Copy Constructor help.

[Andrew Gary]

I'm having trouble making a copy constructor for this ListType class, the list type class implements the GeneralList interface, which is down below the ListType class.

I have to create a copy constructor that accepts a reference to another GeneralList Object.

Here is the ListType class

public class ListType<E> implements GeneralList<E>
{
   // Constants for the default capacity and
   // the resizing factor.
   private final int DEFAULT_CAPACITY = 10;
   private final int RESIZE_FACTOR = 2;
   
   // Private Fields
   private E[] list;       // The list
   private int elements;   // Number of elements stored
   
   public ListType()
   {
      // The following statement causes a compiler warning.
      // It is a necessary work-around because we cannot
      // instantiate an array of a generic type.
      list = (E[])(new Object[DEFAULT_CAPACITY]);
      elements = 0;
   }

   public ListType(int capacity)
   {
      if (capacity < 1)
         throw new IllegalArgumentException();

      // The following statement causes a compiler warning.
      // It is a necessary work-around because we cannot
      // instantiate an array of a generic type.
      list = (E[])(new Object[capacity]);
      elements = 0;
   }

   public void add(E element)
   {
      // If the list is full, resize it.
      if (elements == list.length)
         resize();
      
      // Add element to the end of the list.
      list[elements] = element;
      
      // Adjust the number of elements.
      elements++;
   }
  
   public void add(int index, E element)
   {
      // First make sure the index is valid.
      if (index >= elements || index < 0)
         throw new IndexOutOfBoundsException();

      // If the list is full, resize it.
      if (elements == list.length)
         resize();
         
      // Shift the elements starting at index
      // to the right one position.
      for (int index2 = elements; index2 > index; index2--)
         list[index2] = list[index2 - 1];
      
      // Add the new element at index.
      list[index] = element;
      
      // Adjust the number of elements.
      elements++;
   }

   public void clear()
   {
      for (int index = 0; index < list.length; index++)
         list[index] = null;
      elements = 0;
   }
   
   public boolean contains(E element)
   {
      int index = 0;          // Index counter
      boolean found = false;  // Search flag
      
      // Step through the list. When the element
      // is found, set found to true and stop.
      while (!found && index < elements)
      {
         if (list[index].equals(element))
            found = true;
         index++;
      }
      
      // Return the status of the search.
      return found;
   }
   
   public E get(int index)
   {
      if (index >= elements || index < 0)
         throw new IndexOutOfBoundsException();
      return list[index];
   }

   public int indexOf(E element)
   {
      int index = 0;          // Index counter
      boolean found = false;  // Search flag
      
      // Step through the list. When the element
      // is found, set found to true and stop.
      while (!found && index < elements)
      {
         if (list[index].equals(element))
            found = true;
         else
            index++;
      }
      
      // Return the index of element or -1.
      if (!found)
         index = -1;
      return index;
   }
 
   public boolean isEmpty()
   {
      return (elements == 0);
   }
   
   public boolean remove(E element)
   {
      int index = 0;          // Index counter
      boolean found = false;  // Search flag
      
      // Perform a sequential search for element. When it is
      // found, remove it and stop searching.
      while (!found && index < elements)
      {
         if (list[index].equals(element))
         {
            list[index] = null;
            found = true;
         }
         index++;
      }
      
      // If the value was found, shift all subsequent
      // elements toward the front of the list.
      if (found)
      {
         while(index < elements)
         {
            list[index - 1] = list[index];
            index++;
         }
         
         // Adjust the number of elements.
         elements--;
      }
      
      // Return the status of the search.
      return found;
   }
   
   public E remove(int index)
   {
      if (index >= elements || index < 0)
         throw new IndexOutOfBoundsException();

      // Save the element, but remove it from the list.
      E temp = list[index];
      list[index] = null;
      index++;

      // Shift all subsequent elements toward
      // the front of the list.
      while(index < elements)
      {
         list[index - 1] = list[index];
         index++;
      }
      
      // Adjust the number of elements.
      elements--;
      
      // Return the element that was removed.
      return temp;
   }
   
   private void resize()
   {
      // Calculate the new length, which is the current
      // length multiplied by the resizing factor.
      int newLength = list.length * RESIZE_FACTOR;
      
      // Create a new list.
      // Note: This statement causes a compiler warning.
      // It is a necessary work-around because we cannot
      // instantiate an array of a generic type.
      E[] tempList = (E[])(new Object[newLength]);
      
      // Copy the existing elements to the new list.
      for (int index = 0; index < elements; index++)
         tempList[index] = list[index];
      
      // Replace the existing list with the new one.
      list = tempList;
   }
  
   public E set(int index, E element)
   {
      if (index >= elements || index < 0)
         throw new IndexOutOfBoundsException();

      // Save the existing element at that index.
      E temp = list[index];
      
      // Replace the element with element.
      list[index] = element;
      
      // Return the previously stored element.
      return temp;
   }
   
   public int size()
   {
      return elements;
   }
   
   public String[] toStringArray()
   {
      // Create a String array large enough to hold
      // all the elements of the list.
      String[] strArray = new String[elements];
      
      // Store each element's toString() return value
      // as an element in the String array.
      for (int index = 0; index < elements; index++)
         strArray[index] = list[index].toString();
      
      // Return the String array.
      return strArray;
   }
   
   public int getCapacity()
   {
	   return list.length;
   }
   
   public void ensureCapacity(int minCapacity)
   {
	   if (minCapacity < list.length)
		   throw new IllegalArgumentException();
	   
	   E[] tempList = (E[])(new Object[minCapacity]);
	   
	   for (int index = 0; index < elements; index++)
		   tempList[index] = list[index];
	   
	   list = tempList;
   }
   
   public void trimToSize()
   {
	   E[] tempList = (E[])(new Object[elements]);
	   
	   for (int index = 0; index < elements; index++)
		   tempList[index] = list[index];
	   
	   list = tempList;
   }
   
   public int lastIndexOf(E element)
   {
	   int index = elements - 1;
	   boolean found = false;
	   
	   while (!found && index >= 0)
	   {
		   if (list[index].equals(element))
			   found = true;
		   else
			   index--;
	   }
	   if(!found)
		   index = -1;
	   return index;
   }
}

And here is the GeneralList interface

public interface GeneralList<E>
{
   public void add(E element);
   
   public void add(int index, E element);

   public void clear();
   
   public boolean contains(E element);
   
   public E get(int index);
   
   public int indexOf(E element);
   
   public boolean isEmpty();
   
   public boolean remove(E element);
   
   public E remove(int index);
   
   public E set(int index, E element);

   public int size();
}

Here is what I have come up with, but it does not work.
Could someone help me understand why this copy constructor does not work, and how to modify it so that it does work?

public ListType(GeneralList ilist)
   {
    list = (E[])(new Object[ilist.size()]);
    for(int index = 0; index < ilist.size(); index++)
    list[index] = (E)ilist.get(index);
   }

Thanks for any help, it's much appreciated.

[Praveen Kumar M K]

Hi Andrew,

In the copy constructor, you are not setting the value of the number of elements. Although the "list" array has the right values, elements is still = 0.

[Andrew Gary]

Oh thanks! I never catch my own stupid mistakes ha. Appreciate the help!!