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jdk1.8 HashMap源码解读

程序员文章站 2022-06-04 18:54:13
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HashMap继承关系

jdk1.8 HashMap源码解读

HashMap几个关键常量

 //默认初始化数量
 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
 static final float DEFAULT_LOAD_FACTOR = 0.75f;
 //树节节点阈值
 static final int TREEIFY_THRESHOLD = 8;
 //tree调整大小最大数
 static final int UNTREEIFY_THRESHOLD = 6;
 //树节点容量
 static final int MIN_TREEIFY_CAPACITY = 64;

HashMap 构造方法

1. 无参构造
public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }
  当不指定初始化容量时,可以看出初始化为 16x0.75
2. 有参构造
2.1 指定初始容量
public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
 }
 可以看出,我们可以自定义初始容量,默认加载因子为0.75
2.2 指定初始容量、加载因子
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);
    }
    这里其实也就是做一些判断,进行一些异常处理

HashMap.put

下面我们看一下核心的put方法,看懂了put();其他的putAll();也是一样的

1. put()解析
public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
}
这里很简单,将我传入的K,V进行存储;这里包含了两个方法:putVal()、hash();
我们先看hash(),在看putVal()
2. hash()解析
static final int hash(Object key) {
   int h;
   return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
 }

这里不难看出,HashMap是可以存储K为null的数据,这里进行了位移运算,计算出每一个hash值,这里就看出了hashCode()方法的重要性了,关于位移运算有兴趣的可以研究,知道hash返回int即可;
但是这里引申出一个问题,如果hash冲突怎么办?是直接替换value吗?
答案是不会替换,这是面试加分项,哈哈,这与HashMap的存储模式有关系,下面会说到!

3. putVal()解析

这是核心部分

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //判断tab是否为null,或者长度等于0,如果是则调用resize()方法进行初始化
        //而resize()内有调用了上面提到的核心常量
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //这里是计算hash值,如果为空,就新增
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            //对比p的hash与k的hash是否相同,如果相同就赋值e=p,代表p是我们找到的节点
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //判断p是否是TreeNode类型,如果是,则走putTreeVal()方法,红黑树查找
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {//不是则进行普通的查找
                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
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            //判断hash是否冲突,冲突则进行追加,而非覆盖
            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) //判断是否需要扩容
            resize(); 
        afterNodeInsertion(evict);
        return null;
    }

这里用到了Node<K,V> 那他是什么呢?这里不贴源码了,其实就是一个树
Jdk1.8后HashMap的结果就变成里数据+链表+树的结构,这里就回答了上面的问题,hash冲突时,不会直接替换,而是在链表内在加一个挂载点进行处理存储

4.resize()解析
final Node<K,V>[] resize() {
		//赋值
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
        //如果oldCap大于最大容量值
            if (oldCap >= MAXIMUM_CAPACITY) {
            	//则将threshold 赋值为Integer最大值
                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;
        //下边基本就是赋值,清空Node,对比hash值等操作
        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;
    }
HashMap讲解结束,如有不足之处,还请多多包涵
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jdk1.8 HashMap源码解读