Intermezzo: Problem solving with data structures

1
Intermezzo Problem solving with data structures

• How to layout your program
• How to use the data structures
• This was an ad-hoc lecture, in which I wrote a
  simple (not complete) program using BlueJ.

2
Example Problem

• You are asked to implement a jukebox.
• The jukebox contains a number of CDs.
• The user can select a CD.
• The user can ask for a list of all CDs in the
  jukebox.
• Each CD contains a number of songs.
• The jukebox starts playing the CD from its first
  song.
• The user can ask for the next or previous song.
• The user can ask for a list of all songs on the
  CD.

3
Step 1 Identify Objects

• Objects
• Jukebox
• CD
• Song
• Make a class for each object. In BlueJ, I
  created three classes Song, CD, Jukebox.
• Start with the simplest class.Because Jukebox
  contains CDs and CDs contain Songs, the simplest
  is the Song class.

4
Step 2 Class Song

• public class Song
• // Data we want about songs (suggested by
  students)
• private String title private int length
  // how many minutes
• // Constructor for objects of class Song
• public Song(String t, int l) this.length
  l this.title t
• public void Play() System.out.print("Currently
  playing....") System.out.println(this.title)
  

5
Step 3 Class CD

• public class CD
• // Data we want about CDs
• private String titleprivate Song songs
  new Song5private int current
• // Constructor for objects of class CD
• public CD(String t) // Simplified all CDs
  will have the same songs.. this.title
  t songs0 new Song(My way, 4) songs1
  new Song(yesterday, 2) songs2 new
  Song("no way", 1) songs3 new
  Song("horrible", 7) songs4 new
  Song("boring", 2)
• public void Start() this.current
  0 songsthis.current.Play()

6
Step 3 Class CD continued

• public void Next() // First tried it without
  the If statement, // and you get an
  ArrayIndexOutOfBounds error, when // using Next
  to go to a non-existing slot in the // songs
  array
• if (this.current lt songs.length - 1)
  this.current this.current
  1 songsthis.current.Play() else
  System.out.println("No next track!")
• // Can add a method Previous in the same way..

7
Step 3 Class CD improved

• Rather than hard-coding the songs in the CD class
  (which leads to all CDs having the same songs, we
  want the constructor method to take a parameter
• // Constructor for objects of class CD
• public CD(String t, Song s) // Still
  simplified // assuming all CDs to have five
  songs.. this.title t songs0
  s0 songs1 s1 songs2 s2
  songs3 s3 songs4 s4

8
Step 4 Class Jukebox

• public class Jukebox
• //Data we want about the Jukeboxprivate CD
  cds new CD2private int current
• // Constructor for objects of class Jukebox
• public Jukebox() Song s1 new
  Song5 s10 new Song("my way", 4) s11
  new Song("yesterday", 4) s12 new Song(no
  way", 2) s13 new Song(boring", 7) s14
  new Song(horrible", 3) this.cds0 new
  CD("rubbish", s1)
• Song s2 new Song5 s20 new
  Song("no more way", 4) s21 new Song(no
  more energy", 4) s22 new Song(still
  boring", 7) s23 new Song(no more music",
  4) s24 new Song(very horrible",
  7) this.cds1 new CD("more rubbish", s2)

9
Step 4 Class Jukebox continued

• public void Play (int y) // Very simplified
  // no error handling for numbers out of
  range // Would be better if user could select
  from list this.cdsy.Start() this.current
  y
• public void NextTrack() this.cdsthis.curren
  t.Next()

10
Comments at end

• This code is not guaranteed to be error free!
• And it is not complete..
• But it may give you some idea of how to start
  your assignment (if you have not done so yet).
• Just think of a Castle as being a bit like a
  JukeBox, and a Room as being a bit like a CD