Map Collections and
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: Employee database using
employee ID number
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
Retrieving Values by Key
Use the keys collection to retrieve the keys
in a map.
Iterator keyIterator =
Keys collection holds “key” objects.
Must cast key values to the appropriate type
Integer key =
Retrieves the next key from keyIterator
(collection of keys) then casts it to an Integer
and assigns it to the variable key
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
A linked list is a type of collection, similar to
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
Can be iterated in both directions
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