HashMap源码阅读（一）-Toy模板网

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HashMap继承抽象类AbstractMap，AbstractMap抽象类实现了Map接口

一、HashMap中的静态常量

//默认初始容量
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//最大长度
static final int MAXIMUM_CAPACITY = 1 << 30;
//负载因子，map中存储的数据在达到负载因子时需要进行扩容
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//哈希桶中存储的链表长度的阈值，当链表长度达到阈值时会转化为红黑树-->树化
static final int TREEIFY_THRESHOLD = 8;
//当哈希桶中存储的链表的长度小于该阈值时，如果发生了树化，则会将树砖换成链表-->反树化
static final int UNTREEIFY_THRESHOLD = 6;
//用于指示哈希表进行重新哈希操作时，何时会将链表转换为红黑树
static final int MIN_TREEIFY_CAPACITY = 64;

二、HashMap中的Node节点

Node节点是用于存储存储哈希表中的键值对的结构通过nent变量，将出现冲突的元素连成一个链表

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;//元素的hash值
    final K key;//元素的Key
    V value;//元素的值
    Node<K,V> next;//该元素所连接的下一个节点
	//构造方法
    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);
    }
	//替换Node里面的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());
    }
}

三、HashMap中的成员变量

//用于存储所有链表的头节点
transient Node<K,V>[] table;
//保存缓存
transient Set<Map.Entry<K,V>> entrySet;
//map的实际长度
transient int size;
transient int modCount;
//用于调整容量的笑一个容量值
int threshold;
//实际的负载因子
final float loadFactor;

四、HashMap的构造函数

public HashMap(int initialCapacity, float loadFactor) {
    //判断初始长度是否合法
    if (initialCapacity < 0)
    	throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
    //判断设置的初始长度是否大于hash表所设置的最大的长
    if (initialCapacity > MAXIMUM_CAPACITY)
    	initialCapacity = MAXIMUM_CAPACITY;
    //判断负载因子是否合法
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
    	throw new IllegalArgumentException("Illegal load factor: " +loadFactor);
    this.loadFactor = loadFactor;
    //初始长度需要是2^n形式
    this.threshold = tableSizeFor(initialCapacity);
}
public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
//构造方法中传入一个map创建对象
public HashMap(Map<? extends K, ? extends V> m) {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    //将参数中的map添加到当前的map中
    putMapEntries(m, false);
}

五、Map中的简单方法

1、将一个map里面的多有值添加到当前map中

final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
    int s = m.size();
    if (s > 0) {
        if (table == null) { // 初始化
            //获取数组的最小长度
            float ft = ((float)s / loadFactor) + 1.0F;
            //判断获取的最小长度是否大于map所支持的最大长度
            int t = ((ft < (float)MAXIMUM_CAPACITY) ?
                     (int)ft : MAXIMUM_CAPACITY);
            if (t > threshold)
                threshold = tableSizeFor(t);
        } else {
            //判断传入的map的长度是否大于实际的，并进行扩容
            while (s > threshold && table.length < MAXIMUM_CAPACITY)
                resize();
        }
		//遍历参数中的map并将map里面的键值对存储在当前的map中
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
            K key = e.getKey();
            V value = e.getValue();
            putVal(hash(key), key, value, false, evict);
        }
    }
}

2、查询map的长度和判空

//查询map长度
public int size() {
    return size;
}
//判断map是否为空
public boolean isEmpty() {
    return size == 0;
}

3、根据键进行查询

3.1根据键查找值

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

3.2根据键查找Node

final Node<K,V> getNode(Object key) {
    Node<K,V>[] tab; Node<K,V> first, e; int n, hash; K k;
    //判断table[(n - 1) & (hash = hash(key))]中是否为null,并对数据进行赋值
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & (hash = hash(key))]) != null) {
        //判断第一个结点
        if (first.hash == hash && 
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        if ((e = first.next) != null) {
            //判断该节点是否树化
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            //没有树化，通过遍历查找key
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;
}

3.3判断键是否存在

public boolean containsKey(Object key) {
    return getNode(key) != null;
}

4、添加

4.1添加

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}
/**
*	hash:key的hash值
*	key
*	value
*	onlyIfAbsent：为true是，出现一样的key不会覆盖value
*	evict：为true时，处于创建模式
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict) {
    //p为当前节点-->currentNode
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //判断table是否为空，若为空执行resize方法进行初始化
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    //判断(n - 1) & hash下标处是否为空，若为空则添加节点为头节点，直接创建
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))
            //判断key与头节点的key是否相同
            e = p;
        else if (p instanceof TreeNode)
            //判断链表是否树化,直接向树中添加节点
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                //判断是否是最后一个节点
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    //判断是否达到树化的阈值，对链表树化，或对数组进行扩容
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                //判断当前节点的key是否与key相同
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                //e = p.next,改行代码等价于p = p.next
                p = e;
            }
        }
        //e != null => map中存在key：将旧值改为新值
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    //判断是否达到扩容的阈值
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

4.2将一个map添加到当前map中

public void putAll(Map<? extends K, ? extends V> m) {
    putMapEntries(m, true);
}

5、扩容和初始化方法

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    //旧的table的长度
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    //threshole：下一个容量
    int oldThr = threshold;
    //newCal:新table的长度，newThr：新的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;
    @SuppressWarnings({"rawtypes","unchecked"})
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    //将就的值全部填入新的数组中
    if (oldTab != null) {
        for (int j = 0; j < oldCap; ++j) {
            Node<K,V> e;
            if ((e = oldTab[j]) != null) {
                oldTab[j] = null;//垃圾回收
                if (e.next == null)//只有一个节点
                    newTab[e.hash & (newCap - 1)] = e;
                else if (e instanceof TreeNode)//已树化，将树进行拆分
                    ((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;
}

6、链表长度过长，树化或扩容

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);
    }
}

7、删除

public V remove(Object key) {
    Node<K,V> e;
    return (e = removeNode(hash(key), key, null, false, true)) == null ?
        null : e.value;
}
//matchValue：当value相同时删除否则不删除
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;
    //判断是否为空（数组、key所对应的下标处）
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (p = tab[index = (n - 1) & hash]) != null) {
        Node<K,V> node = null, e; K k; V v;
        //判断头节点key
        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);
            }
        }
        //判断key对应的node是否存在
        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;
        }
    }
    return null;
}

8、清空链表

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;
    }
}

9、判断链表中是否存在某个值文章来源地址https://www.toymoban.com/news/detail-693716.html

public boolean containsValue(Object value) {
    Node<K,V>[] tab; V v;
    if ((tab = table) != null && size > 0) {
        for (Node<K,V> e : tab) {
            for (; e != null; e = e.next) {
                if ((v = e.value) == value ||
                    (value != null && value.equals(v)))
                    return true;
            }
        }
    }
    return false;
}