Thread: 3 classes - Simple school assignment - ArrayLists

Hello everyone,

I was given an assignment in a JAVA class that seems relatively simple. However, to an inexperienced programmer like myself, this is very confusing. All I need to do is complete the classes for CDCollection and Tunes. Here comes the "What have you tried?" question. Folks, I honestly have no idea where to start. I don't expect much help but anything is appreciated. I will update this frequently with what I have so far.


What I see is that there is an array list of type CD called collection. When the default constructor runs (no parameters) in CDCollection, it just initializes some values. Looking at the main class (Tunes), when I create an object of type CD Collection, it has no parameters. So it will use that default constructor. Perhaps after I create an object in the main class, I can run music.addCD("Magnolia", "Turnover", 10.0, 11); --- this will call the addCD method, but now I have to create that.

//  Tunes.java  
//  Demonstrating the use of an array list of objects.
 
public class Tunes
{
   //-----------------------------------------------------------------
   //  Creates a CDCollection object and tests the methods that add, remove, and
   //  report on the status of the collection.
   //-----------------------------------------------------------------
   public static void main (String[] args)
   {
      CDCollection music = new CDCollection ();
 
      // To be implemented
 
      System.out.println (music);
   }
}

import java.util.ArrayList ;
//  Represents a collection of compact discs.
 
public class CDCollection
{
   private ArrayList<CD> collection;
   private int count;
   private double totalCost;
 
   //-----------------------------------------------------------------
   //  Creates an initially empty collection.
   //-----------------------------------------------------------------
   public CDCollection ()
   {
      collection = new ArrayList<CD>();
      count = 0;
      totalCost = 0.0;
   }
 
   //-----------------------------------------------------------------
   //  Adds a CD to the collection, increasing the size of the
   //  collection if necessary.
   //-----------------------------------------------------------------
   public void addCD (String title, String artist, double cost,
                      int tracks)
   {
     // To be implemented
   }
 
   //-----------------------------------------------------------------
   //  Removes a CD from the collection.  Takes appropriate action if the
   //  CD to be removed is not in the collection.
   //-----------------------------------------------------------------
   public String removeCD (String title)
   { String result = null ;
     // To be implemented
     return result ;
   }
 
   //-----------------------------------------------------------------
   //  Returns a report describing the CD collection.
   //-----------------------------------------------------------------
   public String toString()
   {
     String report = null ;
       // To be implemented
     return report ;
   }
}

 
//  CD.java      
//  Represents a compact disc.
 
public class CD
{
   private String title, artist;
   private double cost;
   private int tracks;
 
   //-----------------------------------------------------------------
   //  Creates a new CD with the specified information.
   //-----------------------------------------------------------------
   public CD (String name, String singer, double price, int numTracks)
   {
      title = name;
      artist = singer;
      cost = price;
      tracks = numTracks;
   }
 
   //-----------------------------------------------------------------
   //  Returns a description of this CD.
   //-----------------------------------------------------------------
   public String toString()
   {
      String description;
 
      description ="\n cost: " + cost + "\ttracks:" + tracks + "\t";
      description += title + "\t" + artist;
 
      return description;
   }
}

Why do you have no idea where to start? Did you just wake up after 4 months and realize you were in a Java class when you thought you were a History major? We can't teach you what you've missed or hold your hand through all that you must do to complete this assignment.

I suggest you read the assignment, review the provided skeletons of all 3 classes so that you have a rough idea of how they work together, and then take each class one at a time and fill in the blanks. Review the instance variables in each class to give you hints what the methods should do. For example, adding a CD to the collection should probably do something with the 'count' variable. You should probably start with CD, then complete CDCollection, and then start testing, starting with the constructor and then every method.

If you run into problems while testing, come back with code, errors, and questions.

Originally Posted by GregBrannon

Why do you have no idea where to start? Did you just wake up after 4 months and realize you were in a Java class when you thought you were a History major? We can't teach you what you've missed or hold your hand through all that you must do to complete this assignment.

I suggest you read the assignment, review the provided skeletons of all 3 classes so that you have a rough idea of how they work together, and then take each class one at a time and fill in the blanks. Review the instance variables in each class to give you hints what the methods should do. For example, adding a CD to the collection should probably do something with the 'count' variable. You should probably start with CD, then complete CDCollection, and then start testing, starting with the constructor and then every method.

If you run into problems while testing, come back with code, errors, and questions.

Thank you, I'm working on it as I type. Yes, that is true. I guess I will just have to take more risks trying things before posting questions asking for help

--- Update ---

Here's what I have so far:

 public static void main (String[] args)
   {
      CDCollection music = new CDCollection ();
 
      // confused by this whole aspect. Should I be creating new objects for each CD ,or is that not necessary?
 
 
      // CD cdOne = new CD("Help!", "The Beatles", 10.0, 14);
 
       // here I have called the addCD method, passing it some parameters. 
        music.addCD("Help!", "The Beatles", 10.0, 14);
 
 
       System.out.println (music);
   }
}

 
//addCD method that I called in the main class 
   public void addCD (String title, String artist, double cost,
                      int tracks)
   {
       count++;      // increment count, now there is 1 cd
 
       totalCost = totalCost + cost;      // the total cost of the cd collection is  (whatever it was before), plus the cost of this current CD
 
       collection.add(new CD (title, artist, cost, tracks));          // Add a new CD to the ArrayList, passing it those arguments
 
 
   }

Now I am trying to access this arraylist. Will check back with more code in a few...

confused by this whole aspect. Should I be creating new objects for each CD ,or is that not necessary?

Since the method to add a CD in the CDCollection class takes all of the parameters needed to create the CD, you can assume that addCD():

1. Creates the CD object
2. Adds the resulting object to the collection

Can the CD object be anonymous (without a reference that can be used later) in that process? Yes.