HashMap代码解析
hashmap (jdk 1.7)使用 “数组-链表” 方式进行存储，图形化表示如下：
即，前面是一个数组，后面跟一个链表，那么数据结构这个对应到HashMap的代码里面是什么样子的呢？
在HashMap中定义了一个类型为Entry<K,V>的数组table，上图就是显示了这个table。
/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
类型Entry<K,V>的定义如下：
static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;
int hash;
// 省略构造/get/set等函数
}
由Entry<K,V>的定义可知，上图每个节点中其实存了4个变量：
key表示键，即存入map的键值
value表示值，即存入map的值
next表示下一个Entry节点
hash表示key的哈希值。
那么上图准确表示应该是：
对于HashMap，最常用的莫过于直接使用默认构造函数创建一个Map对象了
Map<int, String> map = new HashMap<>();
这里HashMap调用了
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
其中，DEFAULT_INITIAL_CAPACITY是
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
DEFAULT_LOAD_FACTOR是
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
this()调用的是
/**
* 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);
this.loadFactor = loadFactor;
threshold = initialCapacity;
init();
}
了解了基本结构之后，看一下HashMap的put()和get()方法是如何实现的。
首先，看put()方法，再了解put()方法之前，先了解几个put()方法会调用的几个辅助方法：
1. inflateTable()，给表充气 or 让表膨胀？原来table对象是空的，所以需要将table对象初始化
/**
* Inflates the table.
*/
private void inflateTable(int toSize) {
// Find a power of 2 >= toSize
// capacity 表示HashpMap的容量，必须是2的倍数
int capacity = roundUpToPowerOf2(toSize);
// threshold 表示需要resize的阈值
threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
// 初始化大小为capacity的table对象
table = new Entry[capacity];
// 初始化 hashSeed
initHashSeedAsNeeded(capacity);
}
2. putForNullKey()
/**
* Offloaded version of put for null keys
*/
private V putForNullKey(V value) {
// 遍历table[0]，如果已经有key为null的元素，直接返回对应的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;
}
}
// 如果没有key为null的元素，
// 将HashMap的修改次数+1
modCount++;
// 将key为null的元素添加到HashMap中
addEntry(0, null, value, 0);
return null;
}
疑问：key为null的元素的hash值一定为0吗？
3. hash()，求对象的hash值
/**
* Retrieve object hash code and applies a supplemental hash function to the
* result hash, 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.
*/
final int hash(Object k) {
int h = hashSeed;
if (0 != h && k instanceof String) {
return sun.misc.Hashing.stringHash32((String) k);
}
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).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
4. indexFor()，根据对象的hash值以及HashMap table的长度，寻找该对象的索引位置
/**
* Returns index for hash code h.
*/
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);
}
这里没有使用hash值h对长度length取余，而是使用的位运算？其实两者结果是一样的，h % length == h & (length -1)
5. addEntry()
/**
* 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) {
// size是HashMap中元素的个数
// threshhold = capacity * load factor，表示需要扩容resize的阈值
// 如果size > threshold，并且table在当前索引处有元素，不为null，则需要扩容HashMap，并从新计算索引值
if ((size >= threshold) && (null != table[bucketIndex])) {
resize(2 * table.length);
hash = (null != key) ? hash(key) : 0;
bucketIndex = indexFor(hash, table.length);
}
// 将元素加入到HashMap
createEntry(hash, key, value, bucketIndex);
}
6. createEntry()
/**
* Like addEntry except that this version is used when creating entries
* as part of Map construction or "pseudo-construction" (cloning,
* deserialization). This version needn't worry about resizing the table.
*
* Subclass overrides this to alter the behavior of HashMap(Map),
* clone, and readObject.
