java 中的集合（十） LinkedHashSet源码分析

LinkedHashSet实现了Set接口，由记录顺序的哈希表（实际上是一个LinkedHashMap实例，LinkedHashMap参考：链接）支持。相比HashSet，它记录了set的顺序。LinkedHashSet也允许使用null元素（很显然，LinkedHashMap也允许使用空值key）。

LinkedHashSet继承于HashSet（同样的，LinkedHashMap继承于HashMap），所以基本上和HashSet类似。（HashSet参考：链接）

来看看源码：

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);
    }
}

可以看到，实际上所有的构造方法，最终都调用了HashSet中的这个方法：

  HashSet(int initialCapacity, float loadFactor, boolean dummy) {
	map = new LinkedHashMap<E,Object>(initialCapacity, loadFactor);
    }

由于LinkedHashMap记录key的顺序，所以LinkedHashSet自然是有序的。

另外，注意一下，在初始化时，若将Collection作为参数，对Map初始大小设置的区别。

这个是HashSet的：

  public HashSet(Collection<? extends E> c) {
	map = new HashMap<E,Object>(Math.max((int) (c.size()/.75f) + 1, 16));
	addAll(c);
    }

这个是LinkedHashSet的：

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

由于LinkedHashMap和HashMap，都采用了和HashSet类似的方法，这让LinkedHashSet显得比较特殊。（而且似乎应该把11换成16，不然和HashTable类似了）

同时看看ArrayList的：

  public ArrayList(Collection<? extends E> c) {
	elementData = c.toArray();
	size = elementData.length;
	// c.toArray might (incorrectly) not return Object[] (see 6260652)
	if (elementData.getClass() != Object[].class)
	    elementData = Arrays.copyOf(elementData, size, Object[].class);
    }

ArrayList没有载入因子，初始化时没有扩容。

参考地址：http://blog.csdn.net/u010156024/article/details/48394475