package java.util;
import java.io.InvalidObjectException;
public class HashSet<E>
extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable{
//序列化ID
static final long serialVersionUID = -5024744406713321676L;
//HashSet 底层用 HashMap 来存放数据
//Key值由外部传入，Value则由 HashSet 内部来维护
private transient HashMap<E,Object> map;
//HashMap 中所有键值对都共享同一个值
//即所有存入 HashMap 的键值对都是使用这个对象作为值
private static final Object PRESENT = new Object();
//无参构造函数，HashMap 使用默认的初始化大小和装载因子
public HashSet() {
map = new HashMap<>();
}
//使用默认的装载因子，并以此来计算 HashMap 的初始化大小
//+1 是为了弥补精度损失
public HashSet(Collection<? extends E> c) {
map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
addAll(c);
}
//为 HashMap 自定义初始化大小和装载因子
public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}
//为 HashMap 自定义初始化大小
public HashSet(int initialCapacity) {
map = new HashMap<>(initialCapacity);
}
//此构造函数为包访问权限，只用于对 LinkedHashSet 的支持
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}
//将对 HashSet 的迭代转换为对 HashMap 的 Key 值的迭代
public Iterator<E> iterator() {
return map.keySet().iterator();
}
//获取集合中的元素数量
public int size() {
return map.size();
}
//判断集合是否为空
public boolean isEmpty() {
return map.isEmpty();
}
//判断集合是否包含指定元素
public boolean contains(Object o) {
return map.containsKey(o);
}
//如果 HashSet 中不包含元素 e，则添加该元素，并返回 true
//如果 HashSet 中包含元素 e，则不会影响 HashSet ，并返回 false
//该方法将向 HashSet 添加元素 e 的操作转换为向 HashMap 添加键值对
//如果 HashMap 中包含 key 值与 e 相等的结点（hashCode() 方法返回值相等，通过 equals() 方法比较也返回 true）
//则新添加的结点的 value 会覆盖原有数据，但 key 不会有所改变
//因此如果向 HashSet 添加一个已存在的元素时，元素不会被存入 HashMap 中
//从而实现了 HashSet 元素不重复的特征
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}
//移除集合中的元素 o
//如果集合不包含元素 o，则返回 false
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}
//清空集合中的元素
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
*/
//为了并行遍历数据源中的元素而设计的迭代器
public Spliterator<E> spliterator() {
return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
}
}