hashmap源码分析

hashmap的源码分析，关于树的方法，不展开

//默认容量，位运算
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//最大容量，位运算
static final int MAXIMUM_CAPACITY = 1 << 30;
//默认负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//链表转树界限
static final int TREEIFY_THRESHOLD = 8;
//树转链表界限
static final int UNTREEIFY_THRESHOLD = 6;
//链表转树的容量最小限制
static final int MIN_TREEIFY_CAPACITY = 64;
//hashmap的数组，1.8的hashmap是数组+链表和红黑树的结构，每个数组元素保存链表的头节点或者红黑树的根节点
transient Node[] table;

transient int size;

int threshold;
//负载因子
final float loadFactor;

//无参构造方法，将负载因子设为默认的负载因子，初始化hashmap不在这里，在put方法
	public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }
	//设置容量和负载因子，容量最大为最大容量，
	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);//保证容量是2的幂
    }
	public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

//控制容量是2的幂
static final int tableSizeFor(int cap) {
    int n = cap - 1;
    n |= n >>> 1;
    n |= n >>> 2;
    n |= n >>> 4;
    n |= n >>> 8;
    n |= n >>> 16;
    return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}

//计算key的hash，并调用真实设值方法
	public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
	//真实的设值方法
	final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
		//hashmap的临时变量
        Node[] tab; Node p; 
		//n,数组长度；i，key在数组的下标
		int n, i;
		//如果hashmap没有初始化则初始化
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //如果对应数组元素没有值，则直接设值
		if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {//对应数组元素有值，需要区分，是链表还是红黑树，还是直接覆盖
            Node e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //如果是树，则走树的方法
			else if (p instanceof TreeNode)
                e = ((TreeNode)p).putTreeval(this, tab, hash, key, value);
            //走链表的方法
			else {
				//遍历链表，直到找到链表末端，将Node放入链表末端，判断是否是否应该树化
                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;
                }
            }
			//覆盖原来Node的value
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;//修改次数
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }
	
	static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

final Node[] resize() {
		//保存到临时变量
        Node[] oldTab = table;
		//旧的容量
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        //旧的扩容阈值
		int oldThr = threshold;
		//新的容量，新的扩容阈值
        int newCap, newThr = 0;
		//hashmap不为空
        if (oldCap > 0) {
			//如果容量已达上限，将扩容阈值设为比容量上限更大的值，避免再扩容
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
			//旧容量*2小于最大容量并且旧容量大于等于默认容量，新容量设为旧容量的两倍
            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 threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            //初始化
			newCap = 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);
        }
		//设置新的扩容阈值
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
		//初始化一个新的空数组 容量原来的两倍，为扩容后的hashmap数据结构
            Node[] newTab = (Node[])new Node[newCap];
        table = newTab;
		//hashmap原来有值，进行扩容
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
						//原下标只有一个元素，将其放入新数组的对应下标
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
						//原下标是树，走树的拆分方法
                        ((TreeNode)e).split(this, newTab, j, oldCap);
                    else { // preserve order
						//链表高低链，新数组的前半部分是低链，后半部分是高链，原数组的元素会被拆分到新数组，尾插法
                        Node loHead = null, loTail = null;
                        Node hiHead = null, hiTail = null;
                        Node next;
                        do {
                            next = e.next;
							//将链表才分为两个链表，下面分析
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            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;
    }

//计算hash并调用getNode方法
public V get(Object key) {
        Node e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }
//真正执行get的方法
final Node getNode(int hash, Object key) {
        Node[] tab; Node first, e; int n; K k;
		//hashmap的数组初始化了并且hash对应数组下标有值
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            //数组下标对应元素就是要寻找的元素，直接返回
			if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
			//查找树或者遍历链表，获取被查找对象
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)
                    return ((TreeNode)first).getTreeNode(hash, key);
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }

//Node，静态内部类
static class Node implements Map.Entry {
        final int hash;//node的hash
        final K key;//Node的key值
        V value;//Node的value
        Node next;//下一个节点

        Node(int hash, K key, V value, Node next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

        public final K getKey()        { return key; }
        public final V getValue()      { return value; }
        public final String toString() { return key + "=" + value; }
	
        public final int hashCode() {
            return Objects.hashCode(key) ^ Objects.hashCode(value);
        }
		//设置值，返回旧的值
        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        public final boolean equals(Object o) {
            if (o == this)
                return true;
            if (o instanceof Map.Entry) {
                Map.Entry e = (Map.Entry)o;
                if (Objects.equals(key, e.getKey()) &&
                    Objects.equals(value, e.getValue()))
                    return true;
            }
            return false;
        }
    }

链表的拆分根据Node的hash进行拆分，hash按位与就数组容量，为0，放在低链，不为0，放在高链
比如旧的容量是16，新的容量是32
旧的容量的二进制是 00000000 000000000 00000000 00010000
Node1的hash是xxxxxxxx xxxxxxxx xxxxxxxx xxx01xxx
按位与后的结果 00000000 00000000 00000000 00000000 为0
Node2的hash是xxxxxxxx xxxxxxxx xxxxxxxx xxx11xxx
按位与后的结果 00000000 00000000 00000000 00010000 不为0
但是这两个在旧的hashmap的数组存放数据时计算hash是和(旧容量-1)按位与，结果是一样的
(旧容量-1) = 16-1 = 15 = 00000000 00000000 00000000 00001111
Node1和(旧容量-1)按位与结果是 00000000 00000000 00000000 00001000
Node2和(旧容量-1)按位与结果是 00000000 00000000 00000000 00001000
因此这两个都会放在相同的数组位置