hashMap源码--JDK1.8
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重要的filed
默认容量为16
/** * The default initial capacity - MUST be a power of two. * 建议容量为 2的n次幂 */ static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
默认负载因子
/** * The load factor used when none specified in constructor. */ static final float DEFAULT_LOAD_FACTOR = 0.75f;
最大容量 2^30
/** * The maximum capacity, used if a higher value is implicitly specified * by either of the constructors with arguments. * MUST be a power of two <= 1<<30. */ static final int MAXIMUM_CAPACITY = 1 << 30;
数据bin转换成红黑树的阈值
/** * hash桶的存储方式由list转为tree的转换阈值(插入第9个元素时,list转为tree) * 该阈值必须大于2,并且至少应为8才能与树删除中的假设(收缩时转换回普通箱)相啮合 */ static final int TREEIFY_THRESHOLD = 8; /** * 在调整hash桶大小的操作中,取消hash桶的树化存储的计数阈值 * (当一个hash桶中的元素小于该值时,转换成链表存储) * 应该小于TREEIFY_THRESHOLD,且最大为6,用于删除操作后的收缩检查 */ static final int UNTREEIFY_THRESHOLD = 6; /** * hash桶树化存储的table的最小容量。(否则,如果hash桶中的节点过多,将调整table的大小。) * 应至少为 4 * TREEIFY_THRESHOLD,以避免调整大小和树化阈值之间的冲突。 * 因为一个比较小,比较满的散列表的性能不如一个比较大,比较空的散列表, * 这种请款先考虑变大,而不是树化存储 */ static final int MIN_TREEIFY_CAPACITY = 64;
数据table
/** * 第一次使用时初始化,根据需要调整大小(始终为2的n次幂) * 在某些操作中,我们还允许长度为零,以允许使用当前不需要的引导机制。 */ transient Node<K,V>[] table; //键值对的数量 transient int size; //结构修改的次数 transient int modCount; //下一个要调整大小的大小值 (容量*负载系数) //如果hash桶数组没有初始化,则该字段持有出事容量,或者是0(表示使用 DEFAULT_INITIAL_CAPACITY) int threshold; //负载因子 final float loadFactor;
数据节点类型
static class Node<K,V> implements Map.Entry<K,V> { final int hash; //用来定位数组索引位置 final K key; V value; Node<K,V> next; //链表的下一个node Node(int hash, K key, V value, Node<K,V> next) { ... } public final K getKey(){ ... } public final V getValue() { ... } public final String toString() { ... } public final int hashCode() { ... } public final V setValue(V newValue) { ... } public final boolean equals(Object o) { ... } } static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> { TreeNode<K,V> parent; // 父 TreeNode<K,V> left; // 左 TreeNode<K,V> right; // 右 TreeNode<K,V> prev; // needed to unlink next upon deletion boolean red; // 判断颜色 TreeNode(int hash, K key, V val, Node<K,V> next) { super(hash, key, val, next); } // 返回根节点 final TreeNode<K,V> root() { for (TreeNode<K,V> r = this, p;;) { if ((p = r.parent) == null) return r; r = p; }
构造方法
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); // 获得对应容量的 2的n次幂 this.loadFactor = loadFactor; this.threshold = tableSizeFor(initialCapacity); } // 构造新的HashMap 使用Map接口的集合: 使用默认loadFactor(0.75) 足以(最小可用)将映射保存在指定Map中的初始容量 public HashMap(Map<? extends K, ? extends V> m) { this.loadFactor = DEFAULT_LOAD_FACTOR; putMapEntries(m, false); } // 实现 Map.putAll and Map constructor 这俩方法 final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) { int s = m.size(); if (s > 0) { // 判断table是否已经初始化 if (table == null) { // pre-size // capacity * loadFactor = threshold(最小可用 入参map的size=threshold) float ft = ((float)s / loadFactor) + 1.0F; // 得到保存入参map(size)需要的最小 capacity int t = ((ft < (float)MAXIMUM_CAPACITY) ? (int)ft : MAXIMUM_CAPACITY); //根据容量 刷新阈值 if (t > threshold) threshold = tableSizeFor(t); } // 已初始化,并且m元素个数大于阈值,进行扩容处理 else if (s > threshold) resize(); for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) { K key = e.getKey(); V value = e.getValue(); // constructor-evict:false // putAll-evict:true putVal(hash(key), key, value, false, evict); } } } /** * 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; // table未初始化或者长度为0,进行扩容 if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; // (n - 1) & hash 确定元素存放在哪个桶中,桶为空,新生成结点放入桶中(此时,这个结点是放在数组中) if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null); // 桶中已经存在元素 else { Node<K,V> e; K k; // 比较桶中第一个元素(数组中的结点)的hash值相等,key相等 if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) // 将第一个元素赋值给e,用e来记录 e = p; else if (p instanceof TreeNode) e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); else { for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) { p.next = newNode(hash, key, value, null); if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st treeifyBin(tab, hash); break; } if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; p = e; } } if (e != null) { // existing mapping for key V oldValue = e.value; if (!onlyIfAbsent || oldValue == null) e.value = value; afterNodeAccess(e); return oldValue; } } ++modCount; if (++size > threshold) resize(); afterNodeInsertion(evict); return null; } /** * 初始化或加倍数组的大小。如果为空,则根据属性阈值中保持的初始容量目标进行分配。 * 否则,因为我们使用的是2的幂,所以每个bin中的元素必须保持相同的索引,或者在新表中以2的幂偏移。 * * @return the table */ final Node<K,V>[] resize() { Node<K,V>[] oldTab = table; int oldCap = (oldTab == null) ? 0 : oldTab.length; int oldThr = threshold; int newCap, newThr = 0; //扩容前不为空 if (oldCap > 0) { // 超过最大值就不再扩充了,就只好随你碰撞去吧 if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; } // 没超过最大值,就扩充为原来的2倍(翻倍后不能大于最大容量) else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; // double threshold } // 初始化 初始化容量=阈值(2参数构造中赋值的) else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr; // 初始化方式--threshold=0(表示使用默认值 DEFAULT_INITIAL_CAPACITY) else { // zero initial threshold signifies using defaults newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); } // 计算新的resize上限 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"}) 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; }
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