Java HashMap internals

    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
    static final int MAXIMUM_CAPACITY = 1 << 30;
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    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);
    }
        
    static final int tableSizeFor(int cap) {
        int n = cap - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }
    

   /**
     * Computes key.hashCode() and spreads (XORs) higher bits of hash
     * to lower.  Because the table uses power-of-two masking, sets of
     * hashes that vary only in bits above the current mask will
     * always collide. (Among known examples are sets of Float keys
     * holding consecutive whole numbers in small tables.)  So we
     * apply a transform that spreads the impact of higher bits
     * downward. There is a tradeoff between speed, utility, and
     * quality of bit-spreading. Because many common sets of hashes
     * are already reasonably distributed (so don't benefit from
     * spreading), and because we use trees to handle large sets of
     * collisions in bins, we just XOR some shifted bits in the
     * cheapest possible way to reduce systematic lossage, as well as
     * to incorporate impact of the highest bits that would otherwise
     * never be used in index calculations because of table bounds.
     */
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

binary values of 2.0f, 18.0f and 215.0f all have same lower bits.
 
System.out.println(Long.toBinaryString(Float.floatToRawIntBits(2.0f)));
System.out.println(Long.toBinaryString(Float.floatToRawIntBits(18.0f)));
System.out.println(Long.toBinaryString(Float.floatToRawIntBits(215.0f)));

1000000000000000000000000000000
1000001100100000000000000000000
1000011010101110000000000000000

    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;
        
        // 1. Create table here by calling resize()
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
            
        // 2. Find table index based on hash, if that table entry is empty, create new node and add
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
            
        // 3. If table entry exists, then need to append (either to linked-list or the tree)
        else {
            Node<K,V> e; K k;
            
            // 4. If first element in the table is same as new element to be added, get the element in e.
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k)))) 
                // 5. Above: Performance: Comparing int hash which is faster, if same then only call equals
                e = p;
                
            // 6. If this list is already converted to tree, add to element to the tree
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
             
            // 7. Else add to the linked list
            else {
                for (int binCount = 0; ; ++binCount) {
                
                    // 8. Traverse the list until the end (one element at a time)
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        
                        // 9. If elements in linked list are more than threshold, convert to tree
                        if (binCount >= TREEIFY_THRESHOLD - 1) 
                            treeifyBin(tab, hash);
                        break;
                    }
                    
                    // 10. If while traversing, any element equals and element-to-add then, get element in e
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            
            // 11. If element already exists
            if (e != null) { 
                V oldValue = e.value;
                
                // 12. Update with new value and return old value.
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        
        // 13. Increase modificationCount and Size
        ++modCount;
        
        // 14. If after adding element, threshold is reached, increase table size. 
        if (++size > threshold)
            resize();
            
        // 15. Callback used for linkedhashmap not this regular hashmap
        afterNodeInsertion(evict);
        
        return null;
    }

    final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        
        // 1. If table size is less than MIN_TREEIFY_CAPACITY(64), then 
        // instead of creating tree, resize the table. 
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
            resize();
            
        // 2. Convert linked list to tree
        else if ((e = tab[index = (n - 1) & hash]) != null) {
            TreeNode<K,V> hd = null, tl = null;
            do {
            
                // 3. create TreeNode for each element, and link using just prev/next 
                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);
            
            // 4. Convert list of TreeNodes with prev/next to tree
            if ((tab[index] = hd) != null)
                hd.treeify(tab);
        }
    }

    // 5. Method from within TreeNode class.
    final void treeify(Node<K,V>[] tab) {
        TreeNode<K,V> root = null;
        for (TreeNode<K,V> x = this, next; x != null; x = next) {
            next = (TreeNode<K,V>)x.next;
            x.left = x.right = null;
            
            // 6. For root i.e. first element
            if (root == null) {
                x.parent = null;
                x.red = false;
                root = x;
            }
            
            // 7. For all other TreeNodes
            else {
                K k = x.key;
                int h = x.hash;
                Class<?> kc = null;
                for (TreeNode<K,V> p = root;;) {
                    int dir, ph;
                    K pk = p.key;
                    
                    // 8. Sort to left or right, by comparing hash values
                    // 9. Imp: All elements of this tree have same (hash % table-size) values, but their 
                    // actual hashes are different
                    if ((ph = p.hash) > h)
                        dir = -1;
                    else if (ph < h)
                        dir = 1;
                        
                    // 10. If by chance 2 elements have exact same hash, then break tie using System.identityHashCode
                    else if ((kc == null &&
                              (kc = comparableClassFor(k)) == null) ||
                             (dir = compareComparables(kc, k, pk)) == 0)
                        dir = tieBreakOrder(k, pk);

                    // 11. Traverse TreeNode until appropriate (left/right) null child is found
                    TreeNode<K,V> xp = p;
                    if ((p = (dir <= 0) ? p.left : p.right) == null) {
                    
