//默认初始容量为16 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; //默认负载因子为0.75 static final float DEFAULT_LOAD_FACTOR = 0.75f; //Hash数组(在resize()中初始化) transient Node<K,V>[] table; //元素个数 transient int size; //容量阈值(元素个数超过该值会自动扩容) int threshold;
static class Node<K,V> implements Map.Entry<K,V> { final int hash; final K key; V value; Node<K,V> next; Node(int hash, K key, V value, Node<K,V> next) { this.hash = hash; this.key = key; this.value = value; this.next = next; } public final K getKey() { return key; } public final V getValue() { return value; } public final String toString() { return key "=" value; } public final int hashCode() { return Objects.hashCode(key) ^ Objects.hashCode(value);//^表示相同返回0,不同返回1 //Objects.hashCode(o)————>return o != null ? o.hashCode() : 0; } public final V setValue(V newValue) { V oldValue = value; value = newValue; return oldValue; } public final boolean equals(Object o) { if (o == this) return true; if (o instanceof Map.Entry) { Map.Entry<?,?> e = (Map.Entry<?,?>)o; //Objects.equals(1,b)————> return (a == b) || (a != null && a.equals(b)); if (Objects.equals(key, e.getKey()) && Objects.equals(value, e.getValue())) return true; } return false; } }
/*找到大于或等于 cap 的最小2的幂,用来做容量阈值*/ 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; }
/*传入初始容量和负载因子*/ 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); this.loadFactor = loadFactor; this.threshold = tableSizeFor(initialCapacity); }
/*扩容*/ final Node<K,V>[] resize() { Node<K,V>[] oldTab = table; int oldCap = (oldTab == null) ? 0 : oldTab.length; int oldThr = threshold; int newCap, newThr = 0; //1、若oldCap>0 说明hash数组table已被初始化 if (oldCap > 0) { if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; }//按当前table数组长度的2倍进行扩容,阈值也变为原来的2倍 else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; }//2、若数组未被初始化,而threshold>0说明调用了HashMap(initialCapacity)和HashMap(initialCapacity, loadFactor)构造器 else if (oldThr > 0) newCap = oldThr;//新容量设为数组阈值 else { //3、若table数组未被初始化,且threshold为0说明调用HashMap()构造方法 newCap = DEFAULT_INITIAL_CAPACITY;//默认为16 newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);//16*0.75 } //若计算过程中,阈值溢出归零,则按阈值公式重新计算 if (newThr == 0) { float ft = (float)newCap * loadFactor; newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE); } threshold = newThr; //创建新的hash数组,hash数组的初始化也是在这里完成的 Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; table = newTab; //如果旧的hash数组不为空,则遍历旧数组并映射到新的hash数组 if (oldTab != null) { for (int j = 0; j < oldCap; j) { Node<K,V> e; if ((e = oldTab[j]) != null) { oldTab[j] = null;//GC if (e.next == null)//如果只链接一个节点,重新计算并放入新数组 newTab[e.hash & (newCap - 1)] = e; //若是红黑树,则需要进行拆分 else if (e instanceof TreeNode) ((TreeNode<K,V>)e).split(this, newTab, j, oldCap); else { //rehash————>重新映射到新数组 Node<K,V> loHead = null, loTail = null; Node<K,V> hiHead = null, hiTail = null; Node<K,V> next; do { next = e.next; /*注意这里使用的是:e.hash & oldCap,若为0则索引位置不变,不为0则新索引=原索引 旧数组长度*/ 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; }
static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); }
public V get(Object key) { Node<K,V> e; return (e = getNode(hash(key), key)) == null ? null : e.value;//hash(key)不等于key.hashCode } final Node<K,V> getNode(int hash, Object key) { Node<K,V>[] tab; //指向hash数组 Node<K,V> first, e; //first指向hash数组链接的第一个节点,e指向下一个节点 int n;//hash数组长度 K k; /*(n - 1) & hash ————>根据hash值计算出在数组中的索引index(相当于对数组长度取模,这里用位运算进行了优化)*/ if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) { //基本类型用==比较,其它用euqals比较 if (first.hash == hash && ((k = first.key) == key || (key != null && key.equals(k)))) return first; if ((e = first.next) != null) { //如果first是TreeNode类型,则调用红黑树查找方法 if (first instanceof TreeNode) return ((TreeNode<K,V>)first).getTreeNode(hash, key); do {//向后遍历 if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } while ((e = e.next) != null); } } return null; }`
