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LinkedList.java
Java
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// A linked list is a sequence of nodes with efficient
// element insertion and removal.
// This class contains a subset of the methods of the
// standard java.util.LinkedList class.
 
import java.util.NoSuchElementException;
 
public class LinkedList
{
private int count;
//nested class to represent a node
private class Node
{
public Object data;
public Node next;
}
//only instance variable that points to the first node.
private Node first;
 
// Constructs an empty linked list.
public LinkedList()
{
first = null;
count = 0;
}
 
 
// Returns the first element in the linked list.
public Object getFirst()
{
if (first == null)
{
NoSuchElementException ex
= new NoSuchElementException();
throw ex;
}
else
return first.data;
}
 
// Removes the first element in the linked list.
public Object removeFirst()
{
if (first == null)
{
NoSuchElementException ex = new NoSuchElementException();
throw ex;
}
else
{
Object element = first.data;
first = first.next; //change the reference since it's removed.
return element;
}
}
 
// Adds an element to the front of the linked list.
public void addFirst(Object element)
{
//create a new node
Node newNode = new Node();
newNode.data = element;
newNode.next = first;
//change the first reference to the new node.
first = newNode;
count++;
}
 
// Returns an iterator for iterating through this list.
public ListIterator listIterator()
{
return new LinkedListIterator();
}
 
public String toString()
{
String toString = "";
while(first != null) {
for (int i = 0; i < count; i++ )
{
toString += "{" + first.next.data + "}" ;
first.next = first.next.next;
}
}
return toString;
}
 
public boolean isEmpty()
{
boolean flag = true;
if (count == 0)
{
flag = true;
}
else
{
flag = false;
}
return flag;
}
public void addElement(int index, Object element)
{
Node newNode = new Node();
newNode.data = element;
Node current = new Node();
for (int i = 1; i < index && first.next != null; i++)
{
current.next= first.next;
}
newNode.next = current.next;
current.next = newNode;
count++;
}
public void setElement(int index, Object element)
{
Node current = new Node();
current = first;
for (int i = 1; i < index; i++)
{
current = first.next;
}
}
/*********************************************************
Add your methods here
*********************************************************/
 
 
//nested class to define its iterator
private class LinkedListIterator implements ListIterator
{
private Node position; //current position
private Node previous; //it is used for remove() method
 
// Constructs an iterator that points to the front
// of the linked list.
 
public LinkedListIterator()
{
position = null;
previous = null;
}
 
// Tests if there is an element after the iterator position.
public boolean hasNext()
{
if (position == null) //not traversed yet
{
if (first != null)
return true;
else
return false;
}
else
{
if (position.next != null)
return true;
else
return false;
}
}
 
// Moves the iterator past the next element, and returns
// the traversed element's data.
public Object next()
{
if (!hasNext())
{
NoSuchElementException ex = new NoSuchElementException();
throw ex;
}
else
{
previous = position; // Remember for remove
 
if (position == null)
position = first;
else
position = position.next;
 
return position.data;
}
}
 
// Adds an element before the iterator position
// and moves the iterator past the inserted element.
public void add(Object element)
{
if (position == null) //never traversed yet
{
addFirst(element);
position = first;
}
else
{
//making a new node to add
Node newNode = new Node();
newNode.data = element;
newNode.next = position.next;
//change the link to insert the new node
position.next = newNode;
//move the position forward to the new node
position = newNode;
}
//this means that we cannot call remove() right after add()
previous = position;
}
 
// Removes the last traversed element. This method may
// only be called after a call to the next() method.
public void remove()
{
if (previous == position) //not after next() is called
{
IllegalStateException ex = new IllegalStateException();
throw ex;
}
else
{
if (position == first)
{
removeFirst();
}
else
{
previous.next = position.next; //removing
}
//stepping back
//this also means that remove() cannot be called twice in a row.
position = previous;
}
}
 
// Sets the last traversed element to a different value.
public void set(Object element)
{
if (position == null)
{
NoSuchElementException ex = new NoSuchElementException();
throw ex;
}
else
position.data = element;
}
} //end of LinkedListIterator class
} //end of LinkedList class

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