Java Collection 源码分析

HashMap

成员变量

// 默认大小
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

// 最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;

// 加载因子（可通过构造函数设置）
static final float DEFAULT_LOAD_FACTOR = 0.75f;

// 树化的链表长度阈值（当链表数量大于该值变为红黑树）
static final int TREEIFY_THRESHOLD = 8;

// 链表化的树的节点数量阈值（当红黑树节点数量小于该值退为链表）
static final int UNTREEIFY_THRESHOLD = 6;

// 树化的桶的最低数量（只有Node桶的数量大于该值才会树化，否则优先扩容）
static final int MIN_TREEIFY_CAPACITY = 64;

// 桶数组（指向的是该桶的第一个元素，链表或红黑树）
transient Node<K,V>[] table;

// entrySet()的结果缓存
transient Set<Map.Entry<K,V>> entrySet;

// 实际元素数量
transient int size;

// 修改次数 用于fail-fast
transient int modCount;

// (capacity * load factor)
int threshold;

// 负载因子
final float loadFactor;

成员子结构

// 基本链表元素（也是桶数组Table的类型）
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;

        return o instanceof Map.Entry<?, ?> e
                && Objects.equals(key, e.getKey())
                && Objects.equals(value, e.getValue());
    }
}

// 基本红黑树节点元素
static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
    TreeNode<K,V> parent;  // red-black tree links
    TreeNode<K,V> left;
    TreeNode<K,V> right;
    TreeNode<K,V> prev;    // needed to unlink next upon deletion
    boolean red;
    TreeNode(int hash, K key, V val, Node<K,V> next) {
        super(hash, key, val, next);
    }
    // .......
}

其中TreeNode继承LinkedHashMap.Entry继承Node，所以TreeNode继承Node，而table数组是Node类型。所以桶数组（Table）即可以是Node，也可以是TreeNode。

构造函数

// 创建一个空的HashMap，初始化容量和负载因子
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);
}

// 创建一个空的HashMap，初始化容量以及默认的负载因子0.75
public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

// 创建一个空的HashMap，默认容量16,默认负载因子0.75
public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

// 接受Map参数，将Map元素添加到新的HashMap中
public HashMap(Map<? extends K, ? extends V> m) {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    putMapEntries(m, false);
}

HashMap中的table数组只有在第一次插入元素时才会创建table数组。

put方法

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

// 添加元素
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    // 如果table未初始化或长度为0，则调用resize()初始化table并确定容量
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    // 如果该位置没有冲突（桶为空），直接新建一个节点放入table[i]
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    // 如果该位置已有节点（即发生哈希冲突），处理冲突
    else {
        Node<K,V> e; K k;
        // 如果第一个节点的hash和key都相等，e = p，用于后续直接覆盖 value
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        // 如果是红黑树节点，调用putTreeVal插入红黑树
        // 如果插入成功返回null，如果有相同hash和key的元素返回已存在的节点e
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        // 如果p是链表节点，向链表尾部添加新元素或返回hash和key相同的元素e
        else {
            for (int binCount = 0; ; ++binCount) {
                // 到达链表尾部
                if ((e = p.next) == null) {
                    // 将新的节点插入到链表尾部
                    p.next = newNode(hash, key, value, null);
                    // 如果链表长度达到阈值（默认8），尝试将链表转为红黑树
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                // 如果找到hash和key都相同的元素直接返回e
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        // 在桶中找到了key值、hash值与插入元素相等的结点
        // 替换为新value值并返回旧值
        if (e != null) { 
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    // 修改次数增加
    ++modCount;
    // size + 1 如果超出容量需要扩容
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

get方法

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


final Node<K,V> getNode(Object key) {
    Node<K,V>[] tab; Node<K,V> first, e; int n, hash; K k;
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & (hash = hash(key))]) != null) {
        // 根据hash值 判断tab[(n - 1) & (hash = hash(key))]中第一个元素是否相同
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        // 从第二个元素开始判断
        if ((e = first.next) != null) {
            // 红黑树中寻找key
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            // 链表中寻找key
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;
}

扩容方法 resize()

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) {
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
    // 若满足条件，将容量翻倍 int newCap, newThr = 0;
    if (oldCap > 0) {
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            newThr = oldThr << 1; // double threshold
    }
    // 若创建时指定了初始容量或负载因子，会存在threshold并将其赋值给newCap
    else if (oldThr > 0)
        newCap = oldThr;
    // 若创建时没有参数，则使用默认值
    else {    
        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;
    // 创建桶数组table
    @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;
}