Container-HashMap add element, bottom-level analysis source code of array expansion (18)
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We use map.put(), use Ctrl+left mouse button to enter the source code, use Ctrl+Alt in the put() method to select the HashMap interface implementation class to enter the source code, and analyze the added elements.
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Enter the source code of put()
/** * Implements Map.put and related methods * * @param hash hash for key * @param key the key * @param value the value to put * @param onlyIfAbsent if true, don't change existing value * @param evict if false, the table is in creation mode. * @return previous value, or null if none */ final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<K,V>[] tab; Node<K,V> p; int n, i;//定义一些局部变量 if ((tab = table) == null || (n = tab.length) == 0)//判断数组是否为空 n = (tab = resize()).length;//resize()方法,数组初始化、扩容 if ((p = tab[i = (n - 1) & hash]) == null)//我们刚刚计算的哈希值 p=12,也就是判断我们的哈希值有没有元素 tab[i] = newNode(hash, key, value, null);//如果p有元素,这个就不执行了,就走else else { Node<K,V> e; K k;//定义了两个局部变量 if (p.hash == hash &&//p调用的哈希值和新传进来的哈希值是否相等 ((k = p.key) == key || (key != null && key.equals(k)))) //也就是说,我们数组有一个12的索引,这个位置已经有一个节点了,新增的节点的Key和原先节点的Key相同了 e = p;//把p赋值给e else if (p instanceof TreeNode)//又要判断你这个p是否是一个树的节点 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //如果他是一个树了,就要往树上去挂节点 else { //p不是一个树的节点,就不足else,因为满足了p调用的哈希值和新传进来的哈希值是否相等的if ,如果索引12相同,但是Key不相同,则走的是这个Value,把新的节点挂接到老节点的后面 for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) { //p.next为null,就是找到最后一个节点了 p.next = newNode(hash, key, value, null); //把新节点挂在原来节点的后面 if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st //循环因子 TREEIFY_THRESHOLD是一个列表转树的一个域值,TREEIFY_THRESHOLD是8,还有一个条件就是数组的长度超过64,他才会去列表转成树,也就是下面那个 treeifyBin()的方法,我们来看下 treeifyBin(tab, hash); break; } //做节点的挂接 if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; p = e; } } //然后走这个if,这时e肯定是不为空的,就要做Value的替换了 if (e != null) { // existing mapping for key V oldValue = e.value;//e调用的Value赋给了oldValue if (!onlyIfAbsent || oldValue == null) e.value = value;//新传进来的value又赋给了e调用的Value,这就体现了Value的替换,这就是Key相同完成了Value的覆盖 afterNodeAccess(e); return oldValue;//把这个被替换掉的,老的Value返回 } } ++modCount; if (++size > threshold) resize(); afterNodeInsertion(evict); return null; }
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Take a look at treeifyBin(tab, hash)*; the method of making a linked list into a tree
/** * Replaces all linked nodes in bin at index for given hash unless * table is too small, in which case resizes instead. */ final void treeifyBin(Node<K,V>[] tab, int hash) { int n, index; Node<K,V> e; if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY) //MIN_TREEIFY_CAPACITY是64,也就是当数组的长度小于64时,它不会去做链表的转换,而是去做数组的扩容 resize();//做数组的扩容 //当数组的长度大于64,链表做树的转换 else if ((e = tab[index = (n - 1) & hash]) != null) { TreeNode<K,V> hd = null, tl = null; do { TreeNode<K,V> p = replacementTreeNode(e, null); if (tl == null) hd = p; else { p.prev = tl; tl.next = p; } tl = p; } while ((e = e.next) != null); if ((tab[index] = hd) != null) hd.treeify(tab); } }
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Array expansion
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After adding the elements, the array will be expanded, see the end of the putval() method
