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一、前言
????HashMap是常用的Map实现类，其中可以保存一个key为null的键值对和任意多个key！=null但是value为null的键值对。
二、HashMap源代码分析
2.1、类的继承关系
//Map中定义了Map必须支持的一些操作，abstractMap中添加了其默认实现 public class HashMap<K, V> extends AbstractMap<K, V> implements Map<K, V>, ????????Cloneable, Serializable |
2.2内部类
static class Entry<K,V> implements Map.Entry<K,V> { final K key; V value; Entry<K,V> next; int hash; ? /** * Creates new entry. */ Entry(int h, K k, V v, Entry<K,V> n) { value = v; next = n; key = k; hash = h; } ? public final K getKey() { return key; } ? public final V getValue() { return value; } ? public final V setValue(V newValue) { V oldValue = value; value = newValue; return oldValue; } ? 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 (key==null ? 0 : key.hashCode()) ^ (value==null ? 0 : value.hashCode()); } ? 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) { } } |
此内部类定义了一个节点模型；每个节点中包含了：key、value、hash值和指向下一个节点的引用。为HashMap内部存储的数据结构
2.3私有属性
????// 默认初始化容量；必须为2的n次幂 ????static final int DEFAULT_INITIAL_CAPACITY = 16; ????// HashMap的最大容量，为2的30次幂 ????static final int MAXIMUM_CAPACITY = 1 << 30; ????// 默认因子 ????static final float DEFAULT_LOAD_FACTOR = 0.75f; ? ????// Entry数组，用于保存元素；长度必须为2的n次幂 ????transient Entry<K, V>[] table; ? ????// map中元素的个数 ????transient int size; ? ????/** ???? * The next size value at which to resize (capacity * load factor). ???? * 改变size的阈值=capacity*factor ???? */ ????int threshold; ? ????/** ???? * The load factor for the hash table. ???? */ ????final float loadFactor; ????// 结构改变的次数 ????transient int modCount; ? ????/** ???? * The default threshold of map capacity above which alternative hashing is ???? * used for String keys. Alternative hashing reduces the incidence of ???? * collisions due to weak hash code calculation for String keys. ???? * <p/> ???? * This value may be overridden by defining the system property ???? * {@code jdk.map.althashing.threshold}. A property value of {@code 1} ???? * forces alternative hashing to be used at all times whereas {@code -1} ???? * value ensures that alternative hashing is never used. ???? */ ????static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE; |
2.4构造函数
????// 使用 默认的capacity和factor创建空的HashMap ????public HashMap(int initialCapacity, float loadFactor) { ????????// 检查参数的合法性 ????????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); ? ????????// Find a power of 2 >= initialCapacity ????????// 找到第一个大于initialCapacity的2的整次幂 ????????int capacity = 1; ????????while (capacity < initialCapacity) ????????????capacity <<= 1; ? ????????this.loadFactor = loadFactor; ????????// 确定阈值 ????????threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1); ????????// 初始化hash表 ????????table = new Entry[capacity]; ????????useAltHashing = sun.misc.VM.isBooted() ????????????????&& (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); ????????init(); ????} |
此构造函数为核心构造函数，其他构造函数最终都是调用此函数实现
2.5、核心函数
1、get()函数
????/** ???? * Returns the value to which the specified key is mapped, or {@code null} ???? * if this map contains no mapping for the key. ???? */ ????// 返回值为null可能有两种情况：1不存在key-value对；2、key指向null ????public V get(Object key) { ????????// 两种情况key==null和非null ????????if (key == null) ????????????return getForNullKey(); ????????Entry<K, V> entry = getEntry(key); ? ????????return null == entry ? null : entry.getValue(); ????} ? ????private V getForNullKey() { ????????for (Entry<K, V> e = table[0]; e != null; e = e.next) { ????????????if (e.key == null) ????????????????return e.value; ????????