I'm new programming and have run into some errors here.
First problem is: I keep getting a compiler error in DLLApp.java saying "method in class DLL cannot be applied to given types;". I understand what it's saying, because I'm giving it a string Argument, but I can't figure out how to actually make it the correct Generic type.
Secondly, since this is not compiling yet, I can't check my program, but how do my methods in DLL look? I'm still trying to understand the DLL Data structure.
DLL class
public class DLL<E>
{
/** A reference to the first node in this DLL. */
private Node<E> first;
/** A reference to the last node in this DLL. */
private Node<E> last;
/**
* Construct an empty DLL.
*/
public DLL()
{
first = null;
last = null;
}
/**
* Return the first element in this DLL, or null if DLL empty.
*
* @return The first element in this DLL, or null if DLL empty.
*/
public E getFirst()
{
return (first != null ? first.element : null);
}
/**
* Return the last element in this DLL, or null if DLL empty.
*
* @return The last element in this DLL, or null if DLL empty.
*/
public E getLast()
{
return (last != null ? last.element : null);
}
/**
* Return the number of nodes in this DLL.
*
* @return The number of nodes in this DLL.
*/
public int size()
{
int size = 0;
for (Node<E> curr = first; curr != null; curr = curr.next)
{
size++;
}
return size;
}
/**
* Add an element to the start of this DLL.
*
* @param elem An element to add to the start of this DLL.
*/
public void addFirst(E elem)
{
insert(elem, null);
}
/**
* Add an element to the end of this DLL.
*
* @param elem An element to add to the end of this DLL.
*/
public void addLast(E elem)
{
insert(elem, last);
}
/**
* Add a new element after the element 'prev' (or at the end of this DLL if
* 'prev' isn't found).
*
* @param elem A new element to add to this DLL.
* @param prev An element to add 'elem' after.
*/
public void addAfter(E elem, E prev) //use search method then insert
{
//Node<E> newNode = new Node<E>(elem, prev, next); //do i need?
insert(elem, search(prev)); //not sure?
}
/**
* Delete a given element from this DLL.
*
* @param elem An element to delete from this DLL.
*/
public void delete(E elem) //create node etc
{
//Use search then use back and forward links instead of this whole method
while (first != null) //move through list
{
if (first == elem) //if the next element is the deleting number
{
//reassign nodes skipping the first so deallocated(I think problem)
first.prev = first.next;
first.next = first.prev;
}
first = first.next; //move through the list
}
}
/**
* Return a string representation of DLL to allow System.out.println(aList).
*
* @return A String representation of this DLL.
*/
public String toString()
{
String result = "[";
for (Node<E> curr = first; curr != null; curr = curr.next)
{
result += curr;
if (curr != last)
{
result += ", ";
}
}
return result + "]";
}
/**
* Return a string representation of all the elements in this DLL, in
* last-to-first order.
*
* @return A String representation of this DLL in reverse order.
*/
public String reverseString()
{
String result = "[";
for (Node<E> curr = last; curr != null; curr = curr.prev)
{
result += curr;
if (curr != first)
{
result += ", ";
}
}
return result + "]";
}
/**
* Find the first Node which contains the given element.
*
* @param elem The element to search for.
* @return The first Node containing the given element, or null if no Node
* contains the element.
*/
private Node<E> search(E elem) //HELP
{
for (Node<E> curr = first; curr != null; curr = curr.next)
{
if (curr.element.equals(elem))
{
return curr; //am i returning the element like this?
}
}
return null;
}
/**
* Insert a new element after the element 'prev'. If 'prev' is null then
* insert 'elem' at the front of this DLL.
*
* @param elem A new element to insert into this DLL.
* @param prev An element to insert 'elem' after. If 'prev' is null insert
* 'elem' at the start of this DLL.
*/
private void insert(E elem, Node<E> prev) //HELP
{
Node<E> newNode = new Node<E>(elem, prev, null);
//if list is empty, add to front
if (first == null)
{
first= newNode;
last = newNode;
}
else if (prev == null)
{
//addFirst(newNode);
first.prev = newNode;
newNode.next = first;
first = newNode;
}
else if (prev.next == null) //if at end of list
{
prev.next = newNode;
last = newNode;
}
else
{
newNode.next = prev.next;
newNode.next.prev = newNode;
prev.next = newNode;
}
}
/**
* A simple container class which holds an element, and references to
* previous and next Nodes.
*/
private static class Node<E>
{
/** The element this Node holds. */
private E element;
/** A reference to the Node before this one. */
private Node<E> prev;
/** A reference to the Node after this one. */
private Node<E> next;
/**
* Create a new Node with the given element, adjacent nodes.
*
* @param elem The element this Node holds.
* @param prev The Node which comes before this one.
* @param next The Node which comes after this one.
*/
public Node(E elem, Node<E> prev, Node<E> next)
{
this.element = elem;
this.prev = prev;
this.next = next;
}
/**
* Return a string representation of this Node.
*
* @return A string representation of this Node.
*/
public String toString()
{
return element.toString();
}
} // end class Node
} // end class DLL
Test
package lab15;
import java.util.Scanner;
public class DLLApp
{
public static void main(String[]args)
{
Scanner input = new Scanner(System.in);
DLL<String> dll = new DLL<String>();
while (input.hasNext())
{
handleLine(input.nextLine(), dll);
}
}
public static void handleLine(String values, DLL<String> list)
{
Scanner tokens = new Scanner(values);
if (tokens.hasNext("[aesdpr]"))
{
char command = tokens.next().charAt(0);
switch (command)
{
case 'a':
while (tokens.hasNext())
{
String nextToke = tokens.next();
if (tokens.hasNext())
{
list.insert(nextToke, tokens.next());
}
}
break;
case 'e':
while (tokens.hasNext())
{
String nextToke = tokens.next();
if (tokens.hasNext())
{
list.addLast(nextToke, tokens.next());
}
}
break;
case 's':
while (tokens.hasNext())
{
list.addFirst(tokens.next());
}
break;
case 'd':
if (tokens.hasNext())
{
list.delete(tokens.next());
}
break;
case 'p':
while (tokens.hasNext())
{
list.toString(tokens.next());
}
break;
case 'r':
if (list.size() != 0)
{
list.reverseString(tokens.next());
break;
}
}
}
}
}
