[Java知识库]java - HashMap 源码阅读 - 笔记

创建一个dome

    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();  // 初始化里面的table，当插入一个元素的时候，扩容为16 
        map.put("数据1", "数据1 == 1");
        map.put("数据1", "数据1 copy 1");
        for (int i=2; i < 20000000 ; i++ ){
            map.put("数据"+i, "数据"+i+" == "+i);
        }
        for (String key: map.keySet()) {
            map.get(key);
        }
        map.remove("1");
    }

构造函数:
1. 设置负载因子 默认是0.75 （ 3/4 ）
2. 初始化数据

    /**
     * Constructs an empty {@code HashMap} with the specified initial
     * capacity and load factor.   构造一个具有指定初始容量和负载因子的空 {@code HashMap}
     * 
     * @param  initialCapacity the initial capacity
     * @param  loadFactor      the load factor
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     */
    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);
    }

    /**
     * Constructs an empty {@code HashMap} with the specified initial
     * capacity and the default load factor (0.75).
     *
     * @param  initialCapacity the initial capacity.
     * @throws IllegalArgumentException if the initial capacity is negative.
     */
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /**
     * Constructs an empty {@code HashMap} with the default initial capacity
     * (16) and the default load factor (0.75).    使用默认初始容量 (16) 和默认负载因子 (0.75) 构造一个空的 
     */
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

    /**
     * Constructs a new {@code HashMap} with the same mappings as the
     * specified {@code Map}.  The {@code HashMap} is created with
     * default load factor (0.75) and an initial capacity sufficient to
     * hold the mappings in the specified {@code Map}.
     *
     * @param   m the map whose mappings are to be placed in this map
     * @throws  NullPointerException if the specified map is null
     */
    public HashMap(Map<? extends K, ? extends V> m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

添加( put )元素：
1. 数组+链表+ 红黑树
2. 当 链表大于8 && 数组长度大于64 才变树
3. 链表尾插法
4. 扩容的时候位置不变或索引+旧容量大小

    /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with {@code key}, or
     *         {@code null} if there was no mapping for {@code key}.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with {@code key}.)
     */
    public V put(K key, V value) {  //  hash(key) 将key 进行Hash 运算
        return putVal(hash(key), key, value, false, true);
    }

    /**
     * Implements Map.put and related methods.
     *
     * @param hash hash for key
     * @param key the key
     * @param value the value to put
     * @param onlyIfAbsent if true, don't change existing value  如果为 true，则不要更改现有值
     * @param evict if false, the table is in creation mode.  如果为 false，则表处于创建模式
     * @return previous value, or null if none
     */
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i; //  tab = table
        if ((tab = table) == null || (n = tab.length) == 0)  // 判断容量是否为null 或者为 0， 是则初始化
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)           //  判断是否是一个新节点
            tab[i] = newNode(hash, key, value, null);
        else {    //    不是一个新节点，则修改数据
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))  // 判断当前就是一个节点（只有一个元素）
                e = p;
            else if (p instanceof TreeNode)   //  当前是一棵树
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {                            //  当前是一个链表
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {  // 如果链表只有一个元素
                        p.next = newNode(hash, key, value, null); // 新建一个节点在后面插入
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);      // 链表变树
                        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)  // 当大小 > 总容量 * 0.75  则  resize()
            resize();
        afterNodeInsertion(evict);
        return null;
    }

    /**
     * Initializes or doubles table size.  If null, allocates in
     * accord with initial capacity target held in field threshold.
     * Otherwise, because we are using power-of-two expansion, the
     * elements from each bin must either stay at same index, or move
     * with a power of two offset in the new table.
     *
     * @return the table
     */
    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;   //  旧 Table
        int oldCap = (oldTab == null) ? 0 : oldTab.length; // ==================  根据旧容量计算新容量  START
        int oldThr = threshold;
        int newCap, newThr = 0;
        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
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            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);
        }
        threshold = newThr;                    // ==================  根据旧容量计算新容量  END
        @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];   //  新数组数据
        table = newTab;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {    // 遍历所有数组有数据元素
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;    // 数组元素只有一个数据
                    else if (e instanceof TreeNode)           // 数组元素下是一棵树
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order                  // 数组元素下是一个链表
                        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);  // 还有下一个数据
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead; //  扩容的时候位置不变或索引+旧容量大小
                        }
                    }
                }
            }
        }
        return newTab;
    }

    /**
     * Replaces all linked nodes in bin at index for given hash unless
     * table is too small, in which case resizes instead.
     */
    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);
        }
    }

get 元素
定位数组下标: (n - 1) & hash
判断是否是链表或树

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

    /**
     * Implements Map.get and related methods.
     *
     * @param hash hash for key
     * @param key the key
     * @return the node, or null if none
     */
    final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))    //  数组下的链表只有一个元素
                return first;
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)     // 该节点是树
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                do {                               // 节点是链表
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))  // 用  equals 判断 key 是否相等
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }