need help with compareTo()

[Doug Wolfinger]

I am trying to add String items to a sorted array list, using compareTo to find the proper position for each item.
This is the runtime error I get when I run ListPop.java with sortedAdd() defined with a String, instead of Object, in the argument list.
Exception in thread "main" java.lang.NullPointerException
at java.lang.String.compareTo(String.java:789)
at java.lang.String.compareTo(String.java:825)
at SortedList.sortedAdd(SortedList.java:41)
at ListPop.main(ListPop.java:17)
When I try to define sortedAdd() with an Object, instead of a String, in the argument list I get this error message when I compile SortedList.java
SortedList.java:41: cannot resolve symbol
symbol : method compareTo (java.lang.Object)
location: class java.lang.Object
while(item.compareTo((Object)arr[pos]) <= 0) {
1 error
Here is SortedList.java

class SortedList extends FullName implements SortedListInterface {
  private static final int MAX_LIST = 50;
  private Object arr[];  //a sorted array of list items
  private int numItems;
  public SortedList()
   {
    arr = new Object[MAX_LIST];
    numItems = 0;
   }
  public boolean sortedIsEmpty()
   {
     //returns true if list has no items
     return (numItems == 0);
   }
  public int sortedSize()
   {
    return numItems;
   }
  public void sortedAdd(int index, String item) throws
        IndexOutOfBoundsException
   {                                     
    int length = sortedSize();
    int pos;
    if (index > MAX_LIST) throw
           new ListIndexOutOfBoundsException("index is too high");
    //check for duplicity
    for(pos = 0;pos <= length;pos++) {   
      if(item.equals(arr[index])) {
        throw new SortedListDuplicityException(item + " is already in this sorted list");
      }                                  
     }                                   
    if(length == 0) {                    
      arr[0] = item;
      numItems++;
     }                                   
     else {                              
//       for(pos=length;item < arr[pos];pos--) {
       [B]while(item.compareTo((String)arr[pos]) <= 0)[/B] {
         System.out.println("moving along");
        }                                
       arr[pos + 1] = item;
       numItems++;
      }                                  
     } //end sortedAdd
  public Object sortedGet(int index)
  {
    int pos;
    int length=sortedSize();
    for (pos = 0;pos <= length;pos++) {
      if (pos == index) {
        return arr[pos];
       }
//      else {
        
      }
      return null;
/*       if (pos == length) {
 *        throw new ItemNotFoundException("Item at position " + index + " not found");
 *          return null;
 *     }
*/   }
   public void sortedRemove(int index)
   {
       int pos;
       int length = sortedSize();
       for(pos = index;pos <= length;pos++) {
         arr[index] = arr[pos+1];       
       }
  }
   public int locatedIndex(Object item) throws ItemNotFoundException
   {
     //Locates the position of the item.
     //precondition: none
     //postcondition: The index of the object is returned if it exists in the
     //list. If it does not exist and exception is thrown.
     int length=sortedSize();
     for(int i=0;i<=length;i++) {
       if (arr[i] == item) {
         return i;
        }
        else {
          throw new ItemNotFoundException("that item is not in the list");          
//          return 0;
         }
      }
      return 0;
    }

Here is FullName.java, which implements compareTo()

public class FullName implements java.lang.Comparable { 
  private String firstName;
  private String lastName;
  public FullName() {}
  public FullName(String first, String last) {
    firstName = first;
    lastName = last;
  } // end constructor
  public int compareTo(Object rhs) {
    // returns 0 if all fields match
    //    if lastName equals rhs.lastName and 
    //    firstName is greater than rhs.firstName
    // returns -1 if lastName is less than rhs.lastName or 
    //    if lastName equals rhs.lastName and 
    //    firstName is less than rhs.firstName
    if (lastName.compareTo(((FullName)rhs).lastName)==0){
      System.out.println("from the compareTo meth " + firstName + " is first name");
      return firstName.compareTo(((FullName)rhs).firstName);
    } 
    else {
      return lastName.compareTo(((FullName)rhs).lastName);
    } // end if
  } // end compareTo
} // end class FullName

Here is ListPop.java, which attempts to populate the array.

class ListPop {
  static SortedList sl = new SortedList();
  public static void popMeth()
  {
     //populate the array with strings
     sl.sortedAdd(1, new String("Doug"));
     sl.sortedAdd(1, new String("Will"));
     sl.sortedAdd(1, new String("Scott"));
  }
  public static void getMeth()
  {
    //retrieve the item at the specified index
    System.out.println(sl.sortedGet(2));
  }
  public static void main(String args[]) 
  {
     popMeth();
     getMeth();
  }
}

It should put the first item in the array here

if(length == 0) {                    
      arr[0] = item;
      numItems++;
     }                                   

so why is it giving me the NullPointerException for arr[]?
New ones should be added when the while loop in sortedAdd() finishes, right?
Thanks!
[ October 09, 2002: Message edited by: Doug Wolfinger ]
[ October 09, 2002: Message edited by: Doug Wolfinger ]

[Ilja Preuss]

Regarding your compilation problem: Object doesn't define compareTo - you should use Comparable instead.
Regarding the NullPointerException: what does happen if you reach the end of the list in the loop?
BTW, I have the strong feeling that what you try to do would be much easier to accomplish by using an ArrayList instead of an array, in combination with Collections.binarySearch(...)

[Barry Andrews]

Try this:
while(item.compareTo((String)arr[pos - 1]) <= 0)
You are saying that pos = length. If your length is let's say 3, then you are trying to get an item from the array at position 3 which does not exist. The position at the end of the array would be 2 instead of 3. Hope this helps!
Barry

[Doug Wolfinger]

Regarding the end of the loop, I suppose I could add

&& ((arr[pos-1]) != null)

However, I'm now getting an "Incompatible types" compile error from the while statement in sortedAdd(). The compiler says "found: int" "required: boolean", and points to the paren that follows "compareTo". So I've got my work cut out for me.
Btw, how do I capture the console's output. I thought "javac pgmname.java > pgmname.txt" would places the output in the text file, but it's not working. Thanks!

