/*
 * @(#)LinkedHashSet.java	1.7 01/12/03
 *
 * Copyright 2002 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

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 m) {
 *         Set copy = new LinkedHashSet(m);
 *         ...
 *     }
 * </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 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 <tt>Collections.synchronizedSet</tt>method.  This is
 * best done at creation time, to prevent accidental unsynchronized access:
 * <pre>
 *     Set s = Collections.synchronizedSet(new LinkedHashSet(...));
 * </pre>
 *
 * <p>The iterators returned by the 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 remove method, the
 * iterator will throw a <tt>ConcurrentModificationException</tt>.  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>
 *
 * @author  Josh Bloch
 * @version 1.7 12/03/01
 * @see     Object#hashCode()
 * @see     Collection
 * @see     Set
 * @see     HashSet
 * @see     TreeSet
 * @see     Hashtable
 * @since   1.4
 */

public class LinkedHashSet extends HashSet
    implements Set, Cloneable, java.io.Serializable {
    /**
     * 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, 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 default initial
     * capacity (16) and load factor (0.75).
     */
    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.
     */
    public LinkedHashSet(Collection c) {
        super(Math.max(2*c.size(), 11), .75f, true);
        addAll(c);
    }
}
