A music collection printer, questions about 2 String toString methods returned maybe

Hi, well I understand (almost) my biggest program yet. 3 pages !! almost 3 anyway. And my question is, in the code, which I comment the questions in green below, the code has 2 toString methods that output string data. And that is confusing me because it looks like both methods print out the same material. Any help Greatly appreciated. thank you! Derek


here it is.

Code:

---

//********************************************************************
//  CDCollection.java      Author: Lewis/Loftus
//
//  Represents a collection of compact discs.
//********************************************************************

import java.text.NumberFormat;

public class CDCollection
{
  private CD[] collection;
  private int count;
  private double totalCost;

  //-----------------------------------------------------------------
  //  Constructor: Creates an initially empty collection.
  //-----------------------------------------------------------------
  public CDCollection ()
  {
      collection = new CD[100];
      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)
  {
      if (count == collection.length)
        increaseSize();

      collection[count] = new CD (title, artist, cost, tracks);
      totalCost += cost;
      count++;
  }

  //-----------------------------------------------------------------
  //  Returns a report describing the CD collection.
  //-----------------------------------------------------------------
  public String toString()[B][COLOR="darkgreen"]//notice this is the first of 2 outputs of the program[/COLOR][/B]
  {
      NumberFormat fmt = NumberFormat.getCurrencyInstance();

      String report = "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n";
      report += "My CD Collection\n\n";

      report += "Number of CDs: " + count + "\n";
      report += "Total cost: " + fmt.format(totalCost) + "\n";
      report += "Average cost: " + fmt.format(totalCost/count);

      report += "\n\nCD List:\n\n";

      for (int cd = 0; cd < count; cd++)
        report += collection[cd].toString() + "\n";

      return report;
  }

  //-----------------------------------------------------------------
  //  Increases the capacity of the collection by creating a
  //  larger array and copying the existing collection into it.
  //-----------------------------------------------------------------
  private void increaseSize ()
  {
      CD[] temp = new CD[collection.length * 2];

      for (int cd = 0; cd < collection.length; cd++)
        temp[cd] = collection[cd];

      collection = temp;
  }
}

---

and here is the second class

Code:

---

//********************************************************************
//  CD.java      Author: Lewis/Loftus
//
//  Represents a compact disc.
//********************************************************************

import java.text.NumberFormat;

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 string description of this CD.
  //-----------------------------------------------------------------
  public String toString()[B][COLOR="darkgreen"]// notice this seems to output the same information??[/COLOR][/B]
  {
      NumberFormat fmt = NumberFormat.getCurrencyInstance();

      String description;

      description = fmt.format(cost) + "\t" + tracks + "\t";
      description += title + "\t" + artist;

      return description;
  }
}

---

oh wait, I think I see what they do.

EDIT: nope still don't know what the second one does.

The CDCollection classes toString works on an array of cd objects. It creates some display up top then loops through the array of CDs and adds each CDs toString onto it.

Quote:

---

Originally Posted by silverglade

Hi, well I understand (almost) my biggest program yet. 3 pages !! almost 3 anyway. And my question is, in the code, which I comment the questions in green below, the code has 2 toString methods that output string data. And that is confusing me because it looks like both methods print out the same material. Any help Greatly appreciated. thank you! Derek

---

One is the toString method for the CD class -- and as expected, this outputs the data held by a single CD object, and the other toString method for the CDCollection class -- and as expected outputs the data for all the CD's held by the collection. What in particular confuses you about this?

EDIT. I didn't see Fubarable's post. one sec

Best way to test the difference is to create a single cd object and print it, then create a cd collections with > 1 cd and observe how they differ.

The example code never creates a CD object. Only one CDCollection object. The ouput is like this "System.out.println (music)" and the second output is the exact same thing. They never create a CD object or call the toString method for the CD class. Maybe they just forgot to?

Quote:

---

Originally Posted by silverglade

The example code never creates a CD object. Only one CDCollection object. The ouput is like this "System.out.println (music)" and the second output is the exact same thing. They never create a CD object or call the toString method for the CD class. Maybe they just forgot to?

---

Then create a couple CD's yourself add them to a collection and see what happens.

Edit: the collection creates CD's when you call the addCD method

ok be right back

You don't need to explicitly call toString when printing an object,

Code:

---

CD x = ...;
System.out.println(x);

---

Like I said, add to there main method and include the creation of a single cd, then print it. See if you can see a difference.

Ok I did it. The CD toString method just adds a cd at the end of the CdCollection class toString method. That's it. LOL. Oh well.

Here is what I did, I hope it is correct.

Code:

---

//********************************************************************
//  Tunes.java      Author: Lewis/Loftus
//
//  Demonstrates the use of an array of objects.
//********************************************************************

public class Tunes
{
  //-----------------------------------------------------------------
  //  Creates a CDCollection object and adds some CDs to it. Prints
  //  reports on the status of the collection.
  //-----------------------------------------------------------------
  public static void main (String[] args)
  {
      CDCollection music = new CDCollection ();

      music.addCD ("Storm Front", "Billy Joel", 14.95, 10);
    //ALL OTHER CD'S REMOVED




      System.out.println (music);

      //OK HERE IS MY ADDED CODE TO TEST CD

      CD testCD = new CD ("Doggy","Fat Boy", 3.00, 2);

      System.out.println (testCD);
  }
}

---

and here was the output when program ran.

Code:

---

C:\JAVA_PROGRAMMING_CODE\code1\Tunes>java Tunes
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
My CD Collection

Number of CDs: 1
Total cost: $14.95
Average cost: $14.95

CD List:

$14.95  10      Storm Front    Billy Joel

$3.00  2      Doggy  Fat Boy

C:\JAVA_PROGRAMMING_CODE\code1\Tunes>

---