HashMap深入学习
一、简单介绍
HashMap是一种比较常用的容器，其结构是数组加链表。外层数组，每个数组元素是链表。用链表解决hash冲突。结合了数组易寻址及链表易插入删除的优点。
二、HahMap的结构
1、主要属性：
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 16;
/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
transient Entry[] table;
/**
* The number of key-value mappings contained in this map.
*/
transient int size;
/**
* The next size value at which to resize (capacity * load factor).
* @serial
*/
int threshold;
/**
* The load factor for the hash table.
*
* @serial
*/
final float loadFactor;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
*/
transient volatile int modCount;
| 分类 | 属性名 | 详细 |
| 类变量（default static final） | DEFAULT_INITIAL_CAPACITY | 默认初始容量，初始化不设置初始值以16为初始值，初始值必须是2的幂值。（table数组的size） |
| 类变量（default static final） | MAXIMUM_CAPACITY | 最大容量，1>>>30(table数组size) |
| 类变量（default static final） | DEFAULT_LOAD_FACTOR | 默认比例因子 0.75，当初始化没有传值时使用。（怎么计算的待研究） |
| 实例变量（default transient Entry[]） | table | 数组，用来存放hashMap单元数据，每个数组存一个链表结构。数组大小为2的幂值，大小不够可以resize |
| 实例变量（default transient int） | size | HashMap数据总数 |
| 实例变量 （default） | threshold | table resize 的阈值=capacity * load factor |
| 实例变量（final float） | loadFactor | 加载因子 size/capacity, 默认为0.75 |
| 实例变量（transient volatile int） | modCount | 有新增entry就会加1,remove也加1。 用来做简单的并发控制。 |
2、主要方法实现
1、初始化
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
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);
// Find a power of 2 >= initialCapacity
int capacity = 1;
while (capacity < initialCapacity)
capacity <<= 1;
this.loadFactor = loadFactor;
threshold = (int)(capacity * loadFactor);
table = new Entry[capacity];
init();
} 入参： （1）initialCapacity 初始容量，指外层数组的初始大小。 （2）loadFactor 加载因子，当 size/capacity>loadFactor 需要resize扩大数组大小，默认为0.75。
注：数组容量为大于传入初始容量的最小2的幂值。 2、hash方法
/**
* Applies a supplemental hash function to a given hashCode, which
* defends against poor quality hash functions. This is critical
* because HashMap uses power-of-two length hash tables, that
* otherwise encounter collisions for hashCodes that do not differ
* in lower bits. Note: Null keys always map to hash 0, thus index 0.
*/
static int hash(int h) {
// 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).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
} 入参是key值的hashcode，目的是让hash表更松散，具体原理
待研究。 因为hashmap表长度是2的幂，经过indexfor的计算，就是取hashcode的末几位，hash方法的作用，是让末几位受到其他位差异的影响更多，让hashmap更松散。
3、indexfor方法
/**
* Returns index for hash code h.
*/
static int indexFor(int h, int length) {
return h & (length-1);
}
因为hashmap表长度是2的幂，经过indexfor的计算，就是取hashcode的末几位
4、get方法
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
* <p>A return value of {@code null} does not <i>necessarily</i>
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link #containsKey containsKey} operation may be used to
* distinguish these two cases.
*
* @see #put(Object, Object)
*/
public V get(Object key) {
if (key == null)
return getForNullKey();
int hash = hash(key.hashCode());
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.equals(k)))
return e.value;
}
return null;
}
HashMap允许key为null，key为null放在数组第一位。
获取方法就是，先根据key算出数组下标，取到链表，再遍历链表获取到key相同的值，获取其value值。
遍历的时候，会先比较hash值，我理解这是为了提高性能，因为遍历的时候大多数情况应该是不等的，而大多数的不等通过hash值得比较就能过滤掉。不用通过equals比较，因为hash值是算好的，equals还得再算一遍，有些复杂的可能性能消耗不少。
5、put方法
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
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;
} 根据key算出table下标；取到对应链表，遍历链表，获取key值相同的entry更新value值返回oldValue；如果没有新增entry，返回null。 6、addEntry 方法
/**
* Adds a new entry with the specified key, value and hash code to
* the specified bucket. It is the responsibility of this
* method to resize the table if appropriate.
*
* Subclass overrides this to alter the behavior of put method.
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
if (size++ >= threshold)
resize(2 * table.length);
} 根据指定的hash值、key、value、bucketIndex（table下标）新增entry；如果大小超过阈值（capacity*loadFactor），table阔成原来的两倍。 7、resize方法
/**
* Rehashes the contents of this map into a new array with a
* larger capacity. This method is called automatically when the
* number of keys in this map reaches its threshold.
*
* If current capacity is MAXIMUM_CAPACITY, this method does not
* resize the map, but sets threshold to Integer.MAX_VALUE.
* This has the effect of preventing future calls.
*
* @param newCapacity the new capacity, MUST be a power of two;
* must be greater than current capacity unless current
* capacity is MAXIMUM_CAPACITY (in which case value
* is irrelevant).
*/
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);
table = newTable;
threshold = (int)(newCapacity * loadFactor);
} 如果旧表容量已经到MAXIMUM_CAPACITY， threshold设置成最大int值，table不做扩大。 否则根据传入新capacity，创建新table，将旧table数据迁移到新table，重新计算threshold（阈值）。
8、transfer方法，将oldTable数据迁移到newTable
/**
* Transfers all entries from current table to newTable.
*/
void transfer(Entry[] newTable) {
Entry[] src = table;
int newCapacity = newTable.length;
for (int j = 0; j < src.length; j++) {
Entry<K,V> e = src[j];
if (e != null) {
//oldtable数据设置为null,我理解能够快速垃圾回收。
src[j] = null;
do {
Entry<K,V> next = e.next;
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
} while (e != null);
}
}
}
遍历旧table的entry，重新计算index，存入到new table。
9、HashIterator内部类，HashMap迭代器基类。
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;
}
//可以用这个方法一边遍历一边删除。
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();
}
}
private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
public Map.Entry<K,V> next() {
return nextEntry();
}
}
// Subclass overrides these to alter behavior of views' iterator() method
Iterator<K> newKeyIterator() {
return new KeyIterator();
}
Iterator<V> newValueIterator() {
return new ValueIterator();
}
Iterator<Map.Entry<K,V>> newEntryIterator() {
return new EntryIterator();
}
三、优势
1、功能方面：
（1）根据key值寻找对应value，应用范围非常广。
2、性能方面
（1）结合数组寻址快速，链表插入删除快速的优点，寻找删除快速。
四、劣势
1、功能
（1）无序
2、性能
3、可靠性
（1）非线程安全的，modCout参数有简单的防并发功能。
五、应用场景和使用注意
（1）大小已知时，HashMap初始化需要传入长度参数，因为不传参数，HashMap的bucket个数不够会做resize比较耗性能。