Sublist

[Garion Winters]

Hi there. I was wondering if anyone could help me out here... We are currently doing a project that involves creating a new class called sublist. This takes params of (MyList list, int startIndex, int endIndex). It is basically a partial view of the list (MyList is used because we have been making our own). I'm not looking for an answer, just help here.
We are not to change that constructor, and if you edit anything about the sublist, it should change the actual list. (IOW if you look from the 3rd object to the 5th, and add a new one, it should be so in the original, non-sub list as well)
Any help would be appreciated.

[Marilyn de Queiroz]

Have you seen the way that sublist was implemented in the API? List

You can see the source code by looking in (java_home)/src after you unjar src.jar or src.zip (depending on whether you're using 1.3 or 1.4)
[ October 14, 2002: Message edited by: Marilyn de Queiroz ]

[Dirk Schreckmann]

How have you considered approaching the problem?

[Garion Winters]

(Sorry for the delay.. networking troubles)
Well, I have thought about basically extending our class, MyAbstractList, then for the constructor, make a new MyList by iterating through the startIndex and endIndex and add it to the new MyList. Of course, now I am having all null pointer exceptions (We have 10 test cases... all fail at the same point in the constructor)
Here is what I have. Any help would be great Thx again.
BTW... we can't change the constructor params.
The MySubList (what we are writing)

public class MySubList extends MyAbstractList
{
  MyList subList = null;
  public MySubList(MyList list, int startIndex, int endIndex)
  {
    MyListIterator itr = list.listIterator(startIndex);
    if(startIndex < 0 || startIndex > endIndex || endIndex > list.size())
      throw new IndexOutOfBoundsException();
    for(int i=startIndex; i < endIndex; i++)
    {
      itr.next();
      subList.add(i, list.get(i)); 
      //null occurs on the line above!
    }
  }
  public MyListIterator listIterator()
  {
    return subList.listIterator(0);
  }
  public MyListIterator listIterator(int index)
  {
    return subList.listIterator(index);
  }
  public MyList subList(int start, int end)
  {
    return new MySubList(this, start, end);
  }
  public MyIterator iterator()
  {
    return subList.listIterator(0);
  }
  public int size()
  {
    return subList.size();
  }
}

The MyAbstractList (that I extended)

public abstract class MyAbstractList extends MyAbstractCollection implements MyList
{
  protected transient int m_modCount = 0;
  public void add(int index, Object o)
  {
    MyListIterator i = listIterator(index);
    i.add(o);
  }
  public boolean addAll(int index, MyCollection c)
  {
    MyListIterator i = listIterator(index);
    int size = size();
    for (MyIterator j = c.iterator(); j.hasNext(); )
      i.add(j.next());
    return size != size();
  }
  public int indexOf(Object o)
  {
    int i=0;
    for (MyListIterator j=listIterator(); j.hasNext(); ++i) {
      if (o.equals(j.next()))
        return i;
    }
    return -1;
  }
  public int lastIndexOf(Object o)
  {
    int i=size()-1;
    for (MyListIterator j=listIterator(size()); j.hasPrevious(); --i) {
      if (o.equals(j.previous()))
        return i;
    }
    return -1;
  }

  public Object remove(int index)
  {
    MyListIterator i = listIterator(index);
    Object o = i.next();
    i.remove();
    return o;
  }
  public boolean add(Object o)
  {
    int size = size();
    listIterator(size).add(o);
    return size != size();
  }
  public Object get(int index)
  {
    return listIterator(index).next();
  }
  public Object set(int index, Object o)
  {
    MyListIterator i = listIterator(index);
    Object val = i.next();
    i.set(o);
    return val;
  }
  public MyList subList(int startIndex, int endIndex)
  {
    return new MySubList(this, startIndex, endIndex);
  }
}

And the MyList...

public interface MyList extends MyCollection
{
  void add(int index, Object o);
  boolean addAll(int index, MyCollection c);
  Object get(int index);
  int indexOf(Object o);
  int lastIndexOf(Object o);
  MyListIterator listIterator();
  MyListIterator listIterator(int index);
  Object remove(int index);
  Object set(int index, Object o);
  MyList subList(int fromIndex, int toIndex);
}

If you need to see any other files, let me know. We made a lot over the time, but those are the ones I can see as important. Can't get over the nulls...

[Garion Winters]

Hmm.. I wrote like 170 or so lines of code now, and that one part is STILL causing me errors... I tried adding a while hasNext part, to make sure there is a next before going on, but the test cases all still reveal a NULL POINTER EXCEPTION ERROR! ... here is where I am having the trouble

public class MySubList extends MyAbstractList
{
  //Declare Variables
  MyList subList;  //the subList
  MyList fullList; //the list that is the full extend of the subList
//////////////////////////////////////////////////////
// CONSTRUCTOR TO INITIALIZE THE MYSUBLIST OBJECT   //
//////////////////////////////////////////////////////
  public MySubList(MyList list, int startIndex, int endIndex)
  {
    fullList = list;
    MyListIterator itrFull = list.listIterator(startIndex);
    Object toAdd;
    if(startIndex < 0 || startIndex > endIndex || endIndex > list.size())
      throw new IndexOutOfBoundsException();
    if(startIndex == endIndex)
      subList = null;
    else
    {
      int j = 0;
      while(itrFull.hasNext())
      {
        for(int i=startIndex; i < endIndex; i++)
        {
          toAdd = itrFull.next();
          subList.add(j, toAdd); //NULL POINTER!!
          j++;
        }
      }
    }
  }

Silly thing is driving me nuts. Any suggestions?

[Dirk Schreckmann]

You're getting a NullPointerException because you never initialize your sublist to be something other than null. In other words, you never say MyList sublist = new MyList() ;

[Garion Winters]

That is what I thought... but everytime I tried to do that, it says I can't. MyList is an interface, and it won't allow me to initialize it so. Hmmmm =(
I figure looking at the code they have would help greatly... just can't seem to find it for some reason. Thx everyone!

[Marilyn de Queiroz]

Find src.jar or src.zip in your JAVA_HOME directory. (my latest one is in C:\J2SDK_Forte\jdk1.4.0 and the previous one was in C:\jdk1.3.1_02). Unjar/unzip it. Then you can see the source files.

MyList is an interface, and it won't allow me to initialize it so.

I suggest you create a class that implements the MyList interface
for example

class MyPartialList implements MyList
{
// stuff
}

Then you can have MyList sublist = new MyPartialList();

[Garion Winters]

*smacks himself*
Oh, I see. Thank you very much. Helped out a bundle!