前言
以下内容基于jdk1.7.0_79源码；
什么是HashMap
基于哈希表的一个Map接口实现，存储的对象是一个键值对对象(Entry<K,V>)；
HashMap补充说明
基于数组和链表实现，内部维护着一个数组table，该数组保存着每个链表的表头结点；查找时，先通过hash函数计算key的hash值，再根据key的hash值计算数组索引（取余法），然后根据索引找到链表表头结点，然后遍历查找该链表；
HashMap数据结构
画了个示意图，如下，左边的数组索引是根据key的hash值计算得到，不同hash值有可能产生一样的索引，即哈希冲突，此时采用链地址法处理哈希冲突，即将所有索引一致的节点构成一个单链表；
HashMap继承的类与实现的接口
Map接口，方法的含义很简单，基本上看个方法名就知道了，后面会在HashMap源码分析里详细说明
AbstractMap抽象类中定义的方法
HashMap源码分析，大部分都加了注释
package java.util; import java.io.*; public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable { /** * 默认初始容量，默认为2的4次方 = 16，2的n次方是为了加快hash计算速度，；；减少hash冲突，，，h & (length-1)，，1111111 */ static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 /** * 最大容量，默认为2的30次方， */ static final int MAXIMUM_CAPACITY = 1 << 30; /** * 默认负载因子，默认为0.75 */ static final float DEFAULT_LOAD_FACTOR = 0.75f; /** *当数组表还没扩容的时候，一个共享的空表对象 */ static final Entry<?,?>[] EMPTY_TABLE = {}; /** * 数组表，大小可以改变，且大小必须为2的幂 */ transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE; /** * 当前Map中key-value映射的个数 */ transient int size; /** * 下次扩容阈值，当size > capacity * load factor时，开始扩容 */ int threshold; /** * 负载因子 */ final float loadFactor; /** * Hash表结构性修改次数，用于实现迭代器快速失败行为 */ transient int modCount; /** * 容量阈值，默认大小为Integer.MAX_VALUE */ static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE; /** * 静态内部类Holder，存放一些只能在虚拟机启动后才能初始化的值 */ private static class Holder { /** * 容量阈值，初始化hashSeed的时候会用到该值 */ static final int ALTERNATIVE_HASHING_THRESHOLD; static { //获取系统变量jdk.map.althashing.threshold String altThreshold = java.security.AccessController.doPrivileged( new sun.security.action.GetPropertyAction( "jdk.map.althashing.threshold")); int threshold; try { threshold = (null != altThreshold) ? Integer.parseInt(altThreshold) : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT; // jdk.map.althashing.threshold系统变量默认为-1，如果为-1，则将阈值设为Integer.MAX_VALUE if (threshold == -1) { threshold = Integer.MAX_VALUE; } //阈值需要为正数 if (threshold < 0) { throw new IllegalArgumentException("value must be positive integer."); } } catch(IllegalArgumentException failed) { throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed); } ALTERNATIVE_HASHING_THRESHOLD = threshold; } } /** * 计算hash值的时候需要用到 */ transient int hashSeed = 0; /** * 生成一个空的HashMap,并指定其容量大小和负载因子 * */ public HashMap(int initialCapacity, float loadFactor) { //保证初始容量大于等于0 if (initialCapacity < 0) throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity); //保证初始容量不大于最大容量MAXIMUM_CAPACITY if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; //loadFactor小于0或为无效数字 if (loadFactor <= 0 || Float.isNaN(loadFactor)) throw new IllegalArgumentException("Illegal load factor: " + loadFactor); //负载因子 this.loadFactor = loadFactor; //下次扩容大小 threshold = initialCapacity; init(); } /** * 生成一个空的HashMap,并指定其容量大小，负载因子使用默认的0.75 * */ public HashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } /** * 生成一个空的HashMap,容量大小使用默认值16，负载因子使用默认值0.75 */ public HashMap() { this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR); } /** * 根据指定的map生成一个新的HashMap,负载因子使用默认值，初始容量大小为Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,DEFAULT_INITIAL_CAPACITY) */ public HashMap(Map<? extends K, ? extends V> m) { this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR); inflateTable(threshold); putAllForCreate(m); } //返回>=number的最小2的n次方值，如number=5，则返回8 private static int roundUpToPowerOf2(int number) { // assert number >= 0 : "number must be non-negative"; return number >= MAXIMUM_CAPACITY ? MAXIMUM_CAPACITY : (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1; } /** * 对table扩容 */ private void inflateTable(int toSize) { // Find a power of 2 >= toSize //找一个值（2的n次方，且>=toSize） int capacity = roundUpToPowerOf2(toSize); //下次扩容阈值 threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1); table = new Entry[capacity]; initHashSeedAsNeeded(capacity); } // internal