Understanding how Java HashMap works

Question

I know following thing about Java HashMap put():

1. Calling put() starts by inserting key-values into HashMap.
2. Initially we have linked list per bin, but once we have more than `TREEFY_THRESHOLD entries in a bin, bin contents are rearranged from linked list into balanced trees.

Similarly, get() should work as follows:

1. If the bin in which the key hashes to is in the form of linked list, traverse it to check if the input key exists in the list or not. If it does then return its value.
2. If the bin is in the form balanced tree, search the key in the tree and return its value

This is very high level abstract understanding after reading some articles, but I am unable to see how exactly this is happening in code. I tried to google it a bit and found these good articles: 1, 2, 3. But they target HashMap from java 7 (which does not involve balanced tree logic) and not java 8 (which involves balanced tree logic). Actually I am not struggling with balanced tree logic, but with the rest. Java 7 implementation was quite easy, but not the java 8 implementation. The whole code is quite involved and difficult to understand. It will be great if someone explain important methods of HashMap. If not, below are some specific doubts:

1. The Java 8 HashMap.get() ^source:

   1.     public V get(Object key) {
   2.         Node<K,V> e;
   3.         return (e = getNode(hash(key), key)) == null ? null : e.value;
   4.     }
   5. 
   6.     /**
   7.      * Implements Map.get and related methods
   8.      *
   9.      * @param hash hash for key
   10.      * @param key the key
   11.      * @return the node, or null if none
   12.      */
   13.     final Node<K,V> getNode(int hash, Object key) {
   14.         Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
   15.         if ((tab = table) != null && (n = tab.length) > 0 &&
   16.             (first = tab[(n - 1) & hash]) != null) {
   17.             if (first.hash == hash && // always check first node
   18.                 ((k = first.key) == key || (key != null && key.equals(k))))
   19.                 return first;
   20.             if ((e = first.next) != null) {
   21.                 if (first instanceof TreeNode)
   22.                     return ((TreeNode<K,V>)first).getTreeNode(hash, key);
   23.                 do {
   24.                     if (e.hash == hash &&
   25.                         ((k = e.key) == key || (key != null && key.equals(k))))
   26.                         return e;
   27.                 } while ((e = e.next) != null);
   28.             }
   29.         }
   30.         return null;
   31.     }
   

   (a) Why to always check first node explicitly on line 17?
   (b) What does tab[(n - 1) & hash] do on line 16?
   (c) May I will need to understance hash()^source also:

   static final int hash(Object key) {
      int h;
      return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
   }
   

2. Java 8 HashMap.put() ^source:

   1.     public V put(K key, V value) {
   2.         return putVal(hash(key), key, value, false, true);
   3.     }
   4. 
   5.     final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
   6.                    boolean evict) {
   7.         Node<K,V>[] tab; Node<K,V> p; int n, i;
   8.         if ((tab = table) == null || (n = tab.length) == 0)
   9.             n = (tab = resize()).length;
   10.         if ((p = tab[i = (n - 1) & hash]) == null)
   11.             tab[i] = newNode(hash, key, value, null);
   12.         else {
   13.             Node<K,V> e; K k;
   14.             if (p.hash == h ash &&
   15.                 ((k = p.key) == key || (key != null && key.equals(k))))
   16.                 e = p;
   17.             else if (p instanceof TreeNode)
   18.                 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
   19.             else {
   20.                 for (int binCount = 0; ; ++binCount) {
   21.                     if ((e = p.next) == null) {
   22.                         p.next = newNode(hash, key, value, null);
   23.                         if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
   24.                             treeifyBin(tab, hash);
   25.                         break;
   26.                     }
   27.                     if (e.hash == hash &&
   28.                         ((k = e.key) == key || (key != null && key.equals(k))))
   29.                         break;
   30.                     p = e;
   31.                 }
   32.             }
   33.             if (e != null) { // existing mapping for key
   34.                 V oldValue = e.value;
   35.                 if (!onlyIfAbsent || oldValue == null)
   36.                     e.value = value;
   37.                 afterNodeAccess(e);
   38.                 return oldValue;
   39.             }
   40.         }
   41.         ++modCount;
   42.         if (++size > threshold)
   43.             resize();
   44.         afterNodeInsertion(evict);
   45.         return null;
   46.     }
   

   (a) I understand putTreeVal() in line 18 inserts key-value in balanced tree. But I dont find logic to put key-value in the linked list as in line 27 of getNode(), which involves traversing till end of the linked list.
   (b) What does if on lines 10 and 14 do?
   (c) I also dont get full logic completely clear. Can you provide comments for each important line. In fact I want to know if there is any documentation / source code repo having comment for on each important line explaining what it is trying to understand, because there are many tricky complex methods in this class like, resize(), tableSizeFor(int)