本文主要记录下自己学习hashmap
从构造函数看
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);
}
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
主要逻辑其实就在这两个函数，其他都是弟弟
第一个构造方法
initialCapacity：初始的容量
loadFactor：负载因子
重点是这个方法
tableSizeFor(initialCapacity);
这个方法主要是返回一个2的幂次方比的整书刚好比你给的整数大，也就是传入12 返16。因为HashMap要求容量必须是2的幂。
threshold 不是resize的界限吗？如果负载因子是0.75 返回的应该是12 现在确是16.
threshold 稍后会在put中重新定义
第二个构造方法
重点逻辑
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
//获取元素
int s = m.size();
if (s > 0) {
if (table == null) { // pre-size
//如果没有talbe 重新建一个，根据插入的size 重新计算threshold
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t);
}
//如果已经简历 ，但是小于传进来的map，就需要扩容
else if (s > threshold)
resize();
//接着就是熟悉的便利插入元素
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
}
}
}
接下来再看上面的 resize（）源码
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
//老得容量最大值 了就直接返回
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//翻倍小于最大值，就果断翻倍
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
//table 为空，而oldThr 有数据，还记得上面的构造函数吗，此时oldThr就代表的是容量且talbe 为null
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
//都没有构造过，就取默认值吧
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//新阈值为0 ,算一哈
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
//来个新talbe 放
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
//如果老得是空的
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
//若节点是单个节点
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
// 如果是treenode
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
//若是链表
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
//一个长度为8的，在1位置上 一个1 一个9， 1的位置不变，9就加一个长度
do {
next = e.next;
//为0的不变
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
//为1的变化
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;
}
偷张图
image.png
我们在看putVal的源码
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, I;
//如果空的话resize
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//找到key值对应的槽并且是第一个，直接加入
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
//碰撞了，要么替换 要么链表 要么红黑树
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//没找到一样的，加在最后，同时判断是否转树
for (int binCount = 0; ; ++binCount) {
//如果一直为空，加在最后
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//超过了设置长度8就转换成红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//如果e不为空就替换旧的oldValue值
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)
resize();
afterNodeInsertion(evict);
return null;
}