[Jim Yingst]

I agree with Ilja - it seems like it would probably be easier to use an ArrayList and methods like Collections.sort() and Collections.binarySearch(). Or use a TreeSet which may be even simpler. But anyway...
However, I'm now getting an "Incompatible types" compile error from the while statement in sortedAdd(). The compiler says "found: int" "required: boolean", and points to the paren that follows "compareTo". So I've got my work cut out for me.
Sounds like you need to look carefully at all the parentheses in the while statement. If this continues to trouble you, show us exactly what that while statement currently looks like.
Btw, how do I capture the console's output. I thought "javac pgmname.java > pgmname.txt" would places the output in the text file, but it's not working. Thanks!
That command only redirects standard output - not error messages. Try
javac pgmname.java > pgmname.txt 2>&1
The 2 refers to the error stream, while 1 refers to the standard output. 2>&1 means "send errors to the same place standard output is going."

[Doug Wolfinger]

Hi,
I am still trying to figure out this method compareTo.
Node2.java implements Comparable. So, in using Node2 to cast in the comparison, I would think that I'm fulfilling the obligation to use a Comparable object to do that. Everything I am comparing is an Object, so I can't see what the problem is. The 3 compiler errors are "can't resolve symbol."
Thanks!
Here is Node2, which contains the compareTo method, making the Node2 type Comparable.

public class Node2 implements Comparable {
  private Object item;
  private Node2 next;
  private Integer firstName;
  private Integer lastName;
  public Node2() {}
  public Node2(Integer first, Integer last) {
    firstName = first;
    lastName = last;
   }
  public Node2(Object newItem) {
    item = newItem;
    next = null;
  } // end constructor
  public Node2(Object newItem, Node2 nextNode2) {
    item = newItem;
    next = nextNode2;
  } // end constructor
  public void setItem(Object newItem) {
    item = newItem;
  } // end setItem
  public Object getItem() {
    return item;
  } // end getItem
  public void setNext(Node2 nextNode2) {
    next = nextNode2;
  } // end setNext
  public Node2 getNext() {
    return next;
  } // end getNext
  public int compareTo(Object rhs)
  {
    //returns 0 if all fields match
    //   if lastName equals rhs.lastName and
    //   firstName is greater than rhs.firstName
    //returns -1 if lastName is less than rhs.lastName or
    //   if lastName equals rhs.lastName and
    //   firstName is less than rhs.firstName
    if (lastName.compareTo(((Node2)rhs).lastName)==0) {
      System.out.println("from the compareTo meth " + firstName + " is first name ");
      return firstName.compareTo(((Node2)rhs).firstName);
     }
    else {
      return lastName.compareTo(((Node2)rhs).lastName);
     }
    }
} // end class Node2

Here is Noding6, which extends Node2 and implements compareTo.

class Noding6 extends Node2 {
  //this is needed to make the object in main()
  public Noding6() {}
  Node2 newValue = new Node2(new Integer(16));
  Node2 n1, n2, n3, n4, n5;
  private Node2 head;
  private Node2 tail;
  private Node2 list;        //to reference the last Node2 in a circular list
  private Node2 curr;
  private Node2 prev;
  private Node2 nextNode2;
  private Node2 getCurr;     //used to retrieve a Node2 by specified index in find(int)
  private Object currNode2;
  private Object nextNode2Comp;
  public Noding6(Object first, Object last)
  {
    currNode2 = first;
    nextNode2Comp = last;      //last is the next node that is already in the list
   }
  public Node2 Node2Meth() 
  {
      n5 = new Node2(new Integer(84));
      n4 = new Node2(new Integer(13),n5);
      n3 = new Node2(new Integer(1),n4);
      n2 = new Node2(new Integer(5),n3);
      n1 = new Node2(new Integer(9),n2);
     int i;
     n4.setNext(n5);
     n3.setNext(n4);
     n2.setNext(n3);
     n1.setNext(n2); //link the Node2s
     head = n1;
     curr = head;
     return curr;
   }
   public void tryCompare()
   {
    n1.getItem();
    head = head.getNext();
    for ( prev = null, curr = head; (curr != null)
       && (newValue.compareTo(curr.getItem()) > 0);
       prev = curr, curr = curr.getNext() ) {
      System.out.println("newValue > current item");
    }
   } 
 public int compareTo(Object rhs)
   {
   // returns 0 if all fields match
   //    if nextNode2 equals rhs.nextNode2 and 
   //    currNode2 is greater than rhs.currNode2
   // returns -1 if nextNode2 is less than rhs.nextNode2 or 
   //    if nextNode2 equals rhs.nextNode2 and 
   //    currNode2 is less than rhs.currNode2
   currNode2 = (Node2)curr;                // gets "incompatible types" error when currNode2 is defined as Integer
   nextNode2Comp =  (Node2)curr.getNext(); // gets "incompatible types" error when currNode2 is defined as Integer
   if (currNode2.compareTo(((Node2)rhs).nextNode2Comp)==0){
     System.out.println("from the compareTo meth " + currNode2 + " is currNode2");
     return currNode2.compareTo(((Node2)rhs).nextNode2Comp);
   } 
   else {
     return currNode2.compareTo(((Node2)rhs).nextNode2Comp);
    } // end if
   } // end compareTo
  public static void main(String args[]) {
    Noding6 nod = new Noding6();
    nod.Node2Meth();
    nod.tryCompare();
   }
  }

[ October 18, 2002: Message edited by: Doug Wolfinger ]