HashMap 1.8 源码

本文主要是介绍HashMap 1.8 源码，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

public class HashSet<E> {//1.8版本
    private transient HashMap<E,Object> map;
    //1-1. 创建一个HashMap对象，并且调用无参构造函数
    public HashSet() {
        map = new HashMap<>();
    }
    
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }
    
    //2-1. 添加第一个元素，调用add方法
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }
    
    //-----------HashMap源码---------------------------
    
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    static final int MAXIMUM_CAPACITY = 1 << 30;

    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    
    transient Node<K,V>[] table;
    
    //1-2. 负载因子赋值为默认值0.75
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }
    
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
    
    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);
    }
    
    //2-2. 进入HashMap的put方法中
    public V put(K key, V value) {
        //2-3. key = Student{id=1, name='lili'} value=PRESENT,对key进行hash运算
        //2-5. 走putVal方法
        return putVal(hash(key), key, value, false, true);
    }
    
    //2-4. 扰动函数，避免hash碰撞，每个版本的算法不一样。
    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;
        //2-5. 此时数组为空，所以进if
        if ((tab = table) == null || (n = tab.length) == 0)
            //2-6. 走resize方法,resize方法返回一个长度为16的Node[]
            //2-14. n = 16
            n = (tab = resize()).length;
            //2-15. (n - 1) & hash 根据这个表达式算出元素在数组的下标位置
            //2-16.  (p = tab[i = (n - 1) & hash]) == null 判断数组中该位置是否已经有数据了
        if ((p = tab[i = (n - 1) & hash]) == null)
            //2-17. 创建一个Node，然后放到数组对应下标的位置上
            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;
        //4-1.如果超过扩容边界值12，就扩容
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);//啥也没干
        return null;
    }
    
    final Node<K,V>[] resize() {
        //2-7 oldTab = null;
        //4-2. oldTab = new Node[16];
        Node<K,V>[] oldTab = table;
        //4-3 oldCap = 16
        int oldCap = (oldTab == null) ? 0 : oldTab.length;//oldCap = 0
        //4-4 oldThr = 12
        int oldThr = threshold;// oldThr = 0;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            //4-5. (newCap = oldCap << 1 newCap = oldCap*2=32 数组长度扩展为原来的2倍
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // newThr=24，扩容临界也扩为原来的2倍
        }
        else if (oldThr > 0) 
            newCap = oldThr;
        else {
            //2-8. newCap = 16;            
            newCap = DEFAULT_INITIAL_CAPACITY;
            //2-9. newThr = 12;
            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);
        }
        //2-10. threshold = 12
        //4-6. threshold=24
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
        //2-11. 创建一个newTab=Node[16]的数组
        //4-7 把原来的数组扩展成一个newTab=Node[32]的数组
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        //2-12.table = newTab=Node[16] 主数组长度默认为16
        table = newTab;
        //4-8.进入if
        if (oldTab != null) {
            //4-9.oldCap = 16
            
            /*
            整个循环的意思就是对于之前数组中的元素再次根据e.hash & 数组.length公式算出在扩容后的数组的下标位置,
                1.如果当前元素没有链表，则按照e.hash & (newCap - 1)，算下标位置
                2.如果当前元素下面有链表（链表里每一个元素都重新算），则按照e.hash & oldCap，算下标位置
                     2-1.如果算出(e.hash & oldCap) == 0，则还放在原来在老数组的对应下标位置；
                     2-2.如果算出(e.hash & oldCap) ！= 0，则放在j + oldCap的下标位置中；
            */
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    //1.8 引入的红黑树
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            next = e.next;
                            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;
                        }
                    }
                }
            }
        }
        //2-13. 返回一个长度为16的Node[]
        return newTab;
    }
    
     Node<K,V> newNode(int hash, K key, V value, Node<K,V> next) {
        return new Node<>(hash, key, value, next);
    }
        
    final TreeNode<K,V> putTreeVal(HashMap<K,V> map, Node<K,V>[] tab,
                                       int h, K k, V v) {
            Class<?> kc = null;
            boolean searched = false;
            TreeNode<K,V> root = (parent != null) ? root() : this;
            for (TreeNode<K,V> p = root;;) {
                int dir, ph; K pk;
                if ((ph = p.hash) > h)
                    dir = -1;
                else if (ph < h)
                    dir = 1;
                else if ((pk = p.key) == k || (k != null && k.equals(pk)))
                    return p;
                else if ((kc == null &&
                          (kc = comparableClassFor(k)) == null) ||
                         (dir = compareComparables(kc, k, pk)) == 0) {
                    if (!searched) {
                        TreeNode<K,V> q, ch;
                        searched = true;
                        if (((ch = p.left) != null &&
                             (q = ch.find(h, k, kc)) != null) ||
                            ((ch = p.right) != null &&
                             (q = ch.find(h, k, kc)) != null))
                            return q;
                    }
                    dir = tieBreakOrder(k, pk);
                }

                TreeNode<K,V> xp = p;
                if ((p = (dir <= 0) ? p.left : p.right) == null) {
                    Node<K,V> xpn = xp.next;
                    TreeNode<K,V> x = map.newTreeNode(h, k, v, xpn);
                    if (dir <= 0)
                        xp.left = x;
                    else
                        xp.right = x;
                    xp.next = x;
                    x.parent = x.prev = xp;
                    if (xpn != null)
                        ((TreeNode<K,V>)xpn).prev = x;
                    moveRootToFront(tab, balanceInsertion(root, x));
                    return null;
                }
            }
        }
        
        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);
        }
    }
    
    void afterNodeAccess(Node<K,V> p) { }
    void afterNodeInsertion(boolean evict) { }
}

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