Java Collections:

1
Session 9

• Java Collections
• ArrayList
• HashMap

2
Remember last semester

• One of the last assignments you did was a
  SlideShow class.
• This class read from a file
• The number of pictures
• The filenames for those pictures
• The filename for a Sound file

3
Remember last semester

• One of the last assignments you did was a
  SlideShow class.
• This class read from a file
• The number of pictures WHY???
• The filenames for those pictures
• The filename for a Sound file

4
Using Arrays

• Arrays are fixed size collections of some
  predefined data type.
• Picture slides new Picture5
• slides0 new Picture(picture1.jpg)
• slides1 new Picture(picture2.jpg)
• System.out.println(size slides.length)
• Picture p slides1

5
Using Arrays

• But sometimes you dont know the size when you
  start.
• Picture slides new Picture???
• And what happens when you find one more thing
  that you want to put in the array?
• slides4 new Slide(picture5.jpg)
• ???? new Slide(picture6.jpg)

6
ArrayList

• ArrayLists are flexible sized collections of some
  predefined, referential data type.
• import java.util.ArrayList
• ArrayList slides new
  ArrayList()
• slides.add(new Picture(picture1.jpg))
• slides.add(new Picture(picture2.jpg))
• System.out.println(size slides.size())
• Picture p slides.get(1)

7
Another Collection type

• With Arrays and ArrayLists you look up each
  object by its number.
• What can you do when you dont want to use
  numbers?
• When you lookup my phone number in the directory
  you dont have to worry that my number is the
  3487th number in the book
• Not to mention, you dont want to worry that my
  ranking will change when Dr. Jones retires
  (moving me to 3486.

8
HashMaps

• HashMaps are flexible sized collections of some
  predefined, referential data type.
• They are accessed using some predefined key data
  type.
• For example, a phone directory would contain
  phone number values (probably stored as Strings)
  referenced by some name key (also a String).

9
HashMaps

• import java.util.HashMap
• HashMap directory
• new HashMap()
• directory.put(Schafer,3-2187)
• directory.put(Fienup,3-5918)
• System.out.println(size directory.size())
• String phoneNum directory.get(Schafer)

10
Lets actually see this in action