utilities void init() { } /** * 初始化hashSeed */ final boolean initHashSeedAsNeeded(int capacity) { boolean currentAltHashing = hashSeed != 0; boolean useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); boolean switching = currentAltHashing ^ useAltHashing; if (switching) { hashSeed = useAltHashing ? sun.misc.Hashing.randomHashSeed(this) : 0; } return switching; } /** * 生成hash值 */ final int hash(Object k) { int h = hashSeed; //如果key是字符串，调用un.misc.Hashing.stringHash32生成hash值 //Oracle表示能生成更好的hash分布，不过这在jdk8中已删除 if (0 != h && k instanceof String) { return sun.misc.Hashing.stringHash32((String) k); } //一次散列，调用k的hashCode方法，与hashSeed做异或操作 h ^= k.hashCode(); // This function ensures that hashCodes that differ only by // constant multiples at each bit position have a bounded // number of collisions (approximately 8 at default load factor). //二次散列， h ^= (h >>> 20) ^ (h >>> 12); return h ^ (h >>> 7) ^ (h >>> 4); } /** * 返回hash值的索引，采用除模取余法，h & (length-1)操作 等价于 hash % length操作， 但&操作性能更优 */ static int indexFor(int h, int length) { // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2"; return h & (length-1); } /** * 返回key-value映射个数 */ public int size() { return size; } /** * 判断map是否为空 */ public boolean isEmpty() { return size == 0; } /** * 返回指定key对应的value */ public V get(Object key) { //key为null情况 if (key == null) return getForNullKey(); //根据key查找节点 Entry<K,V> entry = getEntry(key); //返回key对应的值 return null == entry ? null : entry.getValue(); } /** * 查找key为null的value，注意如果key为null，则其hash值为0，默认是放在table[0]里的 */ private V getForNullKey() { if (size == 0) { return null; } //在table[0]的链表上查找key为null的键值对，因为null默认是存在table[0]的桶里 for (Entry<K,V> e = table[0]; e != null; e = e.next) { if (e.key == null) return e.value; } return null; } /** *判断是否包含指定的key */ public boolean containsKey(Object key) { return getEntry(key) != null; } /** * 根据key查找键值对，找不到返回null */ final Entry<K,V> getEntry(Object key) { if (size == 0) { return null; } //如果key为null，hash值为0，否则调用hash方法，对key生成hash值 int hash = (key == null) ? 0 : hash(key); //调用indexFor方法生成hash值的索引，遍历该索引下的链表，查找key“相等”的键值对 for (Entry<K,V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } return null; } /** * 向map存入一个键值对，如果key已存在，则覆盖 */ public V put(K key, V value) { //数组为空，对数组扩容 if (table == EMPTY_TABLE) { inflateTable(threshold); } //对key为null的键值对调用putForNullKey处理 if (key == null) return putForNullKey(value); //生成hash值 int hash = hash(key); //生成hash值索引 int i = indexFor(hash, table.length); //查找是否有key“相等”的键值对，有的话覆盖 for (Entry<K,V> e = table[i]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } //操作次数加一，用于迭代器快速失败行为 modCount++; //在指定hash值索引处的链表上增加该键值对 addEntry(hash, key, value, i); return null; } /** * 存放key为null的键值对，存放在索引为0的链表上，已存在的话，替换 */ private V putForNullKey(V value) { for (Entry<K,V> e = table[0]; e != null; e = e.next) { //已存在key为null，则替换 if (e.key == null) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } //操作次数加一，用于迭代器快速失败行为 modCount++; //在指定hash值索引处的链表上增加该键值对 addEntry(0, null, value, 0); return null; } /** * 添加键值对 */ private void putForCreate(K key, V value) { //生成hash值 int hash = null == key ? 0 : hash(key); //生成hash值索引， int i = indexFor(hash, table.length); /** * key“相等”，则替换 */ for (Entry<K,V> e = table[i]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { e.value = value; return; } } //在指定索引处的链表上创建该键值对 createEntry(hash, key, value, i); } //将制定map的键值对添加到map中 private void putAllForCreate(Map<? extends K, ? extends V> m) { for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) putForCreate(e.getKey(), e.getValue()); } /** * 对数组扩容 */ void resize(int newCapacity) { Entry[] oldTable = table; int oldCapacity = oldTable.length; if (oldCapacity == MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return; } //创建一个指定大小的数组 Entry[] newTable = new Entry[newCapacity]; transfer(newTable, initHashSeedAsNeeded(newCapacity)); //table索引替换成新数组 table = newTable; //重新计算阈值 threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1); } /** * 拷贝旧的键值对到新的哈希表中 */ void transfer(Entry[] newTable, boolean rehash) { int newCapacity = newTable.length; //遍历旧的数组 for (Entry<K,V> e : table) { while(null != e) { Entry<K,V> next = e.next; if (rehash) { e.hash = null == e.key ? 