HashMap与HashSet源码阅读
HashMap
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文件名: java.util.HashMap.class
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原理：用数组表示哈希桶，根据哈希值将对象散列在对应的哈希桶中，如果发生Collision，则有开放地址法和链表法两种处理方式。Java的HashMap采用的是链表法，而且在Java8及其后续版本，将链表改为了红黑树，为此提高查询效率。
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关键的属性和方法
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计算哈希
static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); } -
哈希桶：数组实现，支持O(1)的随机存取
transient Node<K,V>[] table; -
get方法：基于getNode方法实现
public V get(Object key) { Node<K,V> e; return (e = getNode(hash(key), key)) == null ? null : e.value; } -
put方法：基于putVal方法实现
public V put(K key, V value) { return putVal(hash(key), key, value, false, true); } -
getNode：如果无冲突，则返回哈希桶的值，否则根据链表法，到该哈希桶的红黑树中查找
final Node<K,V> getNode(int hash, Object key) { Node<K,V>[] tab; Node<K,V> first, e; int n; K k; if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) { if (first.hash == hash && // always check first node ((k = first.key) == key || (key != null && key.equals(k)))) return first; // 直接从哈希桶中返回 if ((e = first.next) != null) { if (first instanceof TreeNode) return ((TreeNode<K,V>)first).getTreeNode(hash, key); // 哈希桶中发生访问冲突，则先从红黑树中查找 do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } while ((e = e.next) != null); // 红黑树中找不到了，则从链表中查找（好奇怪，似乎是遗留代码，望大佬解答） } } -
putVal：和getNode类似，也是先直接放到哈希桶中，哈希桶中放不下之后再放到红黑树中。与getNode相比，多了扩容操作。
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<K,V>[] tab; Node<K,V> p; int n, i; if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null); else { Node<K,V> e; K k; if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) e = p; // 直接在哈希桶中添加 else if (p instanceof TreeNode) e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); // 如果访问冲突，向该哈希桶对应的红黑树添加 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; } -
默认负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f; -
resize：每当HashMap的负载因子超过了默认负载因子之后，就会发生resize. Java HashMap的哈希桶的个数总是2的次幂，因此每次resize时，总是将哈希桶的个数翻倍。根据哈希的求余性质，哈希桶中的对象要么留在原地，要么向右移动2的次幂。
/** * 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 */ final Node<K,V>[] resize() { Node<K,V>[] oldTab = table; int oldCap = (oldTab == null) ? 0 : oldTab.length; int oldThr = threshold; int newCap, newThr = 0; if (oldCap > 0) { if (oldCap >= MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return oldTab; } else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; // double threshold } else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr; else { // zero initial threshold signifies using defaults newCap = DEFAULT_INITIAL_CAPACITY; newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); } 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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HashSet
文件名：java.util.HashSet.class
HashSet封装了HashMap作为私有属性
private transient HashMap<E,Object> map;
其add, contains方法直接调用了HashMap的方法
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}
public boolean contains(Object o) {
return map.containsKey(o);
}