用LinkedHashMap实现的

    //构造方法，map用的是LinkedHashMap
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

2、源代码

/*
 * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;

/**
 * <p>Hash table and linked list implementation of the <tt>Set</tt> interface,
 * with predictable iteration order.  This implementation differs from
 * <tt>HashSet</tt> in that it maintains a doubly-linked list running through
 * all of its entries.  This linked list defines the iteration ordering,
 * which is the order in which elements were inserted into the set
 * (<i>insertion-order</i>).  Note that insertion order is <i>not</i> affected
 * if an element is <i>re-inserted</i> into the set.  (An element <tt>e</tt>
 * is reinserted into a set <tt>s</tt> if <tt>s.add(e)</tt> is invoked when
 * <tt>s.contains(e)</tt> would return <tt>true</tt> immediately prior to
 * the invocation.)
 *
 * <p>This implementation spares its clients from the unspecified, generally
 * chaotic ordering provided by {@link HashSet}, without incurring the
 * increased cost associated with {@link TreeSet}.  It can be used to
 * produce a copy of a set that has the same order as the original, regardless
 * of the original set's implementation:
 * <pre>
 *     void foo(Set s) {
 *         Set copy = new LinkedHashSet(s);
 *         ...
 *     }
 * </pre>
 * This technique is particularly useful if a module takes a set on input,
 * copies it, and later returns results whose order is determined by that of
 * the copy.  (Clients generally appreciate having things returned in the same
 * order they were presented.)
 *
 * <p>This class provides all of the optional <tt>Set</tt> operations, and
 * permits null elements.  Like <tt>HashSet</tt>, it provides constant-time
 * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and
 * <tt>remove</tt>), assuming the hash function disperses elements
 * properly among the buckets.  Performance is likely to be just slightly
 * below that of <tt>HashSet</tt>, due to the added expense of maintaining the
 * linked list, with one exception: Iteration over a <tt>LinkedHashSet</tt>
 * requires time proportional to the <i>size</i> of the set, regardless of
 * its capacity.  Iteration over a <tt>HashSet</tt> is likely to be more
 * expensive, requiring time proportional to its <i>capacity</i>.
 *
 * <p>A linked hash set has two parameters that affect its performance:
 * <i>initial capacity</i> and <i>load factor</i>.  They are defined precisely
 * as for <tt>HashSet</tt>.  Note, however, that the penalty for choosing an
 * excessively high value for initial capacity is less severe for this class
 * than for <tt>HashSet</tt>, as iteration times for this class are unaffected
 * by capacity.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a linked hash set concurrently, and at least
 * one of the threads modifies the set, it <em>must</em> be synchronized
 * externally.  This is typically accomplished by synchronizing on some
 * object that naturally encapsulates the set.
 *
 * If no such object exists, the set should be "wrapped" using the
 * {@link Collections#synchronizedSet Collections.synchronizedSet}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the set: <pre>
 *   Set s = Collections.synchronizedSet(new LinkedHashSet(...));</pre>
 *
 * <p>The iterators returned by this class's <tt>iterator</tt> method are
 * <em>fail-fast</em>: if the set is modified at any time after the iterator
 * is created, in any way except through the iterator's own <tt>remove</tt>
 * method, the iterator will throw a {@link ConcurrentModificationException}.
 * Thus, in the face of concurrent modification, the iterator fails quickly
 * and cleanly, rather than risking arbitrary, non-deterministic behavior at
 * an undetermined time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:   <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements maintained by this set
 *
 * @author  Josh Bloch
 * @see     Object#hashCode()
 * @see     Collection
 * @see     Set
 * @see     HashSet
 * @see     TreeSet
 * @see     Hashtable
 * @since   1.4
 */
/*
具有可预知迭代顺序的 Set 接口的哈希表和链接列表实现。此实现与 HashSet 的不同之外在于，
后者维护着一个运行于所有条目的双重链接列表。此链接列表定义了迭代顺序，即按照将元素插入
到 set 中的顺序（插入顺序）进行迭代。注意，插入顺序不 受在 set 中重新插入的 元素的影响。
（如果在 s.contains(e) 返回 true 后立即调用 s.add(e)，则元素 e 会被重新插入到 set s 中。）

