Collections and Generics
Java collections
Collection classes

Collection interfaces

Interface	Description
Collection	Defines the basic methods available for all collections.
Set	Defines a type of collection in which no duplicate elements are allowed.
List	Defines a type of collection that maintains the order in which elements were added to the list.
Map	Defines a map, which holds one or more key-value pairs, each consisting of: (1) a key that uniquely identifies an entry and (2) a value that provides data for that key.

Common collection classes

Class	Description
ArrayList	Works much like an array, but can be easily expanded to accommodate new elements. Very efficient for accessing elements in random sequence, but inserting elements into the middle of the list can be inefficient.
LinkedList	Similar to an array list, but with more features. Less efficient than an array list for random access, but more efficient when inserting items into the middle of the list.
HashSet	A collection that stores a set of unique values based on a hash code. Duplicates are not allowed. Objects you add to a hash set must implement a method called hashCode to generate a hash code for the object, which is used to ensure uniqueness.
HashMap	Similar to a hash set, but is based on the Map interface rather than the Set interface. As a result, a hash map stores key-value pairs whose keys must be unique.
TreeMap	Stores key-value pairs in a special arrangement called a tree. Entries are automatically maintained in key sequence.