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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

巴特西

HashMap源码阅读(小白的java进阶)