总结 HashSet：

HashSet的底层是HashMap
并且HashSet中的set相当于在HashMap的key，而value则是new Object出来的常量
扩容条件则是按照HashMap的扩容规则进扩容

源码

  1. public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, java.io.Serializable {
  2.     @java.io.Serial
  3.     static final long serialVersionUID = -5024744406713321676L; // 序列化版本号
  4.     private transient HashMap<E,Object> map;                    // 底层采用HashMap进行存储
  6.     private static final Object PRESENT = new Object();            // 作为HashMap中的value常量，而key则是真正set要存的数据
  9.     public HashSet() {
  10.         map = new HashMap<>(); // 直接创建一个HashMap
  11.     }
  12.     public HashSet(Collection<? extends E> c) {                         // 传入一个集合
  13.         map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));   // 创建一个带初始容量的HashMap
  14.         addAll(c);                                                      // 将集合c中的元素添加进map中
  15.     }
  16.     public HashSet(int initialCapacity, float loadFactor) {
  17.         map = new HashMap<>(initialCapacity, loadFactor);               // 带初始容量和扩容因子的map
  18.     }
  19.     public HashSet(int initialCapacity) {
  20.         map = new HashMap<>(initialCapacity);                           // 带初始容量的map
  21.     }
  23.     HashSet(int initialCapacity, float loadFactor, boolean dummy) {     // 只在内部使用的构造方法，创建一个LinkedHashMap
  24.         map = new LinkedHashMap<>(initialCapacity, loadFactor);
  25.     }
  28.     public Iterator<E> iterator() {
  29.         return map.keySet().iterator();
  30.     }
  31.     public int size() {
  32.         return map.size();              //返回map存有多少个元素
  33.     }
  34.     public boolean isEmpty() {
  35.         return map.isEmpty();           // 判断map是否为空
  36.     }
  37.     public boolean contains(Object o) {
  38.         return map.containsKey(o);      // 判断o是否在map中
  39.     }
  40.     public boolean add(E e) {
  41.         return map.put(e, PRESENT)==null;// 添加元素，将数据作为key
  42.     }
  43.     public boolean remove(Object o) {
  44.         return map.remove(o)==PRESENT;  // 移除元素
  45.     }
  46.     public void clear() {
  47.         map.clear();                    // 清空map
  48.     }
  50.     @SuppressWarnings("unchecked")
  51.     public Object clone() {
  52.         try {
  53.             HashSet<E> newSet = (HashSet<E>) super.clone(); // 拷贝HashSet
  54.             newSet.map = (HashMap<E, Object>) map.clone();  // 拷贝map将map赋值给上面拷贝的HashSet中
  55.             return newSet;                                  // 返回set
  56.         } catch (CloneNotSupportedException e) {
  57.             throw new InternalError(e);
  58.         }
  59.     }
  61.     // 序列化
  62.     @java.io.Serial
  63.     private void writeObject(java.io.ObjectOutputStream s)
  64.         throws java.io.IOException {
  65.         s.defaultWriteObject();
  67.         s.writeInt(map.capacity());
  68.         s.writeFloat(map.loadFactor());
  70.         s.writeInt(map.size());
  71.         for (E e : map.keySet())
  72.             s.writeObject(e);
  73.     }
  75.     // 反序列化
  76.     @java.io.Serial
  77.     private void readObject(java.io.ObjectInputStream s)
  78.         throws java.io.IOException, ClassNotFoundException {
  79.         s.defaultReadObject();
  80.         int capacity = s.readInt();
  81.         if (capacity < 0) {
  82.             throw new InvalidObjectException("Illegal capacity: " + capacity);
  83.         }
  84.         float loadFactor = s.readFloat();
  85.         if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
  86.             throw new InvalidObjectException("Illegal load factor: " + loadFactor);
  87.         }
  88.         int size = s.readInt();
  89.         if (size < 0) {
  90.             throw new InvalidObjectException("Illegal size: " + size);
  91.         }
  92.         capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f), HashMap.MAXIMUM_CAPACITY);
  94.         SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));
  96.         map = (((HashSet<?>)this) instanceof LinkedHashSet ? new LinkedHashMap<>(capacity, loadFactor) : new HashMap<>(capacity, loadFactor));
  98.         for (int i=0; i<size; i++) {
  99.             @SuppressWarnings("unchecked")
  100.                 E e = (E) s.readObject();
  101.             map.put(e, PRESENT);
  102.         }
  103.     }
  105.     public Spliterator<E> spliterator() {
  106.         return new HashMap.KeySpliterator<>(map, 0, -1, 0, 0);  // 获取key的迭代器
  107.     }
  109.     @Override
  110.     public Object[] toArray() {
  111.         return map.keysToArray(new Object[map.size()]);                                             // map的key转换为数组
  112.     }
  114.     @Override
  115.     public <T> T[] toArray(T[] a) {
  116.         return map.keysToArray(map.prepareArray(a));                                                // 带泛型的转换key为数组
  117.     }
  118. }