HashMap源码阅读(小白的java进阶)

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构造方法

//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);
    }

public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

 public HashMap(Map<? extends K, ? extends V> m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

内部类

Node

用Node这个内部类存储键值对

Node<K,V>[] table;
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;
        }

        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);
        }

        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;
        }
    }

TreeNode

继承了 LinkedHashMap.Entry<K,V>,但后者又继承了HashMap.Node,所以可以认为TreeNode是Node的子类

//HashMap
static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V>
//LinkedHashMap
static class Entry<K,V> extends HashMap.Node<K,V>

重要属性

我们还需要区分三个概念

size : 即table这个数组中已经存放的node个数

capacity: 这个数组能够存放的最大node个数,即table.length

threshold:临界值,到达这个临界值后就需要扩容

//默认初始容量(16)
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;
//转换成树时的阈值
static final int TREEIFY_THRESHOLD = 8;
//未转换成树时的阈值
static final int UNTREEIFY_THRESHOLD = 6;
static final int MIN_TREEIFY_CAPACITY = 64;

//用于计算哈希值的函数
static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

put方法

最初调用的是public V put(K key, V value)

但这个方法不是最终实现的,调用的是putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict)这个方法

//将指定的key与value存入,如果已经有了key,就覆盖先前的值
public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

我们来看putVal

//返回值是值的类型
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)
            //当为空时,给n赋值
            n = (tab = resize()).length;
        //判断这个hash值是否存在,不存在则new 一个新的node,此时由于只有一个节点,所以next=null
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {//hash值已经存在了,此时的p就是先前存在的node
            Node<K,V> e; K k;
            //获取已经存在的该节点的key的hash值,如果hash值相等且key值也相等(要么都为null,要么相同)
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //这里是判断元素是否来自LinkedHashMap,暂且不管
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                //到这里以后,说明新插入的数据产生了hash冲突,且不是来自LinkedHashMap
                //采用尾插法,把新的节点接到链表的末尾
                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
                            //到达阈值了,转换为红黑树
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            //已经存在了这个key值,则直接覆盖旧值,并返回原来的值
            if (e != null) { // existing mapping for key

                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                //这里是空实现
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        //到达了阈值进行扩容
        if (++size > threshold)
            resize();
        //这里也是空实现
        afterNodeInsertion(evict);
        return null;
    }

将链表树化

首先判断if (binCount >= TREEIFY_THRESHOLD - 1,看已有的节点是否达到了树化的临界值,到达临界值后调用final void treeifyBin(Node<K,V>[] tab, int hash)进行树化

 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;
            //这个循环做的事就是把链表转换为树结构
            do {
                //这里主要是把Node转化为TreeNode,就是返回了一个TreeNode对象
                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)
                //让目前的树变为红黑树
                hd.treeify(tab);
        }
    }

扩容

final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            //如果大于了最大容量,就不再扩容
            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;
        @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
            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;
                    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;
                        }
                    }
                }
            }
        }
        return newTab;
    }

remove方法

关注点

• 构造方法
• put方法
• remove方法
• get()方法
• 几个内部类的作用
• 扩容机制
• 与红黑树的转换