Download Interface

Computer Science 209
Software Development
Java Collections
Collections
• A collection is a container for 0 or more objects
• Organized in a specific manner (list, stack, queue, set,
map/dictionary, tree, graph)
• Operations include insert, remove, access, size, iteration,
etc.
• Arrays are too restrictive, more like an implementation
structure; collections are smarter objects
The java.util Package
• View documentation or download from
Oracle’s Web site
• Includes docs for all interfaces, classes, and
methods
http://docs.oracle.com/javase/7/docs/api/
Interfaces
• An interface specifies the behavior of a set of
implementing classes
• Really just a name and a set of method headers
• The behavior is abstract and conceptually the
same, regardless of how it is realized in an
implementing class
Example: Lists
• The List interface includes the methods add, remove,
get, and set, among many others
• The ArrayList and LinkedList classes implement
List, so the above methods can be run, in the same
manner, with either class of list
• You just study the interface for the logical behavior, then
choose an implementing class based on its performance
characteristics
Java Collection Interfaces
= extends
<<Interface>>
Iterable
<<Interface>>
Collection
<<Interface>>
Queue
<<Interface>>
List
<<Interface>>
Set
<<Interface>>
Map
<<Interface>>
SortedSet
<<Interface>>
SortedMap
Java Collection
Classes – Lists,
Stacks, Queues
<<Interface>>
Iterable
<<Interface>>
Collection
<<Interface>>
Queue
Abstract
Collection
<<Interface>>
List
= extends
AbstractList
= implements
AbstractSequentialList
LinkedList
ArrayList
Vector
Stack
<<Interface>>
Iterable
Java Collection
Classes - Sets
<<Interface>>
Collection
Abstract
Collection
<<Interface>>
Set
<<Interface>>
SortedSet
AbstractSet
= extends
= implements
HashSet
TreeSet
Java Collection Classes - Maps
<<Interface>>
Map
AbstractMap
= extends
= implements
<<Interface>>
SortedMap
TreeMap
HashMap
Declaration and Instantiation
List<String> names = new ArrayList<String>();
List<Student> students = new LinkedList<Student>();
// Now use the same methods with both lists
Always use an interface name for the type of a
collection variable
The type parameter (in red) restricts the type of
objects that can go into a collection
Select the methods to use from the interface
All Collections Are Iterable
List<String> names = new ArrayList<String>();
List<Student> students = new LinkedList<Student>();
// Now use the same methods with both lists
for (int i = 1; i <= 10; i++)
students.add(new Student("Name" + i, 3));
for (Student s : students)
System.out.println(s);
Syntax of Parameterized Collections
Variable declaration:
interface-name<element-type> variable-name;
List<String> names;
Set<Token> statementHandles;
Object instantiation:
variable-name = class-name<element-type>();
names = new ArrayList<String>();
statementHandles = new HashSet<Token>();
Collections of Numbers are Funky
List<Integer> evens = new ArrayList<Integer>();
for (int i = 1; i <= 10; i++)
evens.add(i * 2);
for (int i : evens)
System.out.println(i);
Need to use a wrapper class for the element type
when it’s a primitive type
Java wraps values before insertion and unwraps
them before access
Lists vs Queues
// List methods (among many others, including Collection)
public E get(int index)
public E remove(int index)
public E set(int index, E newElement)
<<Interface>>
Iterable
// Queue methods (as well as Collection)
public E peek()
public E remove()
public boolean add(E newElement)
<<Interface>>
Collection
<<Interface>>
Queue
<<Interface>>
List
LinkedList
Use Queue to Restrict Access
// All List and Collection methods apply
List<string> list = new LinkedList<String>();
// Only queue and Collection methods apply
Queue<String> queue = new LinkedList<String>();
<<Interface>>
Iterable
<<Interface>>
Collection
<<Interface>>
Queue
<<Interface>>
List
LinkedList
Multiple Type Parameters
Variable declaration:
interface-name<element-type1, …, element-type n> variable-name;
SortedMap<String, Integer> concordance;
The keys are all strings and the values are all integers
Object instantiation:
variable-name = class-name< element-type1, …, element-type n >();
concordance = new TreeMap<String, Integer>();
The Collection Interface
<<Interface>>
Iterable
<<Interface>>
Collection
<<Interface>>
Queue
<<Interface>>
List
A list, queue, set, or
sorted set can be used
wherever a collection is
specified
<<Interface>>
Set
<<Interface>>
SortedSet
Type Conversion
List<String> listOfNames = new ArrayList<String>();
// Add a bunch of names to the list in random order
// Now copy the names to a new sorted set (sorts them
// and removes duplicates)
SortedSet<String> setOfNames =
new SortedSet<String>(listOfNames);
Collection classes that implement Collection usually
include a constructor with a Collection parameter!
The java.util.Collections
Class
• Like the Arrays class, includes class methods for
processing collections that implement the List interface
(array lists, linked lists, stacks)
• Sorting, searching, find the maximum element, etc.
• Some methods assume compareto for collection
elements
Collections.sort(<a List>)
Collections.binarySearch(<a List>)