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;//指向hash数组 Node<K,V> p;//初始化为table中第一个节点 int n, i;//n为数组长度,i为索引 //tab被延迟到插入新数据时再进行初始化 if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; //如果数组中不包含Node引用,则新建Node节点存入数组中即可 if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null);//new Node<>(hash, key, value, next) else { Node<K,V> e; //如果要插入的key-value已存在,用e指向该节点 K k; //如果第一个节点就是要插入的key-value,则让e指向第一个节点(p在这里指向第一个节点) if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) e = p; //如果p是TreeNode类型,则调用红黑树的插入操作(注意:TreeNode是Node的子类) else if (p instanceof TreeNode) e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); else { //对链表进行遍历,并用binCount统计链表长度 for (int binCount = 0; ; binCount) { //如果链表中不包含要插入的key-value,则将其插入到链表尾部 if ((e = p.next) == null) { p.next = newNode(hash, key, value, null); //如果链表长度大于或等于树化阈值,则进行树化操作 if (binCount >= TREEIFY_THRESHOLD - 1) treeifyBin(tab, hash); break; } //如果要插入的key-value已存在则终止遍历,否则向后遍历 if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; p = e; } } //如果e不为null说明要插入的key-value已存在 if (e != null) { V oldValue = e.value; //根据传入的onlyIfAbsent判断是否要更新旧值 if (!onlyIfAbsent || oldValue == null) e.value = value; afterNodeAccess(e); return oldValue; } } modCount; //键值对数量超过阈值时,则进行扩容 if ( size > threshold) resize(); afterNodeInsertion(evict);//也是空函数?回调?不知道干嘛的 return null; }
public V remove(Object key) { Node<K,V> e; return (e = removeNode(hash(key), key, null, false, true)) == null ? null : e.value; } final Node<K,V> removeNode(int hash, Object key, Object value,boolean matchValue, boolean movable) { Node<K,V>[] tab; Node<K,V> p; int n, index; //1、定位元素桶位置 if ((tab = table) != null && (n = tab.length) > 0 && (p = tab[index = (n - 1) & hash]) != null) { Node<K,V> node = null, e; K k; V v; // 如果键的值与链表第一个节点相等,则将 node 指向该节点 if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) node = p; else if ((e = p.next) != null) { // 如果是 TreeNode 类型,调用红黑树的查找逻辑定位待删除节点 if (p instanceof TreeNode) node = ((TreeNode<K,V>)p).getTreeNode(hash, key); else { // 2、遍历链表,找到待删除节点 do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { node = e; break; } p = e; } while ((e = e.next) != null); } } // 3、删除节点,并修复链表或红黑树 if (node != null && (!matchValue || (v = node.value) == value || (value != null && value.equals(v)))) { if (node instanceof TreeNode) ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable); else if (node == p) tab[index] = node.next; else p.next = node.next; modCount; --size; afterNodeRemoval(node); return node; } } return null; }
Map<String, Integer> map = new HashMap<>(); map.put("1", 1); map.put("2", 2); map.put("3", 3); for (String s : map.keySet()) { if (s.equals("2")) map.remove("2"); }
transient int modCount;
public Set<K> keySet() { Set<K> ks = keySet; if (ks == null) { ks = new KeySet(); keySet = ks; } return ks; } final class KeySet extends AbstractSet<K> { public final Iterator<K> iterator() { return new KeyIterator(); } // 省略部分代码 } final class KeyIterator extends HashIterator implements Iterator<K> { public final K next() { return nextNode().key; } } /*HashMap迭代器基类,子类有KeyIterator、ValueIterator等*/ abstract class HashIterator { Node<K,V> next; //下一个节点 Node<K,V> current; //当前节点 int expectedModCount; //修改次数 int index; //当前索引 //无参构造 HashIterator() { expectedModCount = modCount; Node<K,V>[] t = table; current = next = null; index = 0; //找到第一个不为空的桶的索引 if (t != null && size > 0) { do {} while (index < t.length && (next = t[index ]) == null); } } //是否有下一个节点 public final boolean hasNext() { return next != null; } //返回下一个节点 final Node<K,V> nextNode() { Node<K,V>[] t; Node<K,V> e = next; if (modCount != expectedModCount) throw new ConcurrentModificationException();//fail-fast if (e == null) throw new NoSuchElementException(); //当前的链表遍历完了就开始遍历下一个链表 if ((next = (current = e).next) == null && (t = table) != null) { do {} while (index < t.length && (next = t[index ]) == null); } return e; } //删除元素 public final void remove() { Node<K,V> p = current; if (p == null) throw new IllegalStateException(); if (modCount != expectedModCount) throw new ConcurrentModificationException(); current = null; K key = p.key; removeNode(hash(key), key, null, false, false);//调用外部的removeNode expectedModCount = modCount; } }
if (modCount != expectedModCount) throw new ConcurrentModificationException();
`removeNode(hash(key), key, null, false, false);//调用外部的removeNode expectedModCount = modCount;`
Map<String, Integer> map = new HashMap<>(); map.put("1", 1); map.put("2", 2); map.put("3", 3); Iterator<String> iterator = map.keySet().iterator(); while (iterator.hasNext()){ if (iterator.next().equals("2")) iterator.remove(); }
public class Person { Integer id; String name; public Person(Integer id, String name) { this.id = id; this.name = name; } @Override public boolean equals(Object obj) { if (obj == null) return false; if (obj == this) return true; if (obj instanceof Person) { Person person = (Person) obj; if (this.id == person.id) return true; } return false; } public static void main(String[] args) { Person p1 = new Person(1, "aaa"); Person p2 = new Person(1, "bbb"); HashMap<Person, String> map = new HashMap<>(); map.put(p1, "这是p1"); System.out.println(map.get(p2)); } }