源码分析八( hashmap工作原理)

首先从一条简单的语句开始，创建了一个hashmap对象：

Map<String,String> hashmap = new HashMap<String,String>();

调用hashmap类无参数构造方法，使用默认转载因子，

 public HashMap() {
         this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
     }

默认装载因子只有在构造器不指定的情况下使用

 /**
      * The load factor used when none specified in constructor.
      */
     static final float DEFAULT_LOAD_FACTOR = 0.75f;

hashmap还有三个重载的构造方法，分别是：

有装载因子入参的构造方法

 public HashMap(int initialCapacity) {
         this(initialCapacity, DEFAULT_LOAD_FACTOR);
     }

有初始化容器容量以及装载因子的构造方法

 public HashMap(int initialCapacity, float loadFactor) {
         //如果初始容量小于0，则抛出非法初始化容量异常
         if (initialCapacity < 0)
             throw new IllegalArgumentException("Illegal initial capacity: " +
                                                initialCapacity);
         //如果初始容量大于最多容量，则容量为最多容量
         if (initialCapacity > MAXIMUM_CAPACITY)
             initialCapacity = MAXIMUM_CAPACITY;
         //如果装载因子不大于0或者装载因子不是数字，则抛出装载因子非法异常
         if (loadFactor <= 0 || Float.isNaN(loadFactor))
             throw new IllegalArgumentException("Illegal load factor: " +
                                                loadFactor);
         this.loadFactor = loadFactor;
         //根据初始化容量计算hashmap的阈值容量(hashmap真正的容量，必须是2的次方)
         this.threshold = tableSizeFor(initialCapacity);
      }

计算hashmap的容量

 // 根据给定的目标容量，计算一个2的次方
     static final int tableSizeFor(int cap) {
         int n = cap - 1;
         n |= n >>> 1;
         n |= n >>> 2;
         n |= n >>> 4;
         n |= n >>> 8;
         n |= n >>> 16;
         return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
     }

有参数map的构造方法：

 public HashMap(Map<? extends K, ? extends V> m) {
             this.loadFactor = DEFAULT_LOAD_FACTOR;
             putMapEntries(m, false);
      }

将map复制一份到本地，这个复制如果对于数值型则为数值，对于引用型则为地址

 final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
         int s = m.size();
         //原来map的长度大于0处理
         if (s > 0) {
             if (table == null) { // pre-size
                 //根据实际数量计算需要多大的容量
                 float ft = ((float) s / loadFactor) + 1.0F;
                 int t = ((ft < (float) MAXIMUM_CAPACITY) ? (int) ft : MAXIMUM_CAPACITY);
                 //如果预估容量大于现有容量，则需要根据预估容量计算一个2的次方数
                 if (t > threshold)
                     threshold = tableSizeFor(t);
             }
             //如果实际容量大于阈值则需要扩容
             else if (s > threshold)
                 resize();
             //扩容之后，将m的key和value复制一份到本地
             for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
                 K key = e.getKey();
                 V value = e.getValue();
                 putVal(hash(key), key, value, false, evict);
             }
         }
     }

 //第一次调用初始化或者扩容
     final Node<K,V>[] resize() {
         //首次调用时为null,则oldCap为0 | 扩容时，
         Node<K,V>[] oldTab = table;
         int oldCap = (oldTab == null) ? 0 : oldTab.length;
         int oldThr = threshold;
         int newCap, newThr = 0;
         //扩容时,如果目前容量已经超过最多容量，那么默认值为int最大，否则容量为现在2倍
         if (oldCap > 0) {
             if (oldCap >= MAXIMUM_CAPACITY) {
                 threshold = Integer.MAX_VALUE;
                 return oldTab;
             }
             else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                      oldCap >= DEFAULT_INITIAL_CAPACITY)
                 newThr = oldThr << 1; // double threshold
         }
         //初始化时newCap = oldThr = threshold
         else if (oldThr > 0) // initial capacity was placed in threshold
             newCap = oldThr;
         else {               // zero initial threshold signifies using defaults
             newCap = DEFAULT_INITIAL_CAPACITY;
             newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
         }
         //初始化时newThr=newCap * loadFactor
         if (newThr == 0) {
             float ft = (float)newCap * loadFactor;
             newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                       (int)ft : Integer.MAX_VALUE);
         }
         threshold = newThr;
         @SuppressWarnings({"rawtypes","unchecked"})
         //初始化时，创建一个长度为threshold的Node数组,然后返回
             Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
         table = newTab;
         if (oldTab != null) {
             for (int j = 0; j < oldCap; ++j) {
                 Node<K,V> e;
                 if ((e = oldTab[j]) != null) {
                     oldTab[j] = null;
                     if (e.next == null)
                         newTab[e.hash & (newCap - 1)] = e;
                     else if (e instanceof TreeNode)
                         ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                     else { // preserve order
                         Node<K,V> loHead = null, loTail = null;
                         Node<K,V> hiHead = null, hiTail = null;
                         Node<K,V> next;
                         do {
                             next = e.next;
                             if ((e.hash & oldCap) == 0) {
                                 if (loTail == null)
                                     loHead = e;
                                 else
                                     loTail.next = e;
                                 loTail = e;
                             }
                             else {
                                 if (hiTail == null)
                                     hiHead = e;
                                 else
                                     hiTail.next = e;
                                 hiTail = e;
                             }
                         } while ((e = next) != null);
                         if (loTail != null) {
                             loTail.next = null;
                             newTab[j] = loHead;
                         }
                         if (hiTail != null) {
                             hiTail.next = null;
                             newTab[j + oldCap] = hiHead;
                         }
                     }
                 }
             }
         }
         return newTab;
     }