public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
        if (key == null || mappingFunction == null)
            throw new NullPointerException();
        int h = hash(key);
        V val = null;
        int binCount = 0;
        for (Node<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh; K fk; V fv;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            else if ((f = tabAt(tab, i = (n - 1) & h)) == null) {
                Node<K,V> r = new ReservationNode<K,V>(hashingStrategy);
                synchronized (r) {
                    if (casTabAt(tab, i, null, r)) {
                        binCount = 1;
                        Node<K,V> node = null;
                        try {
                            if ((val = mappingFunction.apply(key)) != null)
                                node = new Node<K,V>(h, key, val, hashingStrategy);
                        } finally {
                            setTabAt(tab, i, node);
                        }
                    }
                }
                if (binCount != 0)
                    break;
            }
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else if (fh == h    // check first node without acquiring lock
                     && ((fk = f.key) == key || (fk != null && isEqual(key,fk)))
                     && (fv = f.val) != null)
                return fv;
            else {
                boolean added = false;
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            binCount = 1;
                            for (Node<K,V> e = f;; ++binCount) {
                                K ek;
                                if (e.hash == h &&
                                    ((ek = e.key) == key ||
                                     (ek != null && isEqual(key,ek)))) {
                                    val = e.val;
                                    break;
                                }
                                Node<K,V> pred = e;
                                if ((e = e.next) == null) {
                                    if ((val = mappingFunction.apply(key)) != null) {
                                        if (pred.next != null)
                                            throw new IllegalStateException("Recursive update");
                                        added = true;
                                        pred.next = new Node<K,V>(h, key, val, hashingStrategy);
                                    }
                                    break;
                                }
                            }
                        }
                        else if (f instanceof TreeBin) {
                            binCount = 2;
                            TreeBin<K,V> t = (TreeBin<K,V>)f;
                            TreeNode<K,V> r, p;
                            if ((r = t.root) != null &&
                                (p = r.findTreeNode(h, key, null)) != null)
                                val = p.val;
                            else if ((val = mappingFunction.apply(key)) != null) {
                                added = true;
                                t.putTreeVal(h, key, val);
                            }
                        }
                        else if (f instanceof ReservationNode)
                            throw new IllegalStateException("Recursive update");
                    }
                }
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (!added)
                        return val;
                    break;
                }
            }
        }
        if (val != null)
            addCount(1L, binCount);
        return val;
    }
public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)