// Returns true iff this map contains no key/value mappings.
      
          @Override
          public boolean isEmpty () {
              return size() == 0;
          }
      
          // Returns a set of the key/value pairs in this Map.
      
          public Set entrySet () {
              Iterator<K> it = keyIterator();
              List entries = new ArrayList();
              while (it.hasNext()) {
                  K key = it.next();
                  V value = getVal (key);
                  Entry<K,V> entry = new SimpleImmutableEntry<K,V> (key, value);
                  entries.add (entry);
              }
              return new ListSet<K,V> (entries);
          }
    

The isEmpty method calls size() to compute its result. That's fine if the size method is fast. If size is slow in some implementation, the isEmpty method can be overridden in a subclass that's part of that implementation.

The entrySet method creates an iterator by copying all keys into a List and calling that list's iterator() method. This is a fairly common idiom in Java.

All of the other methods listed in the Map<K,V> interface are defined within and inherited from AbstractMap<K,V>.