iterator: 
    itr.hasNext()  -- are there any values left? 
    itr.next()     -- returns the next value and moves itr forward
    itr.remove()   -- removes the value most recently returned by next

-------------------------------------
set operations:
     s.add(value);    -- adds a value to the set (might already be there)
     s.remove(value); -- remove the given value from the set
     s.contains()     -- whether set contains the value
     s.size()         -- number of values in the set

-------------------------------------
map operations:
    m.put(key, value)   -- associates the given key with the given value
    m.get(key)          -- returns the value for the given key
    m.containsKey(key)  -- whether keySet contains the given key
    m.keySet()          -- returns a Set of the keys in the map
    m.size()            -- number of key-value pairs in the map