/** * 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 * * * * 初始化或者翻倍表大小。 * 如果表为null,则根据存放在threshold变量中的初始化capacity的值来分配table内存 * (这个注释说的很清楚,在实例化HashMap时,capacity其实是存放在了成员变量threshold中, * 注意,HashMap中没有capacity这个成员变量) * 。如果表不为null,由于我们使用2的幂来扩容, * 则每个bin元素要么还是在原来的bucket中,要么在2的幂中 * * 此方法功能:初始化或扩容 */ final Node<K,V>[] resize() { Node<K,V>[] oldTab = table; int oldCap = (oldTab == null) ? 0 : oldTab.length; int oldThr = threshold; //新的容量值,新的扩容阀界值 int newCap, newThr = 0; //oldTab!=null,则oldCap>0 if (oldCap > 0) { //如果此时oldCap>=MAXIMUM_CAPACITY(1 << 30),表示已经到了最大容量,这时还要往map中放数据,则阈值设置为整数的最大值 Integer.MAX_VALUE,直接返回这个oldTab的内存地址。 if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; } //如果(当前容量*2<最大容量&&当前容量>=默认初始化容量(16)) //并将将原容量值<<1(相当于*2)赋值给 newCap else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) //如果能进来证明此map是扩容而不是初始化 //操作:将原扩容阀界值<<1(相当于*2)赋值给 newThr newThr = oldThr << 1; // double threshold } else if (oldThr > 0) // initial capacity was placed in threshold //进入此if证明创建map时用的带参构造:public HashMap(int initialCapacity)或 public HashMap(int initialCapacity, float loadFactor) //注:带参的构造中initialCapacity(初始容量值)不管是输入几都会通过 “this.threshold = tableSizeFor(initialCapacity);”此方法计算出接近initialCapacity参数的2^n来作为初始化容量(初始化容量==oldThr) newCap = oldThr; else { // zero initial threshold signifies using defaults //进入此if证明创建map时用的无参构造: //然后将参数newCap(新的容量)、newThr(新的扩容阀界值)进行初始化 newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); } if (newThr == 0) { //进入此if有两种可能 // 第一种:进入此“if (oldCap > 0)”中且不满足该if中的两个if // 第二种:进入这个“else if (oldThr > 0)” //分析:进入此if证明该map在创建时用的带参构造,如果是第一种情况就说明是进行扩容且oldCap(旧容量)小于16,如果是第二种说明是第一次put float ft = (float)newCap * loadFactor; //计算扩容阀界值 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; //如果“oldTab != null”说明是扩容,否则直接返回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) //如果该元素是TreeNode的实例 ((TreeNode<K,V>)e).split(this, newTab, j, oldCap); else { // preserve order Node<K,V> loHead = null, loTail = null;//此对象接收会放在原来位置 Node<K,V> hiHead = null, hiTail = null;//此对象接收会放在“j + oldCap”(当前位置索引+原容量的值) Node<K,V> next; do { next = e.next; //以下是扩容操作的核心,详情见我的博客:https://www.cnblogs.com/shianliang/p/9204942.html 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; }
原文地址:https://www.cnblogs.com/shianliang/p/9233199.html
时间: 2024-10-08 20:41:50