1.特性
- 数组 + 链表 的底层 结构
- 默认容量：16
- 加载因子：0.75
2.put数据
- 散列算法
- 对key进行hash值计算
- 然后与数组长度取模操作，得到该节点插入的数组下标位置
- 插入链表
- 调用方法 put(K key,V value);
扩容
3.源码分析
3.1.构造函数
//重要属性：
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // HashMap默认容量大小16
static final int MAXIMUM_CAPACITY = 1 << 30; //容量最大值
static final float DEFAULT_LOAD_FACTOR = 0.75f; //加载因子0.75
int threshold; //阈值，数组扩容阈值，大小为当前数组容量 * 加载因子
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
* 默认大小16，加载因子为0.75
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
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;
this.threshold = tableSizeFor(initialCapacity);
}
Node节点
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next; //后继节点
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
3.2.添加元素 put方法
public V put(K key, V value) {
// 1.先调用hash方法获取key的hash值，然后调用putVal方法
return putVal(hash(key), key, value, false, true);
}
//返回Object key值的hash值，为null返回0，
//不为null先获取key的hashCode的值h，然后二进制操作（h >>> 16） 往右移动16位
//-接着再与h 进行异或^操作
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
// Node节点p对象为当前key值中，数组下标的节点
Node<K,V>[] tab; Node<K,V> p; int n, i;
//table为null，调用resize()方法获取初始化创建的tab，并获取tab数组的大小，
//resize方法默认创建一个大小16的Node数组，
if ((tab = table) == null || (n = tab.length) == 0){
n = (tab = resize()).length;
}
// 根据key的hash值定位到数组下标位置 i = (n - 1) & hash
// 如果当前数组下标为null，则新建一个Node节点放到该数组下标i位置
if ((p = tab[i = (n - 1) & hash]) == null){
// 如果数组下标的元素为null，则新建一个Node节点
tab[i] = newNode(hash, key, value, null);
}else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k)))){
e = p; //如果hash值相同，且key值相等，替换
}else if (p instanceof TreeNode){ //红黑树
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
}else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
// 沿着数组下标的第一个节点p，不断往后遍历查找到当前链路的最后一个节点p，
// -新建新节点连接在尾节点后面
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
//当数组节点链表的节点数超过8个时，链表转换成红黑树结果
treeifyBin(tab, hash);
break;
}
// 找到相同的hash值，e节点为相同key值的节点对象
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))){
break;
}
p = e;
}
}
// e不为null，说明存在相同key值的节点，将数据域进行替换即可
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold) //添加元素后超过数组大小阈值，需要进行数组扩容
resize();
afterNodeInsertion(evict);
return null;
}
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
//oldCap表示之前数组的大小，默认为0
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY){
// newCap = oldCap << 1 数组新容量在原数组容量基础上的两倍
// newThr 为扩展阈值，也扩大为之前的两倍
newThr = oldThr << 1; // double threshold
}
}else if (oldThr > 0){ // initial capacity was placed in threshold
//oldThr 为 threshold，用户可以在构造函数中设置threshold的大小
newCap = oldThr;
}else { // zero initial threshold signifies using defaults
//oldCap == 0,数组还每创建呢，
//--第一次Node[]数组table 为null，oldCap为0，命中该条件，newCap设置为16
newCap = DEFAULT_INITIAL_CAPACITY;
// newThr等于数组容量的大小 * 扩展因子，而后会赋值给threshold的
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
//创建Node数组，并给table赋值
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
// 新创建更大容量的数组，需要将原先的数组内容移动到新数组链表节点上去
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
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;
}
3.3.获取数据get() 函数
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
// (n - 1) & hash -->根据hash值获取当前key值在数组中的下标位置
//first = tab[(n - 1) & hash] --》first为数组下标中第一个头节点
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // 判断头节点是否命中
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode){ //红黑树获取节点
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
}
// 从头节点不断获取后继节点，判断是否命中，命中的话直接返回
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
//红黑树中根据key值获取节点对象
static final class TreeNode<K,V> extends LinkedHashMap.LinkedHashMapEntry<K,V> {
final TreeNode<K,V> getTreeNode(int h, Object k) {
return ((parent != null) ? root() : this).find(h, k, null);
}
}
网友评论