Collections

1
Collections Definite Loops

• CSE 115
• Spring 2006
• April 10, 12, 14 2006

2
Discussion of PacMan

• Make note of the requirements listed in the lab
  description.
• Attend recitations for additional advice and
  assistance.
• In class, we will build a game (Tic Tac Toe).

3
Collections

• Storage for many objects.
• Two main types
• Bags
• Associations
• We will discuss use of collections, you will see
  how to write your own collection classes in CSE
  116 250.

4
Java Collections Framework

• Java provides implementations for a number of
  standard collections classes in the java.util
  package.
• The root interface of the collections hierarchy
  is Collection.

5
The CollectionltEgt interface

• Note the ltEgt after the word collection.
• The ltEgt indicates that this class can use a
  generic type (parameterized type).
• When you create an instance of a class with a
  generic type, you specify in the ltgt the actual
  type for the generic.

6
Using Generic Types

• For collections, what you are specifying with the
  generic type is the type of objects you will be
  storing in a collection.
• Eg) I want a bag of cats.
• When you do this, Java ensures that only objects
  of the type specified go in and you can be
  assured that only objects of that type come out.

7
A usable bag

• java.util.ArrayListltEgt
• Note the operations that you can perform on this
  collection.
• The most important will be creating an instance
  of the collection, inserting elements, removing
  elements, and finding if elements are in the
  collection.

8
Another important collection operation

• Iterating over all the elements of a collection
  and performing some operation with/on each
  element of the collection.
• This process can be accomplished by using a
  special object provided by Java called an
  iterator.
• In Java 5, the use of the iterator has been
  replaced with the for-each loop.

9
For-each loop

• Allows access to each element of a collection.
• Syntax
• for(TypeOfElementInCollection giveNameToElement
  NameOfCollection)
• //write code for what to do with each
  //element.

10
ArrayLists and PacMan

• Note that in the CSE115.Pacman.BoardPositions
  class there are ArrayLists for each of the type
  of cells on the PacMan board. Further
  explanation will be provided in recitation.

11
A useful association

• java.util.HashMapltK, Vgt
• Associates a key with a value.
• Both the key and value are objects.
• User specifies what type of key and value is used
  when HashMap is created.

12
Useful operations with HashMaps

• put (put a key/value pair in the HashMap)
• Remove
• Look up a value using its key
• Iterating using the for-each loop

13
Using for-each with HashMaps (note .values())

• java.util.HashMapltPosition, Cellgt _board new
  java.util.HashMap ltPosition, Cellgt()
• //magic happens to put things into board.
• for(Cell c _board.values()
• c.draw()

14
The keyword for

• The for-each is a specialized loop designed to
  work with collections.
• for is a keyword in Java that tells us there is a
  loop.
• You can create a regular for-loop for use in
  your programs.

15
Loops (Iteration/Repetition)

• The ability to do a task repeatedly.
• The functionality of repetition is most often
  implemented in programming languages using loops.

16
Definite Loop

• The for-loop is characterized as a definite
  loop and is normally used when you know how many
  times you want a specific task to be performed.
  It is sometimes referred to as a counting loop.

17
Entry Test Loop

• A for-loop is also characterized as an
  entry-test loop. That is, a condition about
  whether the loop should continue is tested before
  actually doing the work of the loop.

18
Syntax of for-loop

• for (initialization condition increment)
• //loop body
• Usually, the initialization is of a loop counter
  variable that is checked against a bounds in the
  condition and is incremented in the increment
  step.