} ????????return null; ????} ? ????final Entry<K, V> getEntry(Object key) { ????????// 先获取key对应的hash值，若key为null，则hash值为0 ????????int hash = (key == null) ? 0 : hash(key); ????????// 获取table的index=hash&table.length-1 ????????for (Entry<K, V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) { ????????????Object k; ????????????// 判断是否为指定key的map对：1、hash值相等；2、key为同个对象或者值相等 ????????????if (e.hash == hash ????????????????????&& ((k = e.key) == key || (key != null && key.equals(k)))) ????????????????return e; ????????} ????????return null; ????} |
2、put()函数
???? ????// 添加键值对，如果之前已存在，则覆盖之前的值 ????public V put(K key, V value) { ????????if (key == null) ????????????return putForNullKey(value); ????????// 先获取key对应的hash值 ????????int hash = hash(key); ????????// 然后获取hash值对应的index=hash&table.lenth-1 ????????int i = indexFor(hash, table.length); ????????for (Entry<K, V> e = table[i]; e != null; e = e.next) { //如果索引链表中已经存在此节点；则覆盖之前的值 ????????????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; ????????????} ????????} //如果索引处的Entry对象为null，则添加一个元素 ????????modCount++; ????????addEntry(hash, key, value, i); ????????return null; ????} ????private V putForNullKey(V value) { ????????for (Entry<K, V> e = table[0]; e != null; e = e.next) { ????????????if (e.key == null) { ????????????????V oldValue = e.value; ????????????????e.value = value; ????????????????e.recordAccess(this); ????????????????return oldValue; ????????????} ????????} ????????modCount++; ????????addEntry(0, null, value, 0); ????????return null; ????} ????void addEntry(int hash, K key, V value, int bucketIndex) { ????????if ((size >= threshold) && (null != table[bucketIndex])) { ????????????// 调整map的size ????????????resize(2 * table.length); ????????????hash = (null != key) ? hash(key) : 0; ????????????bucketIndex = indexFor(hash, table.length); ????????} ? ????????createEntry(hash, key, value, bucketIndex); ????} ????void createEntry(int hash, K key, V value, int bucketIndex) { ????????Entry<K, V> e = table[bucketIndex]; //新添加的元素指向之前的元素 ????????table[bucketIndex] = new Entry<>(hash, key, value, e); ????????size++; ????} |
说明：当HashMap添加元素时，如果key==null，则存放在table中0索引处，如果0索引出的链表中存在key==null的节点，则将此节点的值更新成最新值，如果不存在，则在链表的首部添加一个key为null的节点。如果key！=null，会首先调用hash()函数获取hash值，然后将调用indexFor函数获取存放table数组中的索引值；其中indexFor函数代码如下：
static int indexFor(int h, int length) { return h & (length-1); } |
此处获取table数组index的方法设计较为巧妙：上面提到length必须为2的n此幂，因此在length大小合法的情况下值肯定是最高位为1其余低位为0，而length-1则为00..111…1的形式，再&h则可以保证所取的index一定是在table.length范围之内的。获取索引值后，对此索引出的链表遍历，如果链表中已经存在hash值相等切key相等则将原先的值覆盖，否则在链表的开头添加一个节点。其中在添加节点的时候会先判断table的大小有没有超过阈值且将要添加的table的index位置不为null，如果两者都满足，则调整table的大小重新获取索引值。
通过上面对put函数的分析，我们可以看出HashMap内部是使用数据+链表的数据结构保存数据的，数组中的每一个index位置都保存了一个链表。其中key==null的元素统一保存在数组索引为0的链表中，但是索引为0的链表中之多有一个节点，当第二次添加key==null的元素时会将之前的节点值覆盖；当key！=null时，通过对key的hash值运算得到索引，当hash值相等时索引的值也是相等的，当索引值相等，但是key不等的时候，就会在同一个索引位置添加多个元素，从而产生链表。另外在HashMap中存取元素都是使用hash值进行定位的，所以效率相对要高。
3、removeEntryForKey(Object key)函数
removeEntryForKey函数可以将指定key值的键值对删除，其代码如下：
final Entry<K,V> removeEntryForKey(Object key) { int hash = (key == null) ? 0 : hash(key); int i = indexFor(hash, table.length); Entry<K,V> prev = table[i]; Entry<K,V> e = prev; ? while (e != null) { Entry<K,V> next = e.next; Object k; 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; } |
删除键值对的步骤是：先找到键值对所在的table的index，然后用删除节点的方式删除键值对。
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原文地址：http://www.cnblogs.com/vitasyuan/p/5456930.html
