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;
*****************************************************************
***************************
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
****************************
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();
}
**************************