0 : hash(e.key); } //根据新的数组长度，重新计算索引， int i = indexFor(e.hash, newCapacity); //插入到链表表头 e.next = newTable[i]; //将e放到索引为i处 newTable[i] = e; //将e设置成下个节点 e = next; } } } /** * 将制定map的键值对put到本map，key“相等”的直接覆盖 */ public void putAll(Map<? extends K, ? extends V> m) { int numKeysToBeAdded = m.size(); if (numKeysToBeAdded == 0) return; //空map，扩容 if (table == EMPTY_TABLE) { inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold)); } /* * 判断是否需要扩容 */ if (numKeysToBeAdded > threshold) { int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1); if (targetCapacity > MAXIMUM_CAPACITY) targetCapacity = MAXIMUM_CAPACITY; int newCapacity = table.length; while (newCapacity < targetCapacity) newCapacity <<= 1; if (newCapacity > table.length) resize(newCapacity); } //依次遍历键值对，并put for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) put(e.getKey(), e.getValue()); } /** * 移除指定key的键值对 */ public V remove(Object key) { Entry<K,V> e = removeEntryForKey(key); return (e == null ? null : e.value); } /** * 移除指定key的键值对 */ final Entry<K,V> removeEntryForKey(Object key) { if (size == 0) { return null; } //计算hash值及索引 int hash = (key == null) ? 0 : hash(key); int i = indexFor(hash, table.length); Entry<K,V> prev = table[i]; Entry<K,V> e = prev; //头节点为table[i]的单链表上执行删除节点操作 while (e != null) { Entry<K,V> next = e.next; Object k; //找到要删除的节点 if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { modCount++; size--; if (prev == e) table[i] = next; else prev.next = next; e.recordRemoval(this); return e; } prev = e; e = next; } return e; } /** * 删除指定键值对对象(Entry对象) */ final Entry<K,V> removeMapping(Object o) { if (size == 0 || !(o instanceof Map.Entry)) return null; Map.Entry<K,V> entry = (Map.Entry<K,V>) o; Object key = entry.getKey(); int hash = (key == null) ? 0 : hash(key); //得到数组索引 int i = indexFor(hash, table.length); Entry<K,V> prev = table[i]; Entry<K,V> e = prev; //开始遍历该单链表 while (e != null) { Entry<K,V> next = e.next; //找到节点 if (e.hash == hash && e.equals(entry)) { modCount++; size--; if (prev == e) table[i] = next; else prev.next = next; e.recordRemoval(this); return e; } prev = e; e = next; } return e; } /** * 清空map，将table数组所有元素设为null */ public void clear() { modCount++; Arrays.fill(table, null); size = 0; } /** * 判断是否含有指定value的键值对 */ public boolean containsValue(Object value) { if (value == null) return containsNullValue(); Entry[] tab = table; //遍历table数组 for (int i = 0; i < tab.length ; i++) //遍历每条单链表 for (Entry e = tab[i] ; e != null ; e = e.next) if (value.equals(e.value)) return true; return false; } /** * 判断是否含有value为null的键值对 */ private boolean containsNullValue() { Entry[] tab = table; for (int i = 0; i < tab.length ; i++) for (Entry e = tab[i] ; e != null ; e = e.next) if (e.value == null) return true; return false; } /** * 浅拷贝，键值对不复制 */ public Object clone() { HashMap<K,V> result = null; try { result = (HashMap<K,V>)super.clone(); } catch (CloneNotSupportedException e) { // assert false; } if (result.table != EMPTY_TABLE) { result.inflateTable(Math.min( (int) Math.min( size * Math.min(1 / loadFactor, 4.0f), // we have limits... HashMap.MAXIMUM_CAPACITY), table.length)); } result.entrySet = null; result.modCount = 0; result.size = 0; result.init(); result.putAllForCreate(this); return result; } //内部类，节点对象，每个节点包含下个节点的引用 static class Entry<K,V> implements Map.Entry<K,V> { final K key; V value; Entry<K,V> next; int hash; /** * 创建节点 */ Entry(int h, K k, V v, Entry<K,V> n) { value = v; next = n; key = k; hash = h; } //获取节点的key public final K getKey() { return key; } //获取节点的value public final V getValue() { return value; } //设置新value，并返回旧的value public final V setValue(V newValue) { V oldValue = value; value = newValue; return oldValue; } //判断key和value是否相同,两个都“相等”，返回true public final boolean equals(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry e = (Map.Entry)o; Object k1 = getKey(); Object k2 = e.getKey(); if (k1 == k2 || (k1 != null && k1.equals(k2))) { Object v1 = getValue(); Object v2 = e.getValue(); if (v1 == v2 || (v1 != null && v1.equals(v2))) return true; } return false; } public final int hashCode() { return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue()); } public final String toString() { return getKey() + "=" + getValue(); } /** * This method is invoked whenever the value in an entry is * overwritten by an invocation of put(k,v) for a key k that's already * in the HashMap. */ void recordAccess(HashMap<K,V> m) { } /** * This method is invoked whenever the entry is * removed from the table. */ void recordRemoval(HashMap<K,V> m) { } } /** * 添加新节点，如有必要，执行扩容操作 */ void addEntry(int hash, K key, V value, int bucketIndex) { if ((size >= threshold) && (null != table[bucketIndex])) { resize(2 * table.length); hash = (null != key) ? hash(key) : 0; bucketIndex = indexFor(hash, table.length); } createEntry(hash, key, value, bucketIndex); } /** * 插入单链表表头 */ void createEntry(int hash, K key, V value, int bucketIndex) { Entry<K,V> e = table[bucketIndex]; table[bucketIndex] = new Entry<>(hash, key, value, e); size++; } //hashmap迭代器 private abstract class HashIterator<E> implements Iterator<E> { Entry<K,V> next; // 下个键值对索引 int expectedModCount; // 用于判断快速失败行为 int index; // current slot Entry<K,V> current; // current entry HashIterator() { expectedModCount = modCount; if (size > 0) { // advance to first entry Entry[] t = table; while (index < t.length && (next = t[index++]) == null) ; } } public final boolean hasNext() { return next != null; } final Entry<K,V> nextEntry() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); Entry<K,V> e = next; if (e == null) throw new NoSuchElementException(); if ((next = e.next) == null) { Entry[] t = table; while (index < t.length && (next = t[index++]) == null) ; } current = e; return e; } public void remove() { if (current == null) throw new IllegalStateException(); if (modCount != expectedModCount) throw new ConcurrentModificationException(); Object k = current.key; current = null; HashMap.this.removeEntryForKey(k); expectedModCount = modCount; } } //ValueIterator迭代器 private final class ValueIterator extends HashIterator<V> { public V next() { return nextEntry().value; } } //KeyIterator迭代器 private final class KeyIterator extends HashIterator<K> { public K next() { return nextEntry().getKey(); } } ////KeyIterator迭代器 private final class EntryIterator extends HashIterator<Map.Entry<K,V>> { public Map.Entry<K,V> next() { return nextEntry(); } } // 返回迭代器方法 Iterator<K> newKeyIterator() { return new KeyIterator(); } Iterator<V> newValueIterator() { return new ValueIterator(); } Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); } // Views private transient Set<Map.Entry<K,V>> entrySet = null; /** * 返回一个set集合，包含key */ public Set<K> keySet() { Set<K> ks = keySet; return (ks != null ? ks : (keySet = new KeySet())); } private final class KeySet extends AbstractSet<K> { public Iterator<K> iterator() { return newKeyIterator(); } public int size() { return size; } public boolean contains(Object o) { return containsKey(o); } public boolean remove(Object o) { return HashMap.this.removeEntryForKey(o) != null; } public void clear() { HashMap.this.clear(); } } /** * 返回一个value集合，包含value */ public Collection<V> values() { Collection<V> vs = values; return (vs != null ? vs : (values = new Values())); } private final class Values extends AbstractCollection<V> { public Iterator<V> iterator() { return newValueIterator(); } public int size() { return size; } public boolean contains(Object o) { return containsValue(o); } public void clear() { HashMap.this.clear(); } } /** * 返回一个键值对集合 */ public Set<Map.Entry<K,V>> entrySet() { return entrySet0(); } private Set<Map.Entry<K,V>> entrySet0() { Set<Map.Entry<K,V>> es = entrySet; return es != null ? es : (entrySet = new EntrySet()); } private final class EntrySet extends AbstractSet<Map.Entry<K,V>> { public Iterator<Map.Entry<K,V>> iterator() { return newEntryIterator(); } public boolean contains(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry<K,V> e = (Map.Entry<K,V>) o; Entry<K,V> candidate = getEntry(e.getKey()); return candidate != null && candidate.equals(e); } public boolean remove(Object o) { return removeMapping(o) != null; } public int size() { return size; } public void clear() { HashMap.this.clear(); } } /** * map序列化,可实现深拷贝 */ private void writeObject(java.io.ObjectOutputStream s) throws IOException { // Write out the threshold, loadfactor, and any hidden stuff s.defaultWriteObject(); // Write out number of buckets if (table==EMPTY_TABLE) { s.writeInt(roundUpToPowerOf2(threshold)); } else { s.writeInt(table.length); } // Write out size (number of Mappings) s.writeInt(size); // Write out keys and values (alternating) if (size > 0) { for(Map.Entry<K,V> e : entrySet0()) { s.writeObject(e.getKey()); s.writeObject(e.getValue()); } } } private static final long serialVersionUID = 362498820763181265L; /** * 反序列化，读取字节码转为对象 */ private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException { // Read in the threshold (ignored), loadfactor, and any hidden stuff s.defaultReadObject(); if (loadFactor <= 0 || Float.isNaN(loadFactor)) { throw new InvalidObjectException("Illegal load factor: " + loadFactor); } // set other fields that need values table = (Entry<K,V>[]) EMPTY_TABLE; // Read in number of buckets s.readInt(); // ignored. // Read number of mappings int mappings = s.readInt(); if (mappings < 0) throw new InvalidObjectException("Illegal mappings count: " + mappings); // capacity chosen by number of mappings and desired load (if >= 0.25) int capacity = (int) Math.min( mappings * Math.min(1 / loadFactor, 4.0f), // we have limits... HashMap.MAXIMUM_CAPACITY); // allocate the bucket array; if (mappings > 0) { inflateTable(capacity); } else { threshold = capacity; } init(); // Give subclass a chance to do its thing. // Read the keys and values, and put the mappings in the HashMap for (int i = 0; i < mappings; i++) { K key = (K) s.readObject(); V value = (V) s.readObject(); putForCreate(key, value); } } // These methods are used when serializing HashSets int capacity() { return table.length; } float loadFactor() { return loadFactor; } } }
简单使用示例
package com.pichen.collection; import java.util.HashMap; import java.util.Map; public class Main { public static void main(String[] args) { Map<String, Integer> map = new HashMap<String, Integer>(); //put方法 map.put("A", 5); map.put("B", 6); map.put("C", 7); map.put("D", 8); //重写了toString方法 System.out.println(map); //size方法 System.out.println(map.size()); System.out.println(map.containsKey("A")); System.out.println(map.containsValue(6)); System.out.println(map.get("B")); //remove map.remove("C"); System.out.println(map); //key集合 for(String str:map.keySet()){ System.out.print(str + " "); } System.out.println(); //value集合 for(Integer obj:map.values()){ System.out.print(obj + " "); } System.out.println(); //key-value集合 for(Map.Entry<String, Integer> entry:map.entrySet()){ System.out.print(entry.getKey() + ": " + entry.getValue() + ", "); } } }
本文转自风一样的码农博客园博客，原文链接：http://www.cnblogs.com/chenpi/p/5280304.html，如需转载请自行联系原作者
