JDK8 HashMap源码剖析

Posted 2020-10-27 zhumian

HashMap底层数据结构为哈希表

初始容量

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

最大容量

static final int MAXIMUM_CAPACITY = 1 << 30;

默认加载因子

static final float DEFAULT_LOAD_FACTOR = 0.75f;

链表最大长度，桶节点数超过最大长度链表将转为红黑树结构

static final int TREEIFY_THRESHOLD = 8;

桶节点数小于该值则转回为链表结果

static final int UNTREEIFY_THRESHOLD = 6;

扩容的临界点，当容量达到该值时则扩容

int threshold;

更新HashMap的次数，每次对HashMap进行操作就会+1

 transient int modCount;

构造方法

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);
    }

put方法

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

putVal方法

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //桶未初始化或者未扩容的时候，先用resize()进行扩容
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //hash函数，，计算key放置的桶的位置，(n-1)&hash实际上算的是hash%n，计算更快
        //如果是空桶，则创建新的键值对节点，放进数组里面
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        //如果不是空桶
        else {
            Node<K,V> e; K k;
            //如果key已经存在，则替代原来的键值对
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //key不存在，按红黑树插入
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            //key不存在，按链表方式插入
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        //如果大于链表最大长度，则转为红黑树结构
                        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;
        //判断是否大于扩容临界点，进行扩容
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

removeNode方法

final Node<K,V> removeNode(int hash, Object key, Object value,
                               boolean matchValue, boolean movable) {
        Node<K,V>[] tab; Node<K,V> p; int n, index;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (p = tab[index = (n - 1) & hash]) != null) {
            Node<K,V> node = null, e; K k; V v;
            //待删除元素未桶的首元素
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            //待删除元素在桶中但不是首元素
            else if ((e = p.next) != null) {
                //在红黑树中
                if (p instanceof TreeNode)
                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
                //在链表中
                else {
                    do {
                        if (e.hash == hash &&
                            ((k = e.key) == key ||
                             (key != null && key.equals(k)))) {
                            node = e;
                            break;
                        }
                        p = e;
                    } while ((e = e.next) != null);
                }
            }
            //matchValue为true，表示value相等才进行删除操作
            //matchValue为false，表示只要key相等就进行删除操作
            if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount;
                --size;
                afterNodeRemoval(node);
                return node;
            }
        }
        return null;
    }