HashSet 是一个没有重复元素的集合。基于HashMap实现。
HashSet定义:
public class HashSet<E>
extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable
它是由HashMap实现的,不保证元素的顺序,而且HashSet允许使用 null 元素。类图结构如下:
HashSet是非同步的。如果多个线程同时访问一个哈希 set,而其中至少一个线程修改了该 set,那么它必须 保持外部同步。这通常是通过对自然封装该 set 的对象执行同步操作来完成的。如果不存在这样的对象,则应该使用 Collections.synchronizedSet
方法来“包装” set。最好在创建时完成这一操作,以防止对该 set 进行意外的不同步访问:
Set s = Collections.synchronizedSet(new HashSet(...));
HashSet通过iterator()
返回的迭代器是fail-fast的。
HashSet是Set的一种实现方式,底层主要使用HashMap来确保元素不重复。
// 内部使用HashMap
private transient HashMap<E,Object> map;
// 虚拟对象,用来作为value放到map中
private static final Object PRESENT = new Object();
(1)HashSet
内部使用HashMap
的key
存储元素,以此来保证元素不重复;
(2)HashSet
是无序的,因为HashMap
的key
是无序的;
(3)HashSet
中允许有一个null
元素,因为HashMap
允许key
为null
;
(4)HashSet
是非线程安全的;
(5)HashSe
t是没有get()
方法的;
彩蛋
(1)你知道初始化HashMap
的时候初始容量怎么传吗?
我们发现有下面这个构造方法,很清楚明白地告诉了我们怎么指定容量。
public HashSet(Collection<? extends E> c) {
map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
addAll(c);
}
假如,我们预估HashMap
要存储n个元素,那么,它的容量就应该指定为((n/0.75f) + 1),如果这个值小于16,那就直接使用16得了。
初始化时指定容量是为了减少扩容的次数,提高效率。
(2)什么是fail-fast?
fail-fast机制是java集合中的一种错误机制。
当使用迭代器迭代时,如果发现集合有修改,则快速失败做出响应,抛出ConcurrentModificationException
异常。这种修改有可能是其它线程的修改,也有可能是当前线程自己的修改导致的,比如迭代的过程中直接调用remove()
删除元素等。
另外,并不是java中所有的集合都有fail-fast的机制。比如,像最终一致性的ConcurrentHashMap
、CopyOnWriterArrayList
等都是没有fast-fail的。
那么,fail-fast是怎么实现的呢?
细心的同学可能会发现,像ArrayList
、HashMap
中都有一个属性叫modCount
,每次对集合的修改这个值都会加1,在遍历前记录这个值到expectedModCount
中,遍历中检查两者是否一致,如果出现不一致就说明有修改,则抛出ConcurrentModificationException
异常。
(1)集合(Collection)和集合(Set)有什么区别?
(2)HashSet怎么保证添加元素不重复?
(3)HashSet是否允许null元素?
(4)HashSet是有序的吗?
(5)HashSet是同步的吗?
(6)什么是fail-fast?
package java.util;
import java.io.InvalidObjectException;
/**
* This class implements the <tt>Set</tt> interface, backed by a hash table
* (actually a <tt>HashMap</tt> 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 <tt>null</tt>
* element.
*
* <p>This class offers constant time performance for the basic operations
* (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>),
* assuming the hash function disperses the elements properly among the
* buckets. Iterating over this set requires time proportional to the sum of
* the <tt>HashSet</tt> instance's size (the number of elements) plus the
* "capacity" of the backing <tt>HashMap</tt> 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 <tt>iterator</tt> 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 <tt>remove</tt>
* 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 <tt>ConcurrentModificationException</tt> 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}/../technotes/guides/collections/index.html">
* 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
{
static final long serialVersionUID = -5024744406713321676L;
private transient HashMap<E,Object> map;
// Dummy value to associate with an Object in the backing Map
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).
*/
public HashSet() {
map = new HashMap<>();
}
/**
* Constructs a new set containing the elements in the specified
* collection. The <tt>HashMap</tt> 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) {
map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
addAll(c);
}
/**
* Constructs a new, empty set; the backing <tt>HashMap</tt> 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 <tt>HashMap</tt> 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);
}
/**
* Constructs a new, empty linked hash set. (This package private
* constructor is only used by LinkedHashSet.) The backing
* HashMap instance is a LinkedHashMap with 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
* @param dummy ignored (distinguishes this
* constructor from other int, float constructor.)
* @throws IllegalArgumentException if the initial capacity is less
* than zero, or if the load factor is nonpositive
*/
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();
}
/**
* Returns <tt>true</tt> if this set contains no elements.
*
* @return <tt>true</tt> if this set contains no elements
*/
public boolean isEmpty() {
return map.isEmpty();
}
/**
* Returns <tt>true</tt> if this set contains the specified element.
* More formally, returns <tt>true</tt> if and only if this set
* contains an element <tt>e</tt> such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*
* @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);
}
/**
* Adds the specified element to this set if it is not already present.
* More formally, adds the specified element <tt>e</tt> to this set if
* this set contains no element <tt>e2</tt> such that
* <tt>(e==null ? e2==null : e.equals(e2))</tt>.
* If this set already contains the element, the call leaves the set
* unchanged and returns <tt>false</tt>.
*
* @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;
}
/**
* Removes the specified element from this set if it is present.
* More formally, removes an element <tt>e</tt> such that
* <tt>(o==null ? e==null : o.equals(e))</tt>,
* if this set contains such an element. Returns <tt>true</tt> 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 <tt>true</tt> 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() {
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
*/
public Spliterator<E> spliterator() {
return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
}
}
https://www.jianshu.com/p/1f7a8dda341b https://www.cnblogs.com/tong-yuan/p/HashSet.html