介绍

HashSet和LinkedHashSet都是不可重复的元素集合，HashSet通过HashMap实现，LinkedHashSet通过LinkedHashMap来实现，所以LinkedHashSet迭代是有顺序的，和add时的顺序一致，HashSet迭代是无序的。
类图如下：

HashSet

底层利用HashMap保存所有的元素，元素都保存在map的key上，因为map的key都是不重复的，所以间接实现了去重的效果，map的value都固定是一个静态的Object对象，称之为PRESENT。

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;
    // 保存所有的元素
    private transient HashMap<E,Object> map;
    // Dummy value to associate with an Object in the backing Map
    // 每个key的value都是它
    private static final Object PRESENT = new Object();
    // 默认初始化
    public HashSet() {
        map = new HashMap<>();
    }
    // 集合转hashset
    public HashSet(Collection<? extends E> c) {
        // 默认根据负载因子0.75计算hashmap的容量再+1，不足16默认16
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }
    // 自定义初始化容量和负载因子
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }
    // 自定义初始化容量
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }
    // dummy实际上没有用到，可以看到没有访问修饰符，这个构造方法是提供给子类LinkedHashSet使用
    // dummy的作用是为了和上面的构造函数区别开来
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }
    public int size() {
        return map.size();
    }
    public boolean isEmpty() {
        return map.isEmpty();
    }
    public boolean contains(Object o) {
        return map.containsKey(o);
    }
    // 添加元素，key为e，value固定为PRESENT
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }
    // 移除元素，元素的value是PRESENT
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }
    public void clear() {
        map.clear();
    }
    @SuppressWarnings("unchecked")
    public Object clone() {
        try {
            HashSet<E> newSet = (HashSet<E>) super.clone();
            newSet.map = (HashMap<E, Object>) map.clone();
            return newSet;
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();
        // Write out HashMap capacity and load factor
        s.writeInt(map.capacity());
        s.writeFloat(map.loadFactor());
        // Write out size
        s.writeInt(map.size());
        // Write out all elements in the proper order.
        for (E e : map.keySet())
            s.writeObject(e);
    }
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in any hidden serialization magic
        s.defaultReadObject();
        // Read capacity and verify non-negative.
        int capacity = s.readInt();
        if (capacity < 0) {
            throw new InvalidObjectException("Illegal capacity: " +
                                             capacity);
        }
        // Read load factor and verify positive and non NaN.
        float loadFactor = s.readFloat();
        if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
            throw new InvalidObjectException("Illegal load factor: " +
                                             loadFactor);
        }
        // Read size and verify non-negative.
        int size = s.readInt();
        if (size < 0) {
            throw new InvalidObjectException("Illegal size: " +
                                             size);
        }
        // Set the capacity according to the size and load factor ensuring that
        // the HashMap is at least 25% full but clamping to maximum capacity.
        capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
                HashMap.MAXIMUM_CAPACITY);
        // Create backing HashMap
        map = (((HashSet<?>)this) instanceof LinkedHashSet ?
               new LinkedHashMap<E,Object>(capacity, loadFactor) :
               new HashMap<E,Object>(capacity, loadFactor));
        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
            @SuppressWarnings("unchecked")
                E e = (E) s.readObject();
            map.put(e, PRESENT);
        }
    }
    public Spliterator<E> spliterator() {
        return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
    }
}

LinkedHashSet

LinkedHashSet完美利用了HashSet的方法，只需要构造函数的不同，构造出底层是LinkedHashMap就行，
因为LinkedHashMap是HashMap的子类，LinkedHashMap的节点之间是有顺序的，所以LinkedHashSet的节点也拥有了顺序。

public class LinkedHashSet<E>
    extends HashSet<E>
    implements Set<E>, Cloneable, java.io.Serializable {
    private static final long serialVersionUID = -2851667679971038690L;
    // 调用HashSet的构造初始化，实际上是new LinkedHashMap(initialCapacity, loadFactor);
    public LinkedHashSet(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor, true);
    }
    // 调用HashSet的构造初始化，默认负载因子0.75
    public LinkedHashSet(int initialCapacity) {
        super(initialCapacity, .75f, true);
    }
    // 默认容量16，负载因子0.75
    public LinkedHashSet() {
        super(16, .75f, true);
    }
    // 初始化一个集合，初始容量是2倍size和11取其中的大值。
    public LinkedHashSet(Collection<? extends E> c) {
        super(Math.max(2*c.size(), 11), .75f, true);
        addAll(c);
    }
    @Override
    public Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
    }
}