e.HashSet与LinkedHashSet源码解析（1.7）

java容器类分为两大类，一部分是Map，用来存放键值对。另一部分是Collection，用来存放独立的元素。Collection分为List与Set，List可以有重复顺序，Set则不行

我们要分析的HashSet与LinkedHashSet底层都是用的HashMap与LinkedHashMap的key来实现的，这里将具体分析下具体的封装

<pre name="code" class="java">public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable

属性： 

<span style="white-space:pre"></span>private transient HashMap<E,Object> map;//没错，就是用它的key来存储元素的

    <span style="white-space:pre"></span>private static final Object PRESENT = new Object();//常量，它的作用基本可以看出来，是用来当做某种初始值，如果往下看的话应该明白，就是作为Map的Value

构造方法：

   //看到map=new HashMap()，往下就是一切尽在不言中了，我们只要看看set与map的联系即可

    public HashSet() {        map = new HashMap<>();    }        public HashSet(Collection<? extends E> c) {        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));        addAll(c);    }       public HashSet(int initialCapacity, float loadFactor) {        map = new HashMap<>(initialCapacity, loadFactor);    }    public HashSet(int initialCapacity) {        map = new HashMap<>(initialCapacity);    }    //注意，这个将会在LinkedHashSet里头用到    HashSet(int initialCapacity, float loadFactor, boolean dummy) {        map = new LinkedHashMap<>(initialCapacity, loadFactor);    }

方法：

迭代器：这里赤裸裸的直接用上了keySet.iterator();

    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

size与isEmpty一样，也是直接调用了map对象的方法

    public int size() {
        return map.size();
    }

    public boolean isEmpty() {
        return map.isEmpty();
    }

往下一个套路，就不再多说了

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

类声明与HashSet大同小异，LinkedHashSet是HashSet的子类
他的构造方法通过调用HashSet的第五个构造方法，创建一个lingkedHashHap，其余与HashSet相同

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);
    }
}