c.hashMap源码解析(1.7)

Hashmap的实现方法，俗称拉链法，其实是一种散列结构，其结构图（网络图片，这张图也挺好直接用了）如下

然后开始进入正题，解析hashmap源码的实现原理（由于1.8的hashmap与linkedHashmap做了比较大的改动，后面将重开一章解析1.8的这两种结构）

首先看他的类声明

public class HashMap<K,V>    extends AbstractMap<K,V>    implements Map<K,V>, Cloneable, Serializable

中规中矩，实现了Cloneable与Serializable

属性

    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; //默认值16，通过移位操作        static final int MAXIMUM_CAPACITY = 1 << 30;//规定最大容量，小于2的30次方    static final float DEFAULT_LOAD_FACTOR = 0.75f;//很熟悉了，加载因子    static final Entry<?,?>[] EMPTY_TABLE = {}; //空数组，一般用来作比较，或者作为初始值用    transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE; //table:正经用来存东西的，默认是空    transient int size;//大小    int threshold;//极限值，扩容时候作比较用    final float loadFactor;//加载因子    transient int modCount;//记录操作数，避免遍历或者多线程访问可能会出现的错误    static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;//没看，先不看了

   transient int hashSeed = 0;//后面看到了再说

   //通过虚拟机配置来修改threshold值      private static class Holder {          static final int ALTERNATIVE_HASHING_THRESHOLD;          static {              String altThreshold = java.security.AccessController.doPrivileged(                  new sun.security.action.GetPropertyAction(                      "jdk.map.althashing.threshold"));//读取配置值              int threshold;              try {                  threshold = (null != altThreshold)//修改threshold值                          ? Integer.parseInt(altThreshold)                          : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;                  if (threshold == -1) {                      threshold = Integer.MAX_VALUE;                  }                  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;          }      }  

构造方法

    //自定义初始容量与加载因子    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;        threshold = initialCapacity;        init();    }  <span style="white-space:pre"></span>//使用默认加载因子0.75    public HashMap(int initialCapacity) {        this(initialCapacity, DEFAULT_LOAD_FACTOR);    }   //使用默认初始容量与默认加载因子    public HashMap() {        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);    }   //根据size算好新map的初始容量，并且使用默认加载因子    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);    }

   //根据最小容量与容量*loadFactor算出比较小的一个作为极限值，创建一个新的Entry数组

<span style="white-space:pre"></span>private void inflateTable(int toSize) {        // Find a power of 2 >= toSize        int capacity = roundUpToPowerOf2(toSize);        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);        table = new Entry[capacity];        initHashSeedAsNeeded(capacity);    }

内部类

因为下面方法有所涉及，所有内部类先分析

Entry

 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;        }<span style="white-space:pre"></span>//键跟值的hash异或        public final int hashCode() {            return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());        }<span style="white-space:pre"></span>        public final String toString() {            return getKey() + "=" + getValue();        }          void recordAccess(HashMap<K,V> m) {        }        void recordRemoval(HashMap<K,V> m) {        }    }  

方法

<span style="font-family: Arial, Helvetica, sans-serif;"> </span><span style="font-family: Arial, Helvetica, sans-serif;"></span><span style="font-family: Arial, Helvetica, sans-serif;">//添加entry，计算是否需要扩容，扩容之后根据hash与table.length计算桶的位置，然后加入进去</span>

    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);    }  <span style="white-space:pre"></span>获取桶此位置上的entry，然后创建新的entry设置进去，这个设置新加入的元素将放在链表的头一个节点    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++;    }

put

  //如果有key，修改值即可，如果没有调用addEntry，导致因加入的都在头结点上    public V put(K key, V value) {        if (table == EMPTY_TABLE) {            inflateTable(threshold);        }        if (key == null)            return putForNullKey(value);        int hash = hash(key);        int i = indexFor(hash, table.length);        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;            }        }        modCount++;        addEntry(hash, key, value, i);        return null;    }

    //初始table    private void inflateTable(int toSize) {        // Find a power of 2 >= toSize        int capacity = roundUpToPowerOf2(toSize);        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);        table = new Entry[capacity];        initHashSeedAsNeeded(capacity);    }

    //也是遍历的方式找到key为null的，recordAccess这里是空，为linkedHashMap做了个铺垫    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;    }

