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众所周知，hashset里面存储的元素都具有无序性，标识唯一性。但最近仔细研究了一下java里面的hashset，发现hashset里面大多数的内容都是在hashmap的基础上进行修改的。

接下来是hashset的源码展示：

package java.util;import java.io.InvalidObjectException;/** * This class implements the <tt>Set</tt> interface, backed by a hash table * (actually a <tt>HashMap</tt> instance).  It makes no guarantees as to the * iteration order of the set; in particular, it does not guarantee that the * order will remain constant over time.  This class permits the <tt>null</tt> * element. * * <p>This class offers constant time performance for the basic operations * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>), * assuming the hash function disperses the elements properly among the * buckets.  Iterating over this set requires time proportional to the sum of * the <tt>HashSet</tt> instance's size (the number of elements) plus the * "capacity" of the backing <tt>HashMap</tt> instance (the number of * buckets).  Thus, it's very important not to set the initial capacity too * high (or the load factor too low) if iteration performance is important. * * <p><strong>Note that this implementation is not synchronized.</strong> * If multiple threads access a hash set concurrently, and at least one of * the threads modifies the set, it <i>must</i> 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 * {@link Collections#synchronizedSet Collections.synchronizedSet} * method.  This is best done at creation time, to prevent accidental * unsynchronized access to the set:<pre> *   Set s = Collections.synchronizedSet(new HashSet(...));</pre> * * <p>The iterators returned by this class's <tt>iterator</tt> method are * <i>fail-fast</i>: if the set is modified at any time after the iterator is * created, in any way except through the iterator's own <tt>remove</tt> * method, the Iterator throws a {@link ConcurrentModificationException}. * 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> * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @param <E> the type of elements maintained by this set * * @author  Josh Bloch * @author  Neal Gafter * @see     Collection * @see     Set * @see     TreeSet * @see     HashMap * @since   1.2 */public class HashSet<E>    extends AbstractSet<E>    implements Set<E>, Cloneable, java.io.Serializable{    static final long serialVersionUID = -5024744406713321676L;    private transient HashMap<E,Object> map;    // Dummy value to associate with an Object in the backing Map    private static final Object PRESENT = new Object();    /**     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has     * default initial capacity (16) and load factor (0.75).     */    public HashSet() {        map = new HashMap<>();    }    /**     * Constructs a new set containing the elements in the specified     * collection.  The <tt>HashMap</tt> is created with default load factor     * (0.75) and an initial capacity sufficient to contain the elements in     * the specified collection.     *     * @param c the collection whose elements are to be placed into this set     * @throws NullPointerException if the specified collection is null     */    public HashSet(Collection<? extends E> c) {        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));        addAll(c);    }    /**     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has     * 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     * @throws     IllegalArgumentException if the initial capacity is less     *             than zero, or if the load factor is nonpositive     */    public HashSet(int initialCapacity, float loadFactor) {        map = new HashMap<>(initialCapacity, loadFactor);    }    /**     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has     * the specified initial capacity and default load factor (0.75).     *     * @param      initialCapacity   the initial capacity of the hash table     * @throws     IllegalArgumentException if the initial capacity is less     *             than zero     */    public HashSet(int initialCapacity) {        map = new HashMap<>(initialCapacity);    }    /**     * 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<>(initialCapacity, loadFactor);    }    /**     * Returns an iterator over the elements in this set.  The elements     * are returned in no particular order.     *     * @return an Iterator over the elements in this set     * @see ConcurrentModificationException     */    public Iterator<E> iterator() {        return map.keySet().iterator();    }    /**     * Returns the number of elements in this set (its cardinality).     *     * @return the number of elements in this set (its cardinality)     */    public int size() {        return map.size();    }    /**     * Returns <tt>true</tt> if this set contains no elements.     *     * @return <tt>true</tt> if this set contains no elements     */    public boolean isEmpty() {        return map.isEmpty();    }    /**     * Returns <tt>true</tt> if this set contains the specified element.     * More formally, returns <tt>true</tt> if and only if this set     * contains an element <tt>e</tt> such that     * <tt>(o==null ? e==null : o.equals(e))</tt>.     *     * @param o element whose presence in this set is to be tested     * @return <tt>true</tt> if this set contains the specified element     */    public boolean contains(Object o) {        return map.containsKey(o);    }    /**     * Adds the specified element to this set if it is not already present.     * More formally, adds the specified element <tt>e</tt> to this set if     * this set contains no element <tt>e2</tt> such that     * <tt>(e==null ? e2==null : e.equals(e2))</tt>.     * If this set already contains the element, the call leaves the set     * unchanged and returns <tt>false</tt>.     *     * @param e element to be added to this set     * @return <tt>true</tt> if this set did not already contain the specified     * element     */    public boolean add(E e) {        return map.put(e, PRESENT)==null;    }    /**     * Removes the specified element from this set if it is present.     * More formally, removes an element <tt>e</tt> such that     * <tt>(o==null ? e==null : o.equals(e))</tt>,     * if this set contains such an element.  Returns <tt>true</tt> if     * this set contained the element (or equivalently, if this set     * changed as a result of the call).  (This set will not contain the     * element once the call returns.)     *     * @param o object to be removed from this set, if present     * @return <tt>true</tt> if the set contained the specified element     */    public boolean remove(Object o) {        return map.remove(o)==PRESENT;    }    /**     * Removes all of the elements from this set.     * The set will be empty after this call returns.     */    public void clear() {        map.clear();    }    /**     * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements     * themselves are not cloned.     *     * @return a shallow copy of this set     */    @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);        }    }    /**     * Save the state of this <tt>HashSet</tt> instance to a stream (that is,     * serialize it).     *     * @serialData The capacity of the backing <tt>HashMap</tt> instance     *             (int), and its load factor (float) are emitted, followed by     *             the size of the set (the number of elements it contains)     *             (int), followed by all of its elements (each an Object) in     *             no particular order.     */    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);    }    /**     * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,     * deserialize it).     */    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);        }    }    /**     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>     * and <em>fail-fast</em> {@link Spliterator} over the elements in this     * set.     *     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and     * {@link Spliterator#DISTINCT}.  Overriding implementations should document     * the reporting of additional characteristic values.     *     * @return a {@code Spliterator} over the elements in this set     * @since 1.8     */    public Spliterator<E> spliterator() {        return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);    }}

静下心来仔细研究了一下，发现hashset是通过将相应的内容存储在了一个hashmap里的key中，然后再去读取的。 
为了保证hashset里面的数据唯一性，这里将hashset存放的元素作为了hashmap里面唯一的key变量，value部分用一个PRESENT对象来存储，也就是源码里面的这一句内容：

private static final Object PRESENT = new Object();

其实hashset底层的很多部分都是引用了hashmap来进行实现的，所以如果想要了解hashset就必须要了解hashmap的原理，在我的另外一篇博客中   有详细说到hashmap的原理。

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