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Java HashMap源码学习

基本使用

包含创建，添加，删除，迭代，打印

val map = java.util.HashMap<Int, Int>()
map.put(1, 2)
map.put(2, 2)
map.put(3, 2)
map.remove(1)
map.forEach {println("it.key=${it.key}, it.value=${it.value}")
}
println(map)

分析源码

一贯原则，没用的帮兄弟们省掉了

简单介绍

package java.util;// AbstractMap，可以随机访问
public class HashMap<K, V> extends AbstractMap<K, V> {// 初始容量static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 最大容量static final int MAXIMUM_CAPACITY = 1 << 30;// 负载因子，当前容量超过最大容量*负载因子，开始扩容static final float DEFAULT_LOAD_FACTOR = 0.75f;// 链表大于8树化static final int TREEIFY_THRESHOLD = 8;// 树小于6链化static final int UNTREEIFY_THRESHOLD = 6;// 树化的最小容量static final int MIN_TREEIFY_CAPACITY = 64;// 存储key，valuestatic class Node<K, V> implements Map.Entry<K, V> {// 通过key中字段计算final int hash;final K key;V value;Node<K, V> next;Node(int hash, K key, V value, Node<K, V> next) {this.hash = hash;this.key = key;this.value = value;this.next = next;}// 省略get，set等模板方法}
}

创建

// 负载因子
public HashMap() {this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

添加

public V put(K key, V value) {// 通过hash()对key进行一次hash()return putVal(hash(key), key, value);
}// 重点分析
final V putVal(int hash, K key, V value) {// tab，桶，就是数组// p，期望对象// n，当前桶容量// i，桶下标Node<K,V>[] tab; Node<K,V> p; int n, i;// 如果是第一次，通过resize()更新初始容量if ((tab = table) == null || (n = tab.length) == 0)n = (tab = resize()).length;// 与桶大小&计算，二次hash()// 刚好桶上没元素，直接在桶上更新if ((p = tab[i = (n - 1) & hash]) == null)tab[i] = newNode(hash, key, value, null);else {// e，辅助期望元素// k，桶下标元素的keyNode<K,V> e; K k;// 如果和桶第一个一样if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k))))e = p;// 已经树化的计算else if (p instanceof TreeNode)e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);// 链化的计算else {for (int binCount = 0; ; ++binCount) {// 没有相同的，在最后添加if ((e = p.next) == null) {p.next = newNode(hash, key, value, null);// 树化 if (binCount >= TREEIFY_THRESHOLD - 1)treeifyBin(tab, hash);break;}// 找到key一样的if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))break;p = e;}}e.value = value;}// 扩容if (++size > threshold)resize();return null;
}// key经过hashCode()与低位^，让高位也参与计算
static final int hash(Object key) {int h;return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}// 二倍扩容
final Node<K,V>[] resize() {Node<K,V>[] oldTab = table;// 当前容量int oldCap = (oldTab == null) ? 0 : oldTab.length;// 当前负载int oldThr = threshold;int newCap, newThr = 0;if (oldCap > 0) {if (oldCap >= MAXIMUM_CAPACITY) {threshold = Integer.MAX_VALUE;return oldTab;}// 二倍扩容else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&oldCap >= DEFAULT_INITIAL_CAPACITY)newThr = oldThr << 1; // double threshold}else if (oldThr > 0) // initial capacity was placed in thresholdnewCap = oldThr;// 第一次扩容else {               // zero initial threshold signifies using defaultsnewCap = DEFAULT_INITIAL_CAPACITY;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;@SuppressWarnings({"rawtypes","unchecked"})// 大小确定，正式开始Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];table = newTab;if (oldTab != null) {// 遍历桶for (int j = 0; j < oldCap; ++j) {Node<K,V> e;if ((e = oldTab[j]) != null) {oldTab[j] = null;if (e.next == null)// 通过hash&新桶容量，计算位置newTab[e.hash & (newCap - 1)] = e;// 树转移else if (e instanceof TreeNode)((TreeNode<K,V>)e).split(this, newTab, j, oldCap);// 链表else { // preserve order// lo，low，hi，high，high=桶容量/2+lowNode<K,V> loHead = null, loTail = null;Node<K,V> hiHead = null, hiTail = null;Node<K,V> next;do {next = e.next;// 如果在下标0位置if ((e.hash & oldCap) == 0) {if (loTail == null)loHead = e;elseloTail.next = e;loTail = e;}else {if (hiTail == null)hiHead = e;elsehiTail.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;
}

总结

1. HashMap可以存储键值对
2. 初始容量是16，每次扩容2倍数
3. 先添加，后扩容
4. 从尾部添加

后续

里面代码确实挺多，其中还包括树，以后分析并尝试重写