static final long serialVersionUID = -5024744406713321676L;
/*底层是HasMap来实现的，transient?不能使用jdk的默认序列化方式了, 但是jdk
在进行对象的序列化的时候, 会通过反射判断对象中是否有重写的writeObject和
readObject方法, 从而调用重写的方法。*/
private transient HashMap<E,Object> map;
/*为什么不直接赋值null?考虑到HashMap put时，返回null时不知道
是第一插入害死真的插入null值*/
private static final Object PRESENT = new Object();
构造方法
代码语言：javascript复制
/*
底层都是围绕HashMap展开的，负载因子，初始容量等
*/
public HashSet() {
map = new HashMap<>();
}
public HashSet(Collection<? extends E> c) {
map = new HashMap<>(Math.max((int) (c.size()/.75f) 1, 16));
addAll(c);
}
public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}
public HashSet(int initialCapacity) {
map = new HashMap<>(initialCapacity);
}
/*
非public，LinkedHashSet专用，boolean dummy无实际意义，仅
作为和其它构造方法的区分
*/
HashSet(int initialCapacity, float loadFactor, boolean dummy无实际意义) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}
增删操作
代码语言：javascript复制
//底层都是调用hashmap的方法，hashmap学会就ok
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}
其它工具方法
代码语言：javascript复制
public int size() {
return map.size();
}
public boolean isEmpty() {
return map.isEmpty();
}
public boolean contains(Object o) {
return map.containsKey(o);
}
public void clear() {
map.clear();
}
为什么map可以实例化
代码语言：javascript复制
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);
// 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.
SharedSecrets.getJavaOISAccess()
.checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(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);
}
}
