HashMap 储值原理
一.当创建HashMap对象时 成员变量的变化
this.loadFactor = DEFAULT_LOAD_FACTOR; ( 加载因子)
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
//底层哈西表内顺序表默认长度
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//底层哈西表 最大长度
static final int MAXIMUM_CAPACITY = 1 << 30; (左移30位)
//加载/负载 因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//树化阈值 (由链表转成树)
static final int TREEIFY_THRESHOLD = 8;
//反树化阈值 (由树转成链表)
static final int UNTREEIFY_THRESHOLD = 6;
//树化 底层哈西表内顺序表最小长度
static final int MIN_TREEIFY_CAPACITY = 64;
//底层哈西表内顺序表 数据类型
transient Node[] table;//null
static class Node implements Map.Entry {
final int hash;
final K key;
V value;
Node next;
Node(int hash, K key, V value, Node next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
//键值对的数量
transient int size;//0
如果顺序表内有价值数量 > 阈值 进行扩容 16*0.75f = 12
int threshold;
//成员变量 加载/负载因子
final float loadFactor;
二.在进行数据添加时 HashMap底层的处理情况
2.0第一次添加
底层Node[] 进行了空间开辟 长度为16 阈值 12
(会把key=null的键值对放到顺序表0的位置) (源码中的table相当于哈希表)
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
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[] tab; Node p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node 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)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
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;
}
}
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;
}
2.1添加的数据 key 为null
将数据存到 顺序表下标为0的位置

2.2添加的数据 key 不为null
(哈希值的计算 目的是减少哈希位置的冲突) 重写hashCode()方法
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
2.2.1 添加数据到指定位置没有值
直接添加 (因为没有哈希值的冲突)
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);

2.2.2 添加数据指定位置有值 hash一样 key 不一样
在现有节点的后面进行追加 七上 八下 jdk7 jdk8
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
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;
}

2.2.3 添加数据指定位置有值 hash一样 key 一样
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
1.将新的value替换旧的value
2.将旧的value返回
2.2.4 扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
普通扩容
容量 << 1
阈值 <<1
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node[] newTab = (Node[])new Node[newCap];
树化 (要求: 1.某一个位置 节点数量>=8 且 底层数组长度>=64)
1.某一个位置 节点数量>=8 (触发树化 进入到树化的方法中还需要判断此时数组的长度是否>=64,如果成立,在进行最后的安全校验添加数据的位置不为null ) 此时才开能树化。
if (binCount >= TREEIFY_THRESHOLD - 1)
treeifyBin(tab, hash);
final void treeifyBin(Node[] tab, int hash) {
int n, index; Node e;
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
else if ((e = tab[index = (n - 1) & hash]) != null) {
TreeNode hd = null, tl = null;
do {
TreeNode p = replacementTreeNode(e, null);
if (tl == null)
hd = p;
else {
p.prev = tl;
tl.next = p;
}
tl = p;
} while ((e = e.next) != null);
if ((tab[index] = hd) != null)
hd.treeify(tab);
}
}
2.底层数组长度>=64
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
3.添加数据的位置不为null (安全期间校验)
DeBug调试代码
public class MyData {
String name;
int age;
public MyData(String name, int age) {
this.name = name;
this.age = age;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
MyData myData = (MyData) o;
return age == myData.age &&
Objects.equals(name, myData.name);
}
@Override
public int hashCode() {
if(age%2==0){
return 1;
}
return age;
}
@Override
public String toString() {
return "MyData{" +
"name='" + name + '\'' +
", age=" + age +
'}';
}
}
public class MapTest {
@Test
public void test03() {
HashMap map = new HashMap<>();
//key = null 将数据添加到下标为0的位置
//第一次添加的时候才会给底层数组开辟空间
map.put(null, 20);
for (int i = 0; i < 1000; i++) {
map.put(new MyData("MyData" + i, i), 10);
}
}
}