/** * Implements Map.put and related methods * * @param hash hash for key * @param key the key * @param value the value to put * @param onlyIfAbsent if true, don't change existing value * @param evict if false, the table is in creation mode. * @return previous value, or null if none */ final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<K,V>[] tab; Node<K,V> p; int n, i;//定义一些局部变量 if ((tab = table) == null || (n = tab.length) == 0)//判断数组是否为空 n = (tab = resize()).length;//resize()方法,数组初始化、扩容 if ((p = tab[i = (n - 1) & hash]) == null)//我们刚刚计算的哈希值 p=12,也就是判断我们的哈希值有没有元素 tab[i] = newNode(hash, key, value, null);//如果p有元素,这个就不执行了,就走else else { Node<K,V> e; K k;//定义了两个局部变量 if (p.hash == hash &&//p调用的哈希值和新传进来的哈希值是否相等 ((k = p.key) == key || (key != null && key.equals(k)))) //也就是说,我们数组有一个12的索引,这个位置已经有一个节点了,新增的节点的Key和原先节点的Key相同了 e = p;//把p赋值给e else if (p instanceof TreeNode)//又要判断你这个p是否是一个树的节点 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //如果他是一个树了,就要往树上去挂节点 else { //p不是一个树的节点,就不足else,因为满足了p调用的哈希值和新传进来的哈希值是否相等的if ,如果索引12相同,但是Key不相同,则走的是这个Value,把新的节点挂接到老节点的后面 for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) { //p.next为null,就是找到最后一个节点了 p.next = newNode(hash, key, value, null); //把新节点挂在原来节点的后面 if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st //循环因子 TREEIFY_THRESHOLD是一个列表转树的一个域值,TREEIFY_THRESHOLD是8,还有一个条件就是数组的长度超过64,他才会去列表转成树,也就是下面那个 treeifyBin()的方法,我们来看下 treeifyBin(tab, hash); break; } //做节点的挂接 if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; p = e; } } //然后走这个if,这时e肯定是不为空的,就要做Value的替换了 if (e != null) { // existing mapping for key V oldValue = e.value;//e调用的Value赋给了oldValue if (!onlyIfAbsent || oldValue == null) e.value = value;//新传进来的value又赋给了e调用的Value,这就体现了Value的替换,这就是Key相同完成了Value的覆盖 afterNodeAccess(e); return oldValue;//把这个被替换掉的,老的Value返回 } } ++modCount; //记录当前哈希容器改变的次数的,你添加、删除元素都会记录 if (++size > threshold)//判断数组是否要做扩容 //size记录当前节点的数量,threshold是一个扩容的域值,也就是数组的长度*扩容的负载因子,也就是16*0.75=12 resize();//进行扩容处理 afterNodeInsertion(evict); return null; }
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Let's continue to look at the expansion processing of resize();
/** * Initializes or doubles table size. If null, allocates in * accord with initial capacity target held in field threshold. * Otherwise, because we are using power-of-two expansion, the * elements from each bin must either stay at same index, or move * with a power of two offset in the new table. * * @return the table */ //当前数组的容量是以2的容量来扩容的 final Node<K,V>[] resize() { Node<K,V>[] oldTab = table;//table就是Node成员变量的数组 int oldCap = (oldTab == null) ? 0 : oldTab.length; //oldTab现在肯定是不为空的,返回的是oldTab的数组长度 int oldThr = threshold;//把扩容的域值赋给了oldThr int newCap, newThr = 0;//定义两个变量为0 if (oldCap > 0) { //oldCap现在肯定是大于0的 if (oldCap >= MAXIMUM_CAPACITY) { //判断oldCap是否大于最大上限值(MAXIMUM_CAPACITY)2的30次方 threshold = Integer.MAX_VALUE; //如果达到最大上限值,把Integer的最大值赋给threshold return oldTab; //然后返回oldTab,因为你已经到达最大容量了,没法在扩容了 } //很显然,现在oldCap是不大于最大上限值的 else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) //newCap = oldCap << 1=16*2=32 // oldCap=16 = DEFAULT_INITIAL_CAPACITY=16,索引他们现在是相等的 newThr = oldThr << 1; // double threshold //oldThr << 1 = threshold*2=12*2=24,所以newThr=24 } else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr; //if满足,else就不走了 else { // zero initial threshold signifies using defaults newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); } // newThr=24就不等于0,这个if也不走 if (newThr == 0) { float ft = (float)newCap * loadFactor; newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE); } threshold = newThr;//把新的扩容域值24赋给扩容因子,之前的扩容因子是12,现在变成24了 @SuppressWarnings({ "rawtypes","unchecked"}) //new了一个数组,然后把newCap=32放入Node数组里面 Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; table = newTab;//然后把新的数据赋给了table //oldTab等于之前的 table是16,是不为空的,继续走下面,就是把旧的数组的元素移到新的数组,做数组的移动 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;//这样就把新的扩容数组返回回去了 }
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