【Java】Map总结和源码注释
- 2020 年 3 月 2 日
- 筆記
前言
Map为一个Java中一个重要的数据结构，主要表示<key, value>的映射关系对。本文包括了相关Map数据结构的总结和源码的阅读注释。
HashMap
初始化，可以选择第二个初始化函数来设置装载能力threshold和装载系数loadFactor：
HashMap()HashMap(int initialCapacity, float loadFactor)
HashMap中定义的一些常量：
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static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;缺省的初始大小
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static final int MAXIMUM_CAPACITY = 1 << 30;最大限定大小，当超过这个值时，会
resize()到Integer.MAX_VALUE -
static final float DEFAULT_LOAD_FACTOR = 0.75f;threshold = capacity*laodFactor
HashMap的大小始终为2的倍数，若插入时超过threshold时，会调用resize()来自动将大小扩大一倍。
值在Node<K,V>[] table中的定位方式为(n-1)&hash(key)，这也是resize的时候直接double的原因
基本方法：
V put(K key, V value)：若key不存在，则插入；若key存在，则更新value值，返回旧的valueV putIfAbsent(K key, V value)V get(Object key)：get不存在的key时会返回null，需要注意NullPointerExceptionint size()
遍历方式
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forEach(lambda)通过lambda表达式进行遍历 -
entrySet().iterator()Iterator iter = map.entrySet().iterator(); while(iter.hasNext()){ Map.Entry e = (Map.Entry)iter.next(); key = e.getKey(); value = e.getValue(); } -
keySet().iterator()Iterator iter = map.keySet().iterator(); while(iter.hasNext()){ key = iter.next(); value = map.get(key); } -
values().iterator()
resize()
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) { // 旧的大小已经达到设置的最大值时不再增加，改变阈值 threshold = Integer.MAX_VALUE; return oldTab; } else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && // 新大小=旧大小*2 oldCap >= DEFAULT_INITIAL_CAPACITY) newThr = oldThr << 1; // 阈值也一起*2 } else if (oldThr > 0) // initial capacity was placed in threshold newCap = oldThr; else { // oldCap为0时处于初始化阶段，进行初始化 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) { // 将旧map移到新map中 for (int j = 0; j < oldCap; ++j) { Node<K,V> e; if ((e = oldTab[j]) != null) { oldTab[j] = null; // 置为null值方便GC if (e.next == null) // 桶中没有链，直接赋值 newTab[e.hash & (newCap - 1)] = e; else if (e instanceof TreeNode) // 如果桶中为红黑树 ((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) { // 若为真，则在原来位置不变 if (loTail == null) loHead = e; else loTail.next = e; loTail = e; } else { // 为假时说明扩容后原链表中的节点位置发生了改变 if (hiTail == null) hiHead = e; else hiTail.next = e; hiTail = e; } } while ((e = next) != null); if (loTail != null) { loTail.next = null; newTab[j] = loHead; // 原链表所在 } if (hiTail != null) { hiTail.next = null; newTab[j + oldCap] = hiHead; // 扩容部分节点位置加上了oldCap } } } } } return newTab; }冲突解决
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; // 数组为空的情况 if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null); // 没有冲突直接放入 else { Node<K,V> e; K k; if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) e = p; // 有冲突但是key相同，则覆盖原来的值 else if (p instanceof TreeNode) e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); // 如果已经拉成红黑树则插入树中 else { for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) { p.next = newNode(hash, key, value, null); // 找到链表尾插入链表中 if (binCount >= TREEIFY_THRESHOLD - 1) // 如果桶的链长度超过阈值则拉成红黑树 treeifyBin(tab, hash); break; } if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; // 在链中找到相同的key则覆盖其值 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; }Hashtable
初始化函数：
public Hashtable() { this(11, 0.75f); }默认下initialCapacity = 11，loadFactor = 0.75。
插入操作put(K,V)
public synchronized V put(K key, V value) { // Make sure the value is not null if (value == null) { throw new NullPointerException(); } // Makes sure the key is not already in the hashtable. Entry<?,?> tab[] = table; int hash = key.hashCode(); int index = (hash & 0x7FFFFFFF) % tab.length; @SuppressWarnings("unchecked") Entry<K,V> entry = (Entry<K,V>)tab[index]; for(; entry != null ; entry = entry.next) { if ((entry.hash == hash) && entry.key.equals(key)) { // 找到相同的key则覆盖原值 V old = entry.value; entry.value = value; return old; } } addEntry(hash, key, value, index); return null; }Hashtable的hash寻址方法为(hash & 0x7FFFFFFF) % tab.length，当插入的key之前有值时返回旧值，否则返回null。
addEntry(hash, key, value, index)，当table的大小不够时，执行rehash()扩大table
