为什么HashMap会遍历linkedList中的所有节点而不是resize()中的head？

Question

The code in hashmap of jdk8 when resize is

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;
        }
        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;
    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;
}

and this part of code

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;
                    }
                }

seems only traverse each bin of old table and finally assign the head to new table. Since these assignment e = oldTab[j] and loHead = e and newTab[j] = loHead already map the old bins to new bins in map, what dose this while loop actually affect? I mean the LinkedList shoule be already linked in old table, however this while loop seems want to linked it once again. Thanks for any comment!

Answer 1

The HashMap maps hash values to bins by using a modulo table size operation (by calculating hash & (table.length-1) , see for example https://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/HashMap.java#l568 and the following line).

That means that for the table of size 16 the following hash values map to the same bin (2):

• 2
• 18
• 34
• 50

Now if the table size is doubled (now 32), only these hash values map to the old bin 2:

• 2
• 34

but the following hash values now map to another bin (18):

• 18
• 50

That means that during the resize() operation the HashMap has to split the linked list of entries into two:

• those with a hash value that still maps to the old bin (which are added to a linked list at loHead )
• those with a hash value that maps to a new bin (which are added to a linked list at hiHead )

This splitting is done in the do {} while (); loop that you show.

• for the hash values to remain at the same bin, e.hash & oldCap returns 0 ( (2 & 16) == 0 and (34 & 16) == 0 ).
• for the hash values to are moved to the new bin, e.hash & oldCap returns 16 ( (18 & 16) == 16 and (50 & 16) == 16 )

After splitting the list, the splitted lists are added to the new table:

            if (loTail != null) {
                loTail.next = null;
                newTab[j] = loHead;
            }
            if (hiTail != null) {
                hiTail.next = null;
                newTab[j + oldCap] = hiHead;
            }

oldCap is the old table size (16 in my example).