HashMap源码

1.默认的初始化长度是16. 是在哪赋的值。

2.扩容机制是2倍。

初始化

class HashMap{
    
    // HashMap底层存储数据的数组。
    Node<K,V>[] table;
    
    // 阈值
    int threshold;
    
    // 加载因子。
    final float loadFactor;
    
    
    public V put(K key, V value) {
        // 计算得到一个int值。 
        return putVal(hash(key), key, value, false, true);
    }
    
    
    //        
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
        
        //  准备了一堆变量
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        
        // table目前没有初始化。  tab=table   肯定是为null
        if ((tab = table) == null || (n = tab.length) == 0)
            // resize()
            n = (tab = resize()).length;
        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))))
                e = p;
            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);
                        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;
    }
    
    // 看到的初始化方法
    final Node<K,V>[] resize() {
        // oldTab = table = null
        Node<K,V>[] oldTab = table;
        
        // oldCap = 0
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        // oldThr = 0;
        int oldThr = threshold;
        // newCap  newThr = 0
        int newCap, newThr = 0;
        
        // oldCap = 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
            // if没进去； else-if 也没进去。  进入else
            // newCap = DEFAULT_INITIAL_CAPACITY = 16
            newCap = DEFAULT_INITIAL_CAPACITY;
            // newThr = (int) (0.75f * 16) = 12
            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 = 12
        threshold = newThr;
        // newTab = new Node[16];
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        // table是一个长度是16 的数组。
        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;
    }
}

扩容

class HashMap{
    
    // HashMap底层存储数据的数组。
    Node<K,V>[] table;
    
    // 阈值
    int threshold;
    
    // 加载因子。
    final float loadFactor;
    
    // table目前是一个长度为16的数组。
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
    
    
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        
        // table = new Node[16]
        // tab = table
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        
        
        // 对数组长度进行取余。   tab[i] == null?
        if ((p = tab[i = (n - 1) & hash]) == null)
            // 当前位置是空，我直接创建节点。 
            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))))
                e = p;//key相等，准备更新值
            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);
                        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;
                }
            }
            // e其实就是找到的相同的。 
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        
        // ++size 大于阈值。  进行扩容。
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }
    
    
    // 扩容方法
    final Node<K,V>[] resize() {
        
        // oldTab = table = new Node[16]
        Node<K,V>[] oldTab = table;
        // oldCap = 16
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        // oldThr = 12
        int oldThr = threshold;
        // newCap, newThr = 0;
        
        int newCap, newThr = 0;
        if (oldCap > 0) {
            // oldCap = 16  符合条件； 进入
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            // newCap = oldCap * 2 = 32
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                // newThr = oldThr * 2 = 24
                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 = 24
        threshold = newThr;
      
        //newTab =  new Node[newCap] = new Node[32]
        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;
    }

}