private void addEntry(int hash, K key, V value, int index) { Entry<?,?> tab[] = table; if (count >= threshold) { // Rehash the table if the threshold is exceeded rehash(); tab = table; hash = key.hashCode(); index = (hash & 0x7FFFFFFF) % tab.length; } // Creates the new entry. @SuppressWarnings("unchecked") Entry<K,V> e = (Entry<K,V>) tab[index]; tab[index] = new Entry<>(hash, key, value, e); count++; modCount++; }rehash():
protected void rehash() { int oldCapacity = table.length; Entry<?,?>[] oldMap = table; // overflow-conscious code int newCapacity = (oldCapacity << 1) + 1; // 新大小=原大小*2+1 if (newCapacity - MAX_ARRAY_SIZE > 0) { if (oldCapacity == MAX_ARRAY_SIZE) // Keep running with MAX_ARRAY_SIZE buckets return; newCapacity = MAX_ARRAY_SIZE; } Entry<?,?>[] newMap = new Entry<?,?>[newCapacity]; modCount++; threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1); // 更新阈值 table = newMap; for (int i = oldCapacity ; i-- > 0 ;) { // 将旧map中的值一道新map for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) { Entry<K,V> e = old; old = old.next; int index = (e.hash & 0x7FFFFFFF) % newCapacity; e.next = (Entry<K,V>)newMap[index]; newMap[index] = e; } } }与HashMap的区别
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HashMap 继承自AbstractMap类，Hashtable继承自Dictionary类
- Hashtable中的方法均用sychronized关键字修饰，为线程安全
- 扩容方法不同，HashMap直接double，使得大小始终是2的倍数，Hashtable在double后加1
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在table中的查找方式不同：HashMap为
hash&(n-1)，Hashtable为(hash & 0x7FFFFFFF) % tab.length
TreeMap
TreeMap的本质是红黑树，红黑树是一种特殊的二叉查找树，所以TreeMap中的节点都是有序的。
TreeMap中节点Entry的定义为
static final class Entry<K,V> implements Map.Entry<K,V> { K key; V value; Entry<K,V> left; Entry<K,V> right; Entry<K,V> parent; boolean color = BLACK; }初始化函数：
public TreeMap() { comparator = null; } public TreeMap(Comparator<? super K> comparator) { this.comparator = comparator; }TreeMap支持自定义的比较器，若是使用空初始化函数，则默认为key的自然顺序
/** * The comparator used to maintain order in this tree map, or * null if it uses the natural ordering of its keys. * * @serial */ private final Comparator<? super K> comparator;插入操作put(K,V)
public V put(K key, V value) { Entry<K,V> t = root; if (t == null) { // root为空则直接new compare(key, key); // type (and possibly null) check root = new Entry<>(key, value, null); size = 1; modCount++; return null; } int cmp; Entry<K,V> parent; // split comparator and comparable paths Comparator<? super K> cpr = comparator; if (cpr != null) { // 自定义comparator时 do { parent = t; cmp = cpr.compare(key, t.key); if (cmp < 0) t = t.left; else if (cmp > 0) t = t.right; else return t.setValue(value); // 如果key相等则直接覆盖value } while (t != null); } else { // 使用key的comparable接口 if (key == null) throw new NullPointerException(); @SuppressWarnings("unchecked") Comparable<? super K> k = (Comparable<? super K>) key; do { parent = t; cmp = k.compareTo(t.key); if (cmp < 0) t = t.left; else if (cmp > 0) t = t.right; else return t.setValue(value); //找到相同的key则直接覆盖value返回 } while (t != null); } Entry<K,V> e = new Entry<>(key, value, parent); // 插入节点 if (cmp < 0) parent.left = e; else parent.right = e; fixAfterInsertion(e); // 红黑树自平衡过程 size++; modCount++; return null; }插入后红黑树的自平衡过程:
private void fixAfterInsertion(Entry<K,V> x) { x.color = RED; // 设插入节点的颜色为红 while (x != null && x != root && x.parent.color == RED) { // 当x.parent为黑时树已经平衡 if (parentOf(x) == leftOf(parentOf(parentOf(x)))) { // x.parent是祖父节点的左子节点 Entry<K,V> y = rightOf(parentOf(parentOf(x))); // x的uncle节点 if (colorOf(y) == RED) { // uncle为红的时候recolor setColor(parentOf(x), BLACK); setColor(y, BLACK); setColor(parentOf(parentOf(x)), RED); x = parentOf(parentOf(x)); // 向上变色直到满足平衡条件 } else { // uncle为黑的时候则需要rotate if (x == rightOf(parentOf(x))) { // 左右的情况，向左旋转 x = parentOf(x); rotateLeft(x); } setColor(parentOf(x), BLACK); setColor(parentOf(parentOf(x)), RED); rotateRight(parentOf(parentOf(x))); } } else { Entry<K,V> y = leftOf(parentOf(parentOf(x))); if (colorOf(y) == RED) { setColor(parentOf(x), BLACK); setColor(y, BLACK); setColor(parentOf(parentOf(x)), RED); x = parentOf(parentOf(x)); } else { if (x == leftOf(parentOf(x))) { // 右左的情况，向右旋转 x = parentOf(x); rotateRight(x); } setColor(parentOf(x), BLACK); setColor(parentOf(parentOf(x)), RED); rotateLeft(parentOf(parentOf(x))); } } } root.color = BLACK; }如有不对请多指正?
