概念

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

   /**
 * 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();
}

initailCapacity，loadFactor会影响到HashMap扩容。
HashMap每次put操作是都会检查一遍 size（当前容量）>initailCapacity*loadFactor 是否成立。如果不成立则HashMap扩容为以前的两倍（数组扩成两倍），
然后重新计算每个元素在数组中的位置，然后再进行存储。这是一个十分消耗性能的操作。
所以如果能根据业务预估出HashMap的容量，应该在创建的时候指定容量，那么可以避免resize().

   /**
 * 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);
}

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。

  /**
 * 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);
        }
    }
}

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，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。

  /**
 * 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;
}

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。

   /**
 * 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);
}

总结

负载因子表示一个散列表的空间的使用程度，有这样一个公式：initailCapacity*loadFactor=HashMap的容量。
所以负载因子越大则散列表的装填程度越高，也就是能容纳更多的元素，元素多了，链表大了，所以此时索引效率就会降低。
反之，负载因子越小则链表中的数据量就越稀疏，此时会对空间造成烂费，但是此时索引效率高。