java基础---HashMap负载因子

红太狼 2022-05-30 07:59 170阅读 0赞

这两天在看HashMap的时候，被负载因子float loadFactor搞得很晕，经过一天的研究，最后理出了自己的一点个人见解。

在HashMap的底层存在着一个名字为table的Entry数组，在实例化HashMap的时候，会输入两个参数，一个是 int initCapacity（初始化数组大小，默认值是16），一个是float loadFactor（负载因子，默认值是0.75），首先会根据initCapacity计算出一个大于或者等于initCapacity且为2的幂的值capacity，例如initCapacity为15，那么capacity就会为16，还会算出一个临界值threshold，也就是capacity \* loadFactor，贴出源代码

![复制代码][copycode.gif]

/**
         * Constructs an empty <tt>HashMap</tt> with the specified initial
         * capacity and load factor.
         *
         * @param  initialCapacity the initial capacity
         * @param  loadFactor      the load factor
         * @throws IllegalArgumentException if the initial capacity is negative
         *         or the load factor is nonpositive
         */
        public HashMap(int initialCapacity, float loadFactor) {
            if (initialCapacity < 0)
                throw new IllegalArgumentException("Illegal initial capacity: " +
                                                   initialCapacity);
            if (initialCapacity > MAXIMUM_CAPACITY)
                initialCapacity = MAXIMUM_CAPACITY;
            if (loadFactor <= 0 || Float.isNaN(loadFactor))
                throw new IllegalArgumentException("Illegal load factor: " +
                                                   loadFactor);
    
            // Find a power of 2 >= initialCapacity
            int capacity = 1;
            while (capacity < initialCapacity)
                capacity <<= 1;
    
            this.loadFactor = loadFactor;
            threshold = (int)(capacity * loadFactor);
            table = new Entry[capacity];
            init();
        }

![复制代码][copycode.gif]

创建完HashMap之后，下面来看put方法，首先会根据key值计算出其HashCode值，然后在通过一个indexFor方法计算出此元素该存放于table数组的哪个数组之中（我猜想可能是通过对table.length的值取余的操作计算出来的），再检测此table的此坐标位置的entry链是否存在此key或者此key值，若存在，则更新此元素的value值。源代码如下：

![复制代码][copycode.gif]

/**
         * Associates the specified value with the specified key in this map.
         * If the map previously contained a mapping for the key, the old
         * value is replaced.
         *
         * @param key key with which the specified value is to be associated
         * @param value value to be associated with the specified key
         * @return the previous value associated with <tt>key</tt>, or
         *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
         *         (A <tt>null</tt> return can also indicate that the map
         *         previously associated <tt>null</tt> with <tt>key</tt>.)
         */
        public V put(K key, V value) {
            if (key == null)
                return putForNullKey(value);
            int hash = hash(key.hashCode());
            int i = indexFor(hash, table.length);
            for (Entry<K,V> e = table[i]; e != null; e = e.next) {
                Object k;
                if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                    V oldValue = e.value;
                    e.value = value;
                    e.recordAccess(this);
                    return oldValue;
                }
            }
    
            modCount++;
            addEntry(hash, key, value, i);
            return null;
        }

![复制代码][copycode.gif]

下面来看看addEntry方法，参数bucketIndex就是当前元素应该插入到entry数组的下标，先取出放在此位置的entry，然后把当前元素放入该数组中，当前元素的next指向之前取出元素，形成entry链表。（描述的不是很清楚，大概就是把新加入的entry当成头放入到数组当中，然后指向之前的链表），放入之后就去判断当前的size是否达到了threshold极限值，若达到了，将会进行扩容。源代码如下：

![复制代码][copycode.gif]

/**
         * Adds a new entry with the specified key, value and hash code to
         * the specified bucket.  It is the responsibility of this
         * method to resize the table if appropriate.
         *
         * Subclass overrides this to alter the behavior of put method.
         */
        void addEntry(int hash, K key, V value, int bucketIndex) {
        Entry<K,V> e = table[bucketIndex];
            table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
            if (size++ >= threshold)
                resize(2 * table.length);
        }

![复制代码][copycode.gif]

下面来看resize方法，方法比较简单就是生成一个新的table数组（entry数组），然后根据新的Capacity和负载因子去生成新的临界值。重点是里面有个transfer方法。源代码如下：

![复制代码][copycode.gif]

/**
         * Rehashes the contents of this map into a new array with a
         * larger capacity.  This method is called automatically when the
         * number of keys in this map reaches its threshold.
         *
         * If current capacity is MAXIMUM_CAPACITY, this method does not
         * resize the map, but sets threshold to Integer.MAX_VALUE.
         * This has the effect of preventing future calls.
         *
         * @param newCapacity the new capacity, MUST be a power of two;
         *        must be greater than current capacity unless current
         *        capacity is MAXIMUM_CAPACITY (in which case value
         *        is irrelevant).
         */
        void resize(int newCapacity) {
            Entry[] oldTable = table;
            int oldCapacity = oldTable.length;
            if (oldCapacity == MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return;
            }
    
            Entry[] newTable = new Entry[newCapacity];
            transfer(newTable);
            table = newTable;
            threshold = (int)(newCapacity * loadFactor);
        }

![复制代码][copycode.gif]

因为table数组的容量增加了，那么相应的table的length也增加了，那么之前存储的元素的位置也就不一样了，比如之前的length是16，现在的length是32，那么hashCode模16和HashCode模32的结果很有可能会不一样，所以就只有重新去计算新的位置，方法是遍历数组，在遍历数组上的entry链。（此时就是所谓的rehash？？）

![复制代码][copycode.gif]

/**
         * Transfers all entries from current table to newTable.
         */
        void transfer(Entry[] newTable) {
            Entry[] src = table;
            int newCapacity = newTable.length;
            for (int j = 0; j < src.length; j++) {
                Entry<K,V> e = src[j];
                if (e != null) {
                    src[j] = null;
                    do {
                        Entry<K,V> next = e.next;
                        int i = indexFor(e.hash, newCapacity);
                        e.next = newTable[i];
                        newTable[i] = e;
                        e = next;
                    } while (e != null);
                }
            }
        }

![复制代码][copycode.gif]

总结：当负载因子较大时，去给table数组扩容的可能性就会少，所以相对占用内存较少（空间上较少），但是每条entry链上的元素会相对较多，查询的时间也会增长（时间上较多）。反之就是，负载因子较少的时候，给table数组扩容的可能性就高，那么内存空间占用就多，但是entry链上的元素就会相对较少，查出的时间也会减少。所以才有了负载因子是时间和空间上的一种折中的说法。所以设置负载因子的时候要考虑自己追求的是时间还是空间上的少。

注意：设置initCapacity的时候，尽量设置为2的幂，这样会去掉计算比initCapactity大，且为2的幂的数的运算。

[copycode.gif]: /images/20220530/4d2a9999b2a949db8f0c2cbedf2a1744.png