Question

What is wrong with my add and remove method (in bold) under the SongList that it is creating a NullPointerException?

public class Song
{
// instance variables
private String m_artist;
private String m_title;
private Song m_link;

// constructor
public Song(String artist, String title)
{
m_artist = artist;
m_title = title;
m_link = null;
}

// getters and setters
public void setArtist(String artist)
{
m_artist = artist;
}

public String getArtist()
{
return m_artist;
}

public void setTitle(String title)
{
m_title = title;
}

public String getTitle()
{
return m_title;
}
  
public void setLink(Song link)
{
m_link = link;
}

public Song getLink()
{
return m_link;
}
}

-------------------------------------------------------------------

public class SongList
{
// instance variables
private Song m_last;
private int m_numElements;

// constructor
// Do not make any changes to this method!
public SongList()
{
m_last = null;
m_numElements = 0;
}

// check whether the list is empty
// Do not make any changes to this method!
boolean isEmpty()
{
if (m_last == null)
return true;
else
return false;
}

// return the size of the list (# of Song nodes)
// Do not make any changes to this method!
public int size()
{
return m_numElements;
}

// add a new Song to the circular linked list with the given artist and
// title, keeping the list sorted by *song title*.
public void add(String artist, String title)
{
// TODO: implement this method
  
Song newNode = new Song(artist, title);

if(m_last == null)
{
m_last = newNode;
m_numElements++;
return;
}

Song previous = null;
Song current = m_last;



while (current != null)
{
//System.out.println(current.getTitle() + " comparedTo " + title + ": " + (current.getTitle()).compareTo(title));
int compareToResult = (current.getTitle()).compareTo(title);

if (compareToResult < 0)
{
previous = current;
current = current.getLink();
//m_numElements++;


}

else
break;


}

if (previous == null)
{
newNode.setLink(m_last);
m_last = newNode;
m_numElements++;
}

else
{
previous.setLink(newNode);
newNode.setLink(current);
m_numElements++;
}   
}

// remove a Song associated with the given artist and title from the list,
// keeping the list sorted by *song title*.
public boolean remove(String artist, String title)
{
  
Song previous = null;
Song current = m_last;
boolean match = false;
  
while (current != null)
{
int compareToTitle = (current.getTitle()).compareToIgnoreCase(title);
int compareToArtist = (current.getArtist()).compareToIgnoreCase(artist);

if (compareToTitle < 0 && compareToArtist < 0)
{
previous = current;
current = current.getLink();
}
else if (compareToTitle == 0 && compareToArtist == 0)
{
match = true;
current = current.getLink();
  
  
if (previous == null)
m_last = current;
else
previous.setLink(current);
}
else
return match;
}
  
//return match;
return true;

}
  

  
// return a string representation of the list
// Do not make any changes to this method!
public String toString()
{   
String listContent = "";
Song current = m_last;
  
if (m_last != null)

do
{
current = current.getLink();
listContent += " [" + current.getArtist() + " - " + current.getTitle() + "]\n";

} while (current != m_last);

return listContent;
}
}

-------------------------------------------------------------------------------------------------------------------

Answer 1

Your both methods are right. There is error in toString() method. It has 2 errors:

1) current node is set to next node before converting its contents to string

2) condition in do while loop is not correct. It should be while(current != null).

Correct code for toString():

    public String toString()
    {   
        String listContent = "";
        Song current = m_last;
    
        if (m_last != null){
            do
            {
                listContent += " [" + current.getArtist() + " - " + current.getTitle() + "]\n";
                current = current.getLink();
        
            } while (current != null);
        }

        return listContent;
        }
    }

I tested it for follwing operations:

  SongList slist = new SongList();
  slist.add("Akon", "I wana love you");
  slist.add("Akosn", "I wana lovsd su");
  System.out.println(slist.toString());
  slist.remove("Akon", "I wana love you");
  System.out.println(slist.toString());

Output:

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