1、源码分析
废话不多说，直接上代码，相关解释请参见注释
代码语言：javascript复制package java.util;
/**
*
* @param <E> the type of elements maintained by this set
*
* @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;
//transient关键字标记的成员变量不参与序列化过程
//HashSet的底层使用HashMap来保存所有元素
private transient HashMap<E,Object> map;
// Dummy value to associate with an Object in the backing Map
// 英文注释的大致意思：定义一个虚拟的Object对象作为HashMap的value
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).
* 基于具有默认的初始容量（16）和负载因子（0.75）HashMap实例构造一个新的空集。
* 可见HashSet是通过HashMap实现
*/
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);
}
/**
*
*/
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();
}
/**
* 实际上调用的是HashMap的的isEmpty()方法
* @return <tt>true</tt> if this set contains no elements
*/
public boolean isEmpty() {
return map.isEmpty();
}
/**
* 实际上调用的是HashMap的的contains()方法
*
* @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);
}
/**
* 底层实际将将该元素作为key放入HashMap
* 对于HashMap，新添加的Entry的value会将覆盖原来Entry的value，但key不会有任何改变，
* 因此如果向HashSet中添加一个已经存在的元素时，新添加的集合元素将不会被放入HashMap中
*
* @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;
}
/**
*
*/
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
*/
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();
}
}
/**
*
*/
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 in HashMap capacity and load factor and create backing HashMap
int capacity = s.readInt();
float loadFactor = s.readFloat();
map = (((HashSet)this) instanceof LinkedHashSet ?
new LinkedHashMap<E,Object>(capacity, loadFactor) :
new HashMap<E,Object>(capacity, loadFactor));
// Read in size
int size = s.readInt();
// Read in all elements in the proper order.
for (int i=0; i<size; i ) {
E e = (E) s.readObject();
map.put(e, PRESENT);
}
}
}2、总结
- HashSet实现Set接口。
- 通过源码可知，HashSet实际上由HashMap支持实现。它不保证set 的迭代顺序；特别是它不保证该顺序恒久不变。
- 相关HashSet的操作，基本上都是直接调用底层HashMap的相关方法来完成。
- 由于HashMap允许null型key，所以HashSet允许使用null元素。
- 对于HashSet中保存的对象，请注意正确重写其equals和hashCode方法，以保证放入的对象的唯一性。
