Understanding Maps and Custom Collections in Java

1. Chapter 14 Map Collections andCustom Collection Classes

2. Maps • An object that maps a key to a value • Key maps to one value • A map cannot have duplicate keys • Often used to associate a short key with a longer value • Example: Dictionary • Example: Employee database using employee ID number

3. Concrete Map Implementations • TreeMap orders the keys. • HashMap stores the keys in a hash table. • HashMap class is generally preferred for its efficiency (speed) unless sorted keys are needed.

4. Retrieving Values by Key • Use the keys collection to retrieve the keys in a map. • Iterator keyIterator = myMap.keySet().iterator(); • Keys collection holds “key” objects. • Must cast key values to the appropriate type • Integer key = (Integer)keyIterator.next(); • Retrieves the next key from keyIterator (collection of keys) then casts it to an Integer and assigns it to the variable key

5. Custom Collection Classes • Custom collections allow you to define the data storage method. • Good way to understand how other collection classes work • Think through issues involved in writing good generic classes

6. Linked List • A linked list is a type of collection, similar to a train. • Each object or node is linked to the next object in the collection. • Singly linked list • Each node knows the node following it • Can only be iterated in one direction • Doubly linked list • Each node knows the node preceding it and the node following it • Can be iterated in both directions

7. Linked List Hierarchy • Head node points to next node in list • Internal node “carries” an object (the value being stored) and points to next node in list • Tail node signals end of the list