                        // 12. Attach TreeNode as appropriate (left/right) child
                        x.parent = xp;
                        if (dir <= 0)
                            xp.left = x;
                        else
                            xp.right = x;
                        
                        // 13. Tree can become unbalanced after addition of TreeNode, rebalance
                        // The root can change during balancing, re-assign to root
                        root = balanceInsertion(root, x);
                        break;
                    }
                }
            }
        }
        
        // 14. If during balancing root node changed, then table[hash % size] != root, 
        // fix this by change table's index to root
        moveRootToFront(tab, root);
    }

    // 15. Called when number of elements in tree are less than threshold
    final Node<K,V> untreeify(HashMap<K,V> map) {
        Node<K,V> hd = null, tl = null;
        for (Node<K,V> q = this; q != null; q = q.next) {
            Node<K,V> p = map.replacementNode(q, null);
            if (tl == null)
                hd = p;
            else
                tl.next = p;
            tl = p;
        }
        return hd;
    }
    

    
    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }
    
    final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        
        // 1. Validate table not null, and hash table entry is not null
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            
            // 2. Check first element, for performance,
            // since in most cases, where hashcodes are unique, there will be only 1 element
            if (first.hash == hash && 
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            
            // 3. If there are more elements, traverse tree or linked-list accordingly
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }  
   
    final TreeNode<K,V> getTreeNode(int h, Object k) {
        return ((parent != null) ? root() : this).find(h, k, null);
    }
    
    // Method within TreeNode class
    final TreeNode<K,V> find(int h, Object k, Class<?> kc) {
        TreeNode<K,V> p = this;
        do {
            int ph, dir; K pk;
            TreeNode<K,V> pl = p.left, pr = p.right, q;
            
            // 4. If node's hash greater than h, store left child tree in p
            if ((ph = p.hash) > h)
                p = pl;
                
            // 5. If node's hash less than h, store right child tree in p
            else if (ph < h)
                p = pr;
                
            // 6. If node's hash and key match, then return the node
            else if ((pk = p.key) == k || (k != null && k.equals(pk)))
                return p;
                
            // 7. If left child tree is null, then store right child tree in p
            else if (pl == null)
                p = pr;
                
            // 8. If left child tree is null, then store right child tree in p
            else if (pr == null)
                p = pl;
                
            // 9. If keys are comparable, compare and decide left or right child tree
            else if ((kc != null ||
                      (kc = comparableClassFor(k)) != null) &&
                     (dir = compareComparables(kc, k, pk)) != 0)
                p = (dir < 0) ? pl : pr;
                
            // 10. Search recursively in right tree first.
            else if ((q = pr.find(h, k, kc)) != null)
                return q;
                
            // 11. Set p as left child tree, and let while loop recursively search with p
            else
                p = pl;
        } while (p != null);
        return null;
    }
    

    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) {
            // 1. Validate less than MAX
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            
            // 2. Double capacity and threshold
            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"})
        // 3. Create table 
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
        
            // 4. Loop through old table to reassign all elements
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    
                    // 5. Just assign for single element
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                        
                    // 6. Split the tree in 2 if a TreeNode
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                        
                    // 7. Split the linked list
                    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;
                            
                            // 8. Take advantage of factor of 2 size of table
                            // Either new index of element will be the same, or oldTableSize + oldIndex
                            // Assign to appropriate list
                            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);
                        
                        // 9. Attach list to appropriate table indexes
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

   // 1. Extra references to keep track of insertion order. 
   static class Entry<K,V> extends HashMap.Node<K,V> {
        Entry<K,V> before, after;
        Entry(int hash, K key, V value, Node<K,V> next) {
            super(hash, key, value, next);
        }
    }
    
    // 2. Adjust doubly linked list once a node is removed
    void afterNodeRemoval(Node<K,V> e) { // unlink
        LinkedHashMap.Entry<K,V> p =
            (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
        p.before = p.after = null;
        if (b == null)
            head = a;
        else
            b.after = a;
        if (a == null)
            tail = b;
        else
            a.before = b;
    }
    
    // 2. Adjust doubly linked list once a node is added
    void afterNodeInsertion(boolean evict) { // possibly remove eldest
        LinkedHashMap.Entry<K,V> first;
        if (evict && (first = head) != null && removeEldestEntry(first)) {
            K key = first.key;
            removeNode(hash(key), key, null, false, true);
        }
    }
    
    // 3. If a node is replaced, move it to the end (its treated as newly added node)
    void afterNodeAccess(Node<K,V> e) { 
        LinkedHashMap.Entry<K,V> last;
        if (accessOrder && (last = tail) != e) {
            LinkedHashMap.Entry<K,V> p =
                (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
            p.after = null;
            if (b == null)
                head = a;
            else
                b.after = a;
            if (a != null)
                a.before = b;
            else
                last = b;
            if (last == null)
                head = p;
            else {
                p.before = last;
                last.after = p;
            }
            tail = p;
            ++modCount;
        }
    }