 按位与

    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);    }

resize

void resize(int newCapacity) {        Entry[] oldTable = table;        int oldCapacity = oldTable.length;        if (oldCapacity == MAXIMUM_CAPACITY) {            threshold = Integer.MAX_VALUE;            return;        }        Entry[] newTable = new Entry[newCapacity];        transfer(newTable, initHashSeedAsNeeded(newCapacity));        table = newTable;        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);    }

transfer

//也是用两个循环，需要经过rehash，算出新的桶的位置，倒序

void transfer(Entry[] newTable, boolean rehash) {        int newCapacity = newTable.length;        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);                }                int i = indexFor(e.hash, newCapacity);                e.next = newTable[i];                newTable[i] = e;                e = next;            }        }    }

putall

      public void putAll(Map<? extends K, ? extends V> m) {        int numKeysToBeAdded = m.size();        if (numKeysToBeAdded == 0)            return;        if (table == EMPTY_TABLE) {            inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));        }       //新加入的map size大于临界值的时候        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);        }<span style="white-space:pre"></span>还是一个个加入        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())            put(e.getKey(), e.getValue());    }

remove

 //删除元素，元素的键值为key      final Entry<K,V> removeEntryForKey(Object key) {          if (size == 0) {              return null;          }          int hash = (key == null) ? 0 : hash(key);//计算Hash值          int i = indexFor(hash, table.length);//定位Hash桶          Entry<K,V> prev = table[i];          Entry<K,V> e = prev;//保存前面一个指针值          while (e != null) {              Entry<K,V> next = e.next;              Object k;              if (e.hash == hash &&                  ((k = e.key) == key || (key != null && key.equals(k)))) {//在Hash桶中定位元素                  modCount++;//更新修改次数                  size--;//元素个数-1                  if (prev == e)//是否是第一个元素                      table[i] = next;                  else                      prev.next = next;//执行的是单链表的删除                  e.recordRemoval(this);                  return e;              }              prev = e;//单链表移动指针              e = next;          }          return e;      }      //删除一个Entry实体，这里通过o的key查找到元素，之后删除，和上面的实现类似      final Entry<K,V> removeMapping(Object o) {          if (size == 0 || !(o instanceof Map.Entry))//参数有效性验证              return null;          Map.Entry<K,V> entry = (Map.Entry<K,V>) o;          Object key = entry.getKey();          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;              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;      }  

clear

 //清空Hash表      public void clear() {          modCount++;//更新修改次数          Arrays.fill(table, null);//底层数组置为null          size = 0;//元素个数为0      }  

浅复制

 //浅复制HashMap  ，只能获取引用值，获取不到值    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;      }  

迭代器

 private abstract class HashIterator<E> implements Iterator<E> {        Entry<K,V> next;        // next entry to return        int expectedModCount;   // For fast-fail        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;        }    }

    private final class ValueIterator extends HashIterator<V> {        public V next() {            return nextEntry().value;        }    }    private final class KeyIterator extends HashIterator<K> {        public K next() {            return nextEntry().getKey();        }    }

keyset与value的迭代器都是用以上实现的。

隔了两个月，用伪代码重新看一遍逻辑，感觉理解的更加通顺一些

hashmap源码分析
*********************************************************
put
1.如果是空table，则根据默认容量初始化
2.如果是空key，调用putForNullKey
3.如果非空key，计算key的哈细值，然后根据table的length计算bucket的index
  遍历该bucket中的链表，如果有key，修改value，并且记录
4.modCount++
5.调用addEntry
*********************
addEntry
1.如果需要扩容，表长度乘以2，并且调用resize
2.计算hash值，当key==null，则hash为0
3.根据扩容后的length与key的哈细值，计算bucket的index
4.不管是否需要扩容，接下来执行创建createEntry
*******************
createEntry
1.Entry e=table[bucketindex];
2.e=table[bucketindex]=new Entry(hash,key,value,e);
3.size++;
********************
Entry 构造方法（每次插入的新元素都在链表头上）
  Entry(int h, K k, V v, Entry<K,V> n) {
  value = v;
  next = n;
   key = k;
   hash = h;

*****************************************************************

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resize
拷贝一份table
有个关于最大容量的判断，不做关注
创建一个resize大小的空数组
调用transfer方法
table=newTable;
threshold=Math.min(newCapacity*loadFactor,Maximun_capacity+1);


transfer();
遍历数组
对于数组第一个元素，循环，重新定位槽，并且放在每个数组第一位
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核心：HashIterator
用于EntrySet遍历
有个modcount，迭代器做操作会修改modcount，在迭代过程中不会报错
迭代过程就是找entry.next, 当next为null，找下一个槽，直到结束
Iterator i=map.entrySet().iterator();
while(i.hasNext()){
Entry e =(Entry) i.next();
e.getKey();
}

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