Java-Collections Framework学习与总结-HashSet和LinkedHashSet

        本篇总结一下两个常用的集合类HashSet和LinkedHashSet。
        它们都实现了相同接口java.util.Set。Set表示一种元素无序且不可重复的集合；之前总结过的java.util.List表示一种元素可重复且有序的集合。它们都扩展自java.util.Collection。

public interface Set<E> extends Collection<E> {
    int size();
    boolean isEmpty();
    boolean contains(Object o);
    Iterator<E> iterator();
    Object[] toArray();
    <T> T[] toArray(T[] a);
    boolean add(E e);
    boolean remove(Object o);
    boolean containsAll(Collection<?> c);
    boolean addAll(Collection<? extends E> c);
    boolean retainAll(Collection<?> c);
    boolean removeAll(Collection<?> c);
    void clear();
    boolean equals(Object o);
    int hashCode();

        可以看到Set的接口描述，和List比，只少了一些与下标有关的特性。

        先来看一下HashSet的实现方式。

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

        HashSet是 可克隆、可序列化的；实现了java.util.Set；继承了java.util.AbstractSet，AbstractSet包含一些骨架方法，很容易看懂。继续往下看。

    private transient HashMap<E,Object> map;

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

        看到这里就基本上明白是怎么实现的了吧！原来HashSet内部是一个HashMap，难怪HashSet用起来很像一个HashMap，只是没有value。所以PRESENT这个对象就用来填充value值了。有了前面 HashMap的分析，后面的基本上不用看了。
        不过有一个小地方要注意一下。

    /**
     * Constructs a new, empty linked hash set.  (This package private
     * constructor is only used by LinkedHashSet.) The backing
     * HashMap instance is a LinkedHashMap with the specified initial
     * capacity and the specified load factor.
     *
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @param      dummy             ignored (distinguishes this
     *             constructor from other int, float constructor.)
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
     */
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
	map = new LinkedHashMap<E,Object>(initialCapacity, loadFactor);
    }

        这个构造方法和其他的不一样。里面的Map实现是LinkedHashMap。从注释上可以看到，这个方法只是由LinkedHashSet使用，方法的第三个参数也只是用来与其他构造方法进行区分，代码中并没有实际用到。

        接下来看一下java.util.LinkedHashSet的实现吧。

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
     */
    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<? extends E> c) {
        super(Math.max(2*c.size(), 11), .75f, true);
        addAll(c);
    }
}

        以上便是LinkedHashSet的全部代码了，基本上没什么可说的了，参考下 LinkedHashMap的总结吧。