理解 HashSet 基于Java1.8

╰+攻爆jí腚メ 2022-02-25 18:38 343阅读 0赞

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title: “理解 HashSet 基于Java1.8”

url: “[https://wsk1103.github.io/][https_wsk1103.github.io]”

tags:

*  Java

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**java -version** ：jdk 1.8.0\_191

## 构造 ##

![image][]

## 类内参数，方法 ##

![image][image 1]

## 实现 ##

基于**HashMap**实现。

当理解 **HashMap** 的时候，也就能理解HashSet了。

因为 **HashSet** 的 **key** 就是 **HashMap** 的 **key** ，而 对应的 **value** 则存储 **new Object()** 。

## 静态参数 ##

private transient HashMap<E,Object> map;
        方法内有这样的一个全局变量，用了存储 key 。
        无法被序列化
        // Dummy value to associate with an Object in the backing Map
        private static final Object PRESENT = new Object();
        对应的HashMap中就是存储这个值

## 构造方法 ##

#### HashSet() ####

默认初始化一个 HashMap

/**
         * 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<>();
        }

#### HashSet(Collection<? extends E> c) ####

默认的负载因子为 16 和 集合的数量 之间的最大值。

/**
         * 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);
        }

#### public HashSet(int initialCapacity, float loadFactor) ####

/**
         * 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);
        }

#### HashSet(int initialCapacity) ####

/**
         * 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);
        }

#### HashSet(int initialCapacity, float loadFactor, boolean dummy) ####

生成一个LinkedHashMap

/**
         * 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);
        }

## add方法 ##

调用 **HashMap** 的 add();

/**
         * 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) {
            //存储到 HashMap 的 key 中
            return map.put(e, PRESENT)==null;
        }

## remove 方法 ##

调用 HashMap 的remove 方法。

/**
         * 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;
        }

## 序列化 writeObject(java.io.ObjectOutputStream s) ##

序列化的时候，也是调用 HashMap 的相关方法，然后遍历HashMap， 把有值的序列化出来，而不是序列化整个HashSet。

/**
         * 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);
        }

## 反序列化readObject(java.io.ObjectInputStream s) ##

/**
         * 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);
            }
        }

## 其他 ##

#### 没有 get 方法。 ####

#### 线程不安全 ####

由于是基于 **HashMap** 实现，所以是线程不安全的。

当要使用线程安全的 **Set** 时，可以使用 **JUC** 包里的 **ConcurrentSkipListSet** （跳表），是基于 **ConcurrentSkipListMap** 实现的。

[https_wsk1103.github.io]: https://wsk1103.github.io/
[image]: https://raw.githubusercontent.com/wsk1103/images/master/set/1.png
[image 1]: https://raw.githubusercontent.com/wsk1103/images/master/set/2.png