幾個重要的變量：

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;//初始大小

static final int MAXIMUM_CAPACITY = 1 << 30;//map中最多可以容納的數(shù)量

static final float DEFAULT_LOAD_FACTOR = 0.75f; //與當(dāng)前容量大小決定擴(kuò)容極限值各吨，如默認(rèn)的容量（16），默認(rèn)的擴(kuò)容極限值（16 * 0.75 = 12）啃洋；如果size達(dá)到12時，進(jìn)行下次擴(kuò)容篡悟，容量和極限值均擴(kuò)容成原來的兩倍致板，32,24；

構(gòu)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);
    }

put(K key, V value)

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

//對key進(jìn)行hash
static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}


transient Node<K,V>[] table;    //Node是單向鏈表的節(jié)點(diǎn)坊夫，When allocated, length is always a power of two.
// @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;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;    //resize(): Initializes or doubles table size.此處時初始化
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);   //如果目標(biāo)index目前的元素是空的則直接賦值就行
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;  //key已存在map中砖第，更新值
            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);   //以鏈表的形式將同一index的數(shù)據(jù)組織起來
                        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;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }


final Node<K, V>[] resize() {
   ...
     //初始大小為static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
     //計(jì)算得出新數(shù)組的大小撤卢，成倍增長 newThr = oldThr << 1
     //如果達(dá)到MAXIMUM_CAPACITY，其他不變梧兼，threshold = Integer.MAX_VALUE放吩，return原數(shù)組
     Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab; //更新成員變量的值
   ...
    //將舊數(shù)組的數(shù)據(jù)按序遷移至新數(shù)組
     
}

put方法中含有的信息較多，可以看到HashMap內(nèi)部是數(shù)組+鏈表的形式來存儲數(shù)據(jù)的羽杰，每個數(shù)據(jù)被組裝成鏈表的節(jié)點(diǎn)渡紫。

get(Object key)

public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

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) {    //取到數(shù)組中對應(yīng)index的鏈表的首節(jié)點(diǎn)
            if (first.hash == hash && // 從首節(jié)點(diǎn)開始遍歷取出目標(biāo)元素
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)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;
    }

如果判斷元素是否是目標(biāo)元素：node.hash == hash && node.key == key || (key != null && key.equals(k))

remove(Object key)

public boolean remove(Object key, Object value) {
        return removeNode(hash(key), key, value, true, true) != null;
    }
//return the node, or null if none
final Node<K,V> removeNode(int hash, Object key, Object value,
                               boolean matchValue, boolean movable) {
        Node<K,V>[] tab; Node<K,V> p; int n, index;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (p = tab[index = (n - 1) & hash]) != null) {    //通過hash快速找到對應(yīng)的index
            Node<K,V> node = null, e; K k; V v;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            else if ((e = p.next) != null) {
                if (p instanceof TreeNode)
                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
                else {
                    do {
                        if (e.hash == hash &&
                            ((k = e.key) == key ||
                             (key != null && key.equals(k)))) {
                            node = e;
                            break;
                        }
                        p = e;
                    } while ((e = e.next) != null);
                }
            }   //遍歷index中的鏈表元素，找到目標(biāo)節(jié)點(diǎn)
            if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount;
                --size;
                afterNodeRemoval(node);
                return node;
            }//將目標(biāo)節(jié)點(diǎn)從鏈表中移除
        }
        return null;
    }

replace(K key, V value)

public V replace(K key, V value) {
        Node<K,V> e;
        if ((e = getNode(hash(key), key)) != null) {
            V oldValue = e.value;
            e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
        return null;
    }

//well考赛，主要分析下面這個方法
public boolean replace(K key, V oldValue, V newValue) {
        Node<K,V> e; V v;
            //getNode()在get()方法中分析過了
        if ((e = getNode(hash(key), key)) != null &&
            ((v = e.value) == oldValue || (v != null && v.equals(oldValue)))) {
            e.value = newValue; //替換目標(biāo)節(jié)點(diǎn)的值
            afterNodeAccess(e);
            return true;
        }
        return false;
    }

clear()

public void clear() {
        Node<K,V>[] tab;
        modCount++;
        if ((tab = table) != null && size > 0) {
            size = 0;
            for (int i = 0; i < tab.length; ++i)
                tab[i] = null;  //清空數(shù)組即可
        }
    }

Q：為什么擴(kuò)容因子是0.75惕澎？