A map associates keys with values. You can add an associtation:

map.put("Fred", 3);

You can look up a value:

int n = (int) map.get("Fred"); // Cast is needed because we store objects

You can change a value:

map.put("Fred", 6);

An efficient implementation of a map is a hash table. But in this exercise, we'll do an inefficient implementation: an array list of key/value entries. Use a static inner class for the entries.