* This class implements the {@code Set} interface, backed by a hash table
 * (actually a {@code HashMap} 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 {@code null}
 * element.
 * <p>This class offers constant time performance for the basic operations
 * ({@code add}, {@code remove}, {@code contains} and {@code size}),
 * assuming the hash function disperses the elements properly among the
 * buckets.  Iterating over this set requires time proportional to the sum of
 * the {@code HashSet} instance's size (the number of elements) plus the
 * "capacity" of the backing {@code HashMap} 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 {@code iterator} 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 {@code remove}
 * 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 {@code ConcurrentModificationException} 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}/java.base/java/util/package-summary.html#CollectionsFramework">
 * 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

实现了 Set 接口,继承自 AbstractSet 抽象类。


Set 的接口方法比较多。

我们主要看一下 add/remove/contians/size 方法。

抽象类的实现方法并不复杂:继承自 AbstractCollection 类,实现了 Set 接口。

public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {
    protected AbstractSet() {

    // Comparison and hashing
    public boolean equals(Object o) {
        if (o == this)
            return true;

        if (!(o instanceof Set))
            return false;

        // 这里的判断方式是: 
        //1. 判断二者的大小相同
        //2. 避免二者的元素全部 contains     
        Collection<?> c = (Collection<?>) o;
        if (c.size() != size())
            return false;
        try {
            return containsAll(c);
        } catch (ClassCastException | NullPointerException unused) {
            return false;

     * Returns the hash code value for this set.  
    public int hashCode() {
        int h = 0;
        Iterator<E> i = iterator();

        // 直接遍历集合中的所有元素,把 hashCode 累加。
        // 这个方法的 TC 是 O(N) 的。
        while (i.hasNext()) {
            E obj = i.next();
            if (obj != null)
                h += obj.hashCode();
        return h;

     * Removes from this set all of its elements that are contained in the
     * specified collection (optional operation).  If the specified
     * collection is also a set, this operation effectively modifies this
     * set so that its value is the <i>asymmetric set difference</i> of
     * the two sets.
    public boolean removeAll(Collection<?> c) {
        boolean modified = false;

        if (size() > c.size()) {
            // 删除元素
            for (Object e : c)
                modified |= remove(e);
        } else {
            // 迭代接口。
            for (Iterator<?> i = iterator(); i.hasNext(); ) {
                if (c.contains(i.next())) {
                    modified = true;
        return modified;



private transient HashMap<E,Object> map;

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


比较神奇的一点就是,HashSet 的底层实现,是基于 HashMap 的。

public HashSet() {
        map = new HashMap<>();

 * Constructs a new set containing the elements in the specified
 * collection.  The {@code HashMap} 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) {
    // 默认是取 16,或者 指定大小/0.75+1 而这种的最大值
    map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));

 * Constructs a new, empty set; the backing {@code HashMap} 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 {@code HashMap} 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);


所以 HashSet 的核心方法都是基于 HashMap 的。


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

     * Returns {@code true} if this set contains no elements.
     * @return {@code true} if this set contains no elements
    public boolean isEmpty() {
        return map.isEmpty();

     * Returns {@code true} if this set contains the specified element.
     * More formally, returns {@code true} if and only if this set
     * contains an element {@code e} such that
     * {@code Objects.equals(o, e)}.
     * @param o element whose presence in this set is to be tested
     * @return {@code true} 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 {@code e} to this set if
     * this set contains no element {@code e2} such that
     * {@code Objects.equals(e, e2)}.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns {@code false}.
     * @param e element to be added to this set
     * @return {@code true} 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 {@code e} such that
     * {@code Objects.equals(o, e)},
     * if this set contains such an element.  Returns {@code true} 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 {@code true} 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() {


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
    // Write out HashMap capacity and load factor
    // Write out size
    // Write out all elements in the proper order.
    for (E e : map.keySet())

    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Consume and ignore stream fields (currently zero).

        // Read capacity and verify non-negative.
        int capacity = s.readInt();
        if (capacity < 0) {
            throw new InvalidObjectException("Illegal 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: " +
        // Clamp load factor to range of 0.25...4.0.
        loadFactor = Math.min(Math.max(0.25f, loadFactor), 4.0f);

        // 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),

        // Constructing the backing map will lazily create an array when the first element is
        // added, so check it before construction. Call HashMap.tableSizeFor to compute the
        // actual allocation size. Check Map.Entry[].class since it's the nearest public type to
        // what is actually created.
                     .checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));

        // Create backing HashMap
        map = (((HashSet<?>)this) instanceof LinkedHashSet ?
               new LinkedHashMap<>(capacity, loadFactor) :
               new HashMap<>(capacity, loadFactor));

        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
                E e = (E) s.readObject();
            map.put(e, PRESENT);