011—HashMap的储值原理详解_hashmap int bincount = 0; ; ++bincount-优快云博客

本文链接：https://blog.youkuaiyun.com/xiaodi2016/article/details/121150738

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<K,V>[] table;//null
static class Node<K,V> implements Map.Entry<K,V> {
        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;
        }
   //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<K,V>[] tab; Node<K,V> 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<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 {
                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<K,V>[] newTab = (Node<K,V>[])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<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            TreeNode<K,V> hd = null, tl = null;
            do {
                TreeNode<K,V> 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<MyData, Integer> 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);

        }
    }
}