HashMap的put源码解读

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        // 判断数组是否初始化
        if ((tab = table) == null || (n = tab.length) == 0)
        // 没有初始化 调用resize方法初始化
            n = (tab = resize()).length;
        // 通过计算hash值判断在数组下标是否有值
        if ((p = tab[i = (n - 1) & hash]) == null)
        	// 没有值直接放在该数组下标位置
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            // 判断该位置数据和传入的值是否一样
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                // 一致 直接覆盖
                e = p;
            // 判断是否是红黑树
            else if (p instanceof TreeNode)
            	// 红黑树的添加逻辑
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
            	// 遍历链表
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                    	// 把新值放入链表尾部
                        p.next = newNode(hash, key, value, null);
                        // 如果当前长度大于7 转为红黑树
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    // 链表中有相同的hash值 如果一样 直接覆盖
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    // 把下一个节点赋值为当前节点
                    p = e;
                }
            }
            // 判断是否是修改的操作，e就是修改操作存放原数据的变量
            if (e != null) { // existing mapping for key
            	// 取出旧的值
                V oldValue = e.value;
                // 一定会执行，onlyIfAbsent传进来的是false
                if (!onlyIfAbsent || oldValue == null)
                	// 将新值赋值当前节点
                    e.value = value;
                afterNodeAccess(e);
                // 返回旧的值
                return oldValue;
            }
        }
        // 计数器。计算当前节点的修改次数
        ++modCount;
        // 当前数组中的数据长度时候大于阈值
        if (++size > threshold)
        	// 扩容
            resize();
        // 空方法
        afterNodeInsertion(evict);
        return null;
    }

下面是扩容的源码解读

final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        // 如果当前数组未null，把旧数组的容量设置为0
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        // 旧的扩容阈值
        int oldThr = threshold;
        int newCap, newThr = 0;
        // 判断数组容量是否大于0 如果大于0 说明数组已经初始化
        if (oldCap > 0) {
        	// 判断当前数组长度是否大于最大数组长度 MAXIMUM_CAPACITY = 1 << 30 = 1073741824
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            // 左移一位 扩大两倍
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
        // 如果大于0 数组已经初始化
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        // 数组未初始化 赋予默认值
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        // 初始化容量小于16时，扩容阈值没有赋值
        if (newThr == 0) {
        	
            float ft = (float)newCap * loadFactor;
            // 判断最新阈值是否大于最大容量
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
        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;
    }