LinkedHashSet.java

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package java.util;

import java.io.Serializable;

/**
 * <p>Hash table and linked list implementation of the {@code Set} interface,
 * with predictable iteration order.  This implementation differs from
 * {@code HashSet} 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 {@code e}
 * is reinserted into a set {@code s} if {@code s.add(e)} is invoked when
 * {@code s.contains(e)} would return {@code true} 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 {@code Set} operations, and
 * permits null elements.  Like {@code HashSet}, it provides constant-time
 * performance for the basic operations ({@code add}, {@code contains} and
 * {@code remove}), assuming the hash function disperses elements
 * properly among the buckets.  Performance is likely to be just slightly
 * below that of {@code HashSet}, due to the added expense of maintaining the
 * linked list, with one exception: Iteration over a {@code LinkedHashSet}
 * requires time proportional to the <i>size</i> of the set, regardless of
 * its capacity.  Iteration over a {@code HashSet} 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 {@code HashSet}.  Note, however, that the penalty for choosing an
 * excessively high value for initial capacity is less severe for this class
 * than for {@code HashSet}, 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 {@code iterator} 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 {@code remove}
 * 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 {@code ConcurrentModificationException} 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}/java.base/java/util/package-summary.html#CollectionsFramework">
 * 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
 */
// LinkedHashSet是有序Set，即元素顺序遵从其加入集合的顺序，其内部通过LinkedHashMap来实现其固有操作
public class LinkedHashSet<E> extends HashSet<E> implements Set<E>, Cloneable, Serializable {
    
    private static final long serialVersionUID = -2851667679971038690L;
    
    /**
     * Constructs a new, empty linked hash set with the default initial
     * capacity (16) and load factor (0.75).
     */
    public LinkedHashSet() {
        super(16, .75f, 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
     */
    public LinkedHashSet(int initialCapacity) {
        super(initialCapacity, .75f, true);
    }
    
    /**
     * 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
     */
    public LinkedHashSet(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor, 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
     */
    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.
     *
     * @return a {@code Spliterator} over the elements in this set
     *
     * @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}.
     * @since 1.8
     */
    // 返回描述此容器中元素的可分割的迭代器
    @Override
    public Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
    }
}