jdk1.8中底层数据结构使用数组+链表+红黑树
查看源码定义
public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable {
private static final long serialVersionUID = 362498820763181265L;
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 默认初始容量 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;//最大容量2^30
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;//默认负载因子
/**
* The bin count threshold for using a tree rather than list for a
* bin. Bins are converted to trees when adding an element to a
* bin with at least this many nodes. The value must be greater
* than 2 and should be at least 8 to mesh with assumptions in
* tree removal about conversion back to plain bins upon
* shrinkage.
*/
static final int TREEIFY_THRESHOLD = 8;//链表转换为红黑树的阈值，也就是8个结点及以上转换为红黑树
/**
* The bin count threshold for untreeifying a (split) bin during a
* resize operation. Should be less than TREEIFY_THRESHOLD, and at
* most 6 to mesh with shrinkage detection under removal.
*/
static final int UNTREEIFY_THRESHOLD = 6;//红黑树转换为链表的阈值，<=8，红黑树转换为链表
/**
* The smallest table capacity for which bins may be treeified.
* (Otherwise the table is resized if too many nodes in a bin.)
* Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
* between resizing and treeification thresholds.
*/
static final int MIN_TREEIFY_CAPACITY = 64;//红黑树的最小容量
/**
* Basic hash bin node, used for most entries. (See below for
* TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
*/
static class Node<K,V> implements Map.Entry<K,V> {
//存储结点
final int hash;//hash值
final K key;//key
V value;//value
Node<K,V> next;//下一个结点
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
先看构造方法
/**
* 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)//初始容量<0,抛出不合法初始容量异常
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)//初始容量>最大容量(2^30)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))//负载因子<=0或负载因子不是合法浮点数
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;//设置负载因子
this.threshold = tableSizeFor(initialCapacity);//返回与initialCapacity最近的>=initialCapacity的2的整数次幂
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
然后看一下tableSizeFor方法
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
//比如有一个二进制数0100 0000 0000 0000 0000 0000 0000 0000
//执行n|=>>1 执行后变为0110 0000 0000 0000 0000 0000 0000 0000
//执行n|=>>2 执行后变为0111 1000 0000 0000 0000 0000 0000 0000
//执行n|=>>4 执行后变为0111 1111 1000 0000 0000 0000 0000 0000
//执行n|=>>8 执行后变为0111 1111 1111 1111 1000 0000 0000 0000
//执行n|=>>16 执行后变为0111 1111 1111 1111 1111 1111 1111 1111
//即该方法使得最高位及后面的位都变为了1
}
然后看put方法
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
然后看一下hash方法
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);//先通过hashCode方法计算出一个hash值，再将该值与该值的右移16做运算，比如计算出的hashCode为0101 0000 1000 1100 0000 0000 0001 0001
//h 0101 0000 1000 1100 0000 0000 0001 0001
//^
//h^16 0000 0000 0000 0000 0101 0000 1000 1100
//result 0101 0000 1000 1100 0101 0000 1001 1101
//这样得出的结果再与length-1(因为很多时候length非常小，所以这使得hashcode只有低位参与运算，所以冲突会很多) 进行&操作时候，就使得hashcode的高位也参与运算，通过这种方法大大降低hashCode的散列型，使得与length-1进行&运算的随机性增加
//为什么用^不用&或|,因为用&或|会偏向0或1，随机性不强，使用^比较均匀
}
然后看一下putVal方法
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)//tab位null时候或tab长度位0时候
n = (tab = resize()).length;//扩容并获取新长度
if ((p = tab[i = (n - 1) & hash]) == null)//(n - 1) & hash的到一个数组下标i，tab[i]为null时候，也就是tab[i]位置没有元素时候
tab[i] = newNode(hash, key, value, null);//插入一个新的结点
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))//要put结点的key等于当前结点的key
e = p;
else if (p instanceof TreeNode)//如果p是红黑树
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
//计算链表结点数目，遍历链表
if ((e = p.next) == null) {
//当遍历到链表结尾时候，此时肯定没有该key
p.next = newNode(hash, key, value, null);//在尾部插入一个结点，尾插法
if (binCount >= TREEIFY_THRESHOLD - 1) // 链表结点>=7，引入以及插入了一个新的结点，所以就是>=8时候
treeifyBin(tab, hash);//链表转换为红黑树
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))//如果该结点存在key
break;
p = e;
}
}
if (e != null) {
//存在key时候
V oldValue = e.value;//保存旧值
if (!onlyIfAbsent || oldValue == null)
e.value = value;//替换旧值
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)//如果添加了元素且当前key-value的数量>=threadshold
resize();//扩容
afterNodeInsertion(evict);
return null;
}
然后看一下resize扩容方法代码
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;//保存旧table
int oldCap = (oldTab == null) ? 0 : oldTab.length;//保存table长度
int oldThr = threshold;//保存旧table阈值
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
//>=最大容量2^30
threshold = Integer.MAX_VALUE;//threshold为0x7fffffff
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)//新容量的两倍小于最大容量2^30
newThr = oldThr << 1; // 新的threshold变为旧table threadshold的两倍
}
else if (oldThr > 0) // initial capacity was placed in threshold，oldCap==0时候且oldThr>0，也就是null table时候
newCap = oldThr;
else {
// zero initial threshold signifies using defaults oldThr==0时候吗，使用默认容量和threshold
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
//新threshold为0，//threshold=newCap * loadFactor
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
//通过以上oldTab，threshold的判断就得到了newCap，newThr
@SuppressWarnings({
"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];//new新的数组
table = newTab;//把新数组赋给table
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
//遍历旧数组
Node<K,V> e;
if ((e = oldTab[j]) != null) {
//如果当前结点不为null
oldTab[j] = null;//将oldTab[j]置为null，便于gc
if (e.next == null)//只有一个结点的时候
newTab[e.hash & (newCap - 1)] = e;//直接将当前结点赋给newTab[i]
else if (e instanceof TreeNode)//如果e是红黑树
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);//在红黑树插入一个结点
else {
// preserve order，如果是链表且>1个结点
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
//遍历链表，将旧链表的元素移到新链表，采用尾插法
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;//返回新数组容量
}