*/
void createEntry(int hash, K key, V value, int bucketIndex) {
// 这里是获取某个链表的第一个节点e，
// 因为每次插入都是往链表的头部插入的，因此e就作为了新节点的next值
Entry<K,V> e = table[bucketIndex];
// e作为新节点的next值
table[bucketIndex] = new Entry<>(hash, key, value, e);
// HashMap的size加1
size++;
}
7. resize()
/**
* 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;
// 如果HashMap中table的长度（这里是指table数组的长度，不是链表的长度）
// 已经达到了MAXIMUN_CAPACITY = 1 << 30，直接将阈值threshold设置为Integer的最大值。
// 不在扩容HashMap
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
// newCapacity是原table数组长度的2倍
Entry[] newTable = new Entry[newCapacity];
// 将原table中的值迁移到扩容后的newTable中
transfer(newTable, initHashSeedAsNeeded(newCapacity));
// 更新table和阈值threshold
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
8. transfer()， 将原HashMap中的元素迁移到扩容后的HashMap中
/**
* Transfers all entries from current table to newTable.
*/
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
// 遍历table数组
for (Entry<K,V> e : table) {
// 这里e是table数组某个链表的第一个元素，后面e会依次指向链表中所有的元素
// 如果table数组的元素不为null
while(null != e) {
Entry<K,V> next = e.next;
if (rehash) {
// 如果e.key是null，hash值是0
e.hash = null == e.key ? 0 : hash(e.key);
}
// 获取元素e在新table中的索引值
int i = indexFor(e.hash, newCapacity);
// 将e的next指向新table的第一个元素（这里还是要记住，插入链表是从头部插入的）
// newTable[i]是链表的第一个元素
e.next = newTable[i];
// 将e赋值给链表的第一个元素newTable[i]，这样e就取代了链表原来的第一个元素，作为链表新的第一个元素，引领链表！
newTable[i] = e;
e = next;
}
}
}
了解了上述n个方法之后，是时候看一下HashMap的put()方法的真面目了！
/**
* 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) {
// 如果table是空的，需要初始化table
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
// 如果key是null，调用putForNullKey方法插入元素
if (key == null)
return putForNullKey(value);
// 求key的hash值
int hash = hash(key);
// 根据key的hash值和table的长度求元素在table中的索引
int i = indexFor(hash, table.length);
// 遍历table[i]引领的链表
// 如果已经存在了相同的key，则更新value并返回old value，否则插入新元素
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
// key已经存在，更新value，返回old value
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
// HashMap的修改次数加1，modCount是modified times
modCount++;
// 插入新元素
addEntry(hash, key, value, i);
// 如果key没有重复，返回值是null
return null;
}
其次，看一下get()方法，再了解get()方法之前，同样先了解几个get()方法会调用的几个辅助方法：
1. getForNullKey()
/**
* Offloaded version of get() to look up null keys. Null keys map
* to index 0. This null case is split out into separate methods
* for the sake of performance in the two most commonly used
* operations (get and put), but incorporated with conditionals in
* others.
*/
private V getForNullKey() {
if (size == 0) {
return null;
}
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
}
2. getEntry()
/**
* Returns the entry associated with the specified key in the
* HashMap. Returns null if the HashMap contains no mapping
* for the key.
*/
final Entry<K,V> getEntry(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
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;
}
了解了上述2个方法之后，get()方法就比较简单了
/**
* 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();
Entry<K,V> entry = getEntry(key);
return null == entry ? null : entry.getValue();
}
最后，再学习一下remove()方法的实现
/**
* Removes the mapping for the specified key from this map if present.
*
* @param key key whose mapping is to be removed from the map
* @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 remove(Object key) {
Entry<K,V> e = removeEntryForKey(key);
return (e == null ? null : e.value);
}
/**
* Removes and returns the entry associated with the specified key
* in the HashMap. Returns null if the HashMap contains no mapping
* for this key.
*/
final Entry<K,V> removeEntryForKey(Object key) {
if (size == 0) {
return null;
}
// 找到hash值
int hash = (key == null) ? 0 : hash(key);
// 求索引位置
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;
// 寻找到key的位置，是当前的e
if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) {
// HashMap的修改次数加1
modCount++;
// HashMap的size减1
size--;
// 如果是链表的第一个元素，next是null，直接将table[i]设置为null
if (prev == e)
table[i] = next;
else
// prev的next是e，next是e.next，即[prev]-> [e] -> [next]
// prev.next = next，即[prev] -> [next]，直接将元素e移除掉了
prev.next = next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}
return e;
}