此实现可以让客户免遭未指定的、由 HashSet 提供的通常杂乱无章的排序工作，而又不致引起与
TreeSet 关联的成本增加。使用它可以生成一个与原来顺序相同的 set 副本，并且与原 set 的实现无关：

     void foo(Set s) {
         Set copy = new LinkedHashSet(s);
         ...
     }
 如果模块通过输入得到一个 set，复制这个 set，然后返回由此副本决定了顺序的结果，
 这种情况下这项技术特别有用。（客户通常期望内容返回的顺序与它们出现的顺序相同。）
此类提供所有可选的 Set 操作，并且允许 null 元素。与 HashSet 一样，它可以为基本操作
（add、contains 和 remove）提供稳定的性能，假定哈希函数将元素正确地分布到存储段中。
由于增加了维护链接列表的开支，其性能很可能会比 HashSet 稍逊一筹，不过，这一点例外：
LinkedHashSet 迭代所需时间与 set 的大小 成正比，而与容量无关。HashSet 迭代很可能支出较大，
因为它所需迭代时间与其容量 成正比。

链接的哈希 set 有两个影响其性能的参数：初始容量 和加载因子。它们与 HashSet 中的定义极其相同。
注意，为初始容量选择非常高的值对此类的影响比对 HashSet 要小，因为此类的迭代时间不受容量的影响。

注意，此实现不是同步的。如果多个线程同时访问链接的哈希 set，而其中至少一个线程修改了该 set，
则它必须 保持外部同步。这一般通过对自然封装该 set 的对象进行同步操作来完成。如果不存在这样的对象，
则应该使用 Collections.synchronizedSet 方法来“包装”该 set。最好在创建时完成这一操作，以防止意外的非同步访问：

     Set s = Collections.synchronizedSet(new LinkedHashSet(...));
 此类的 iterator 方法返回的迭代器是快速失败 的：在迭代器创建之后，如果对 set 进行修改，
 除非通过迭代器自身的 remove 方法，其他任何时间任何方式的修改，迭代器都将抛出
 ConcurrentModificationException。因此，面对并发的修改，迭代器很快就会完全失败，
 而不冒将来不确定的时间任意发生不确定行为的风险。

注意，迭代器的快速失败行为不能得到保证，一般来说，存在不同步的并发修改时，
不可能作出任何强有力的保证。快速失败迭代器尽最大努力抛出 ConcurrentModificationException。
因此，编写依赖于此异常的程序的方式是错误的，正确做法是：迭代器的快速失败行为应该仅用于检测程序错误。

 */
public class LinkedHashSet<E>
    extends HashSet<E>
    implements Set<E>, Cloneable, java.io.Serializable {

    private static final long serialVersionUID = -2851667679971038690L;

    /**
     * Constructs a new, empty linked hash set with the specified initial
     * capacity and load factor.
     *
     * @param      initialCapacity the initial capacity of the linked hash set
     * @param      loadFactor      the load factor of the linked hash set
     * @throws     IllegalArgumentException  if the initial capacity is less
     *               than zero, or if the load factor is nonpositive
     */
    //调用父类HashSet的构造方法
    public LinkedHashSet(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor, true);
    }

    /**
     * Constructs a new, empty linked hash set with the specified initial
     * capacity and the default load factor (0.75).
     *
     * @param   initialCapacity   the initial capacity of the LinkedHashSet
     * @throws  IllegalArgumentException if the initial capacity is less
     *              than zero
     */
    //调用父类的构造方法
    public LinkedHashSet(int initialCapacity) {
        super(initialCapacity, .75f, true);
    }

    /**
     * Constructs a new, empty linked hash set with the default initial
     * capacity (16) and load factor (0.75).
     */
    //调用父类的构造方法
    public LinkedHashSet() {
        super(16, .75f, true);
    }

    /**
     * Constructs a new linked hash set with the same elements as the
     * specified collection.  The linked hash set is created with an initial
     * capacity sufficient to hold the elements in the specified collection
     * and the default load factor (0.75).
     *
     * @param c  the collection whose elements are to be placed into
     *           this set
     * @throws NullPointerException if the specified collection is null
     */
    //调用父类的构造方法
    public LinkedHashSet(Collection<? extends E> c) {
        super(Math.max(2*c.size(), 11), .75f, true);
        addAll(c);
    }

    /**
     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
     * and <em>fail-fast</em> {@code Spliterator} over the elements in this set.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
     * {@link Spliterator#DISTINCT}, and {@code ORDERED}.  Implementations
     * should document the reporting of additional characteristic values.
     *
     * @implNote
     * The implementation creates a
     * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
     * from the set's {@code Iterator}.  The spliterator inherits the
     * <em>fail-fast</em> properties of the set's iterator.
     * The created {@code Spliterator} additionally reports
     * {@link Spliterator#SUBSIZED}.
     *
     * @return a {@code Spliterator} over the elements in this set
     * @since 1.8
     */
    @Override
    public Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
    }
}