plz help me in understanding this code

#1
sweeeeeeety

Posted 28 April 2009 - 04:13 PM

Newbie

Members
2 posts

Hi,,

This was an exaple used at my skool, but I do not undestand exactly what the code is doin. The program is about linked lists. each class is supposed to do something, for example makeEmpty: akes an existing list empty and returns type void. Here is the code:

public class LinkedList2 
{ 
private ListNode head; 
private ListNode tail; 

public boolean isEmpty() 
{ 
return (head == null); 
} 

public void makeEmpty() 
{ 
head = null; 
}//makes the whole list empty 

public void add(Object x) 
{ 
ListNode newNode = new ListNode(x); 
if(head == null) 
{ 
newNode.next = null; 
head = newNode; 
tail = newNode; 
} 
else 
{ 
newNode.next = head; 
head = newNode; 
} 
}//adds element to the list 
public void addLast(Object x) 
{ 
ListNode newNode = new ListNode(x); 
newNode.next = null; 
if(head == null) 
{ 
head = newNode; 
tail = newNode; 
} 
else 
{ 
tail.next = newNode; 
tail = newNode; 
} 
}// adds object at the last 

public void addAfter(Object y, Object x) 
{ 
ListNode newNode = new ListNode(x); 
ListNode node = head; 
while(node != null) 
{ 
if(node.getData() == y) 
{ 
newNode.next = node.next; 
node.next = newNode; 
} 
node = node.getNext(); 
} 
}// adds element after the specified object 

public void remove(Object x) 
{ 
ListNode currentNode = head; 
ListNode previousNode = head; 
while (currentNode != null) 
{ 
if(head.getData() == x) 
head = head.next; 
else if (currentNode.getData() == x) 
previousNode.next = currentNode.next; 
previousNode = currentNode; 
currentNode = currentNode.next; 
} 
}// removes only the first occurence of the object in the list 

public void append(LinkedList list2) 
{ 
tail.next = list2.head; 
}//appends two lists. 

public void removeAll(Object x) 
{ 
ListNode currentNode = head; 
while(currentNode != null) 
{ 
remove(x); 
currentNode = currentNode.next; 
} 
}// removes all the occurences of the object in the list 

public void printList() 
{ 
ListNode node = head; 
while(node != null) 
{ 
System.out.println(node.getData() + ", "); 
node = node.getNext(); 
} 
}// prints the list 

public void printListRev() 
{ 
ListNode currentNode, nextNode, loopNode; 
currentNode = head; 
nextNode = head.next; 
loopNode = null; 
while(nextNode != null) 
{ 
currentNode.next = loopNode; 
loopNode = currentNode; 
currentNode = nextNode; 
nextNode = nextNode.next; 
} 
head = currentNode; 
head.next = loopNode; 
ListNode node = head; 
while(node != null) 
{ 
System.out.println(node.getData() + ", "); 
node = node.getNext(); 
} 
} 



public static void main(String [] args) 
{ 
LinkedList list = new LinkedList(); 
LinkedList list2 = new LinkedList(); 
list.add(12); 
list.add(14); 
list.add(14); 
list.add(13); 
list.add(15); 
list2.add(32); 
list2.add(45); 
list2.add(56); 
System.out.println("The list is empty : " + list.isEmpty()); 
System.out.println("Original List :"); 
list.printList(); 
System.out.println("List after calling addLast():"); 
list.addLast(16); 
list.printList(); 
System.out.println("List after calling addAfter():"); 
list.addAfter(13,9); 
list.printList(); 
System.out.println("List after calling remove():"); 
list.remove(15); 
list.printList(); 
System.out.println("List after calling removeAll():"); 
list.removeAll(14); 
//list.append(list2); 
list.printList(); 
list.append(list2); 
System.out.println("The List after calling append() : "); 
list.printList(); 
System.out.println("List after printListRev() : "); 
list.printListRev(); 
list.makeEmpty(); 
list.printList(); 
}// main function tests data 
}// class LinkedList 


public class ListNode 
{ 

public Object element; 
public ListNode next; 

public ListNode(Object theElement) 
{ 
element = theElement; 
}//ListNode constructor 

public ListNode getNext() 
{ 
return next; 
}//getNext() 

public Object getData() 
{ 
return element; 
}//getData() 
}// class ListNode

I appreciate all your help,

Thank you...

Edited by WingedPanther, 29 April 2009 - 07:43 AM.
add code tags (the # button)

#2
BlaineSch

Posted 28 April 2009 - 06:03 PM

Writes binary right handed and hex left handed

Members
2,448 posts

First two notes:

Formatting is everything
Spell things correctly you sound like a 10 year old when you dont

public class LinkedList2

	{

	private ListNode head;

	private ListNode tail;


	public boolean isEmpty()

		{

		return (head == null);

		}


	public void makeEmpty()

		{

		head = null;

		}//makes the whole list empty


	public void add(Object x)

		{

		ListNode newNode = new ListNode(x);

		if(head == null)

			{

			newNode.next = null;

			head = newNode;

			tail = newNode;

			}

		else

			{

			newNode.next = head;

			head = newNode;

			}

		}//adds element to the list

	public void addLast(Object x)

		{

		ListNode newNode = new ListNode(x);

		newNode.next = null;

		if(head == null)

			{

			head = newNode;

			tail = newNode;

			}

		else

			{

			tail.next = newNode;

			tail = newNode;

			}

		}// adds object at the last


	public void addAfter(Object y, Object x)

		{

		ListNode newNode = new ListNode(x);

		ListNode node = head;

		while(node != null)

			{

			if(node.getData() == y)

				{

				newNode.next = node.next;

				node.next = newNode;

				}

			node = node.getNext();

			}

		}// adds element after the specified object


	public void remove(Object x)

		{

		ListNode currentNode = head;

		ListNode previousNode = head;

		while (currentNode != null)

			{

			if(head.getData() == x)

			head = head.next;

			else if (currentNode.getData() == x)

			previousNode.next = currentNode.next;

			previousNode = currentNode;

			currentNode = currentNode.next;

			}

		}// removes only the first occurence of the object in the list


	public void append(LinkedList list2)

		{

		tail.next = list2.head;

		}//appends two lists.


	public void removeAll(Object x)

		{

		ListNode currentNode = head;

		while(currentNode != null)

			{

			remove(x);

			currentNode = currentNode.next;

			}

		}// removes all the occurences of the object in the list


	public void printList()

		{

		ListNode node = head;

		while(node != null)

			{

			System.out.println(node.getData() + ", ");

			node = node.getNext();

			}

		}// prints the list


	public void printListRev()

		{

		ListNode currentNode, nextNode, loopNode;

		currentNode = head;

		nextNode = head.next;

		loopNode = null;

		while(nextNode != null)

			{

			currentNode.next = loopNode;

			loopNode = currentNode;

			currentNode = nextNode;

			nextNode = nextNode.next;

			}

		head = currentNode;

		head.next = loopNode;

		ListNode node = head;

		while(node != null)

			{

			System.out.println(node.getData() + ", ");

			node = node.getNext();

			}

		}

	public static void main(String [] args)

		{

		LinkedList list = new LinkedList();

		LinkedList list2 = new LinkedList();

		list.add(12);

		list.add(14);

		list.add(14);

		list.add(13);

		list.add(15);

		list2.add(32);

		list2.add(45);

		list2.add(56);

		System.out.println("The list is empty : " + list.isEmpty());

		System.out.println("Original List :");

		list.printList();

		System.out.println("List after calling addLast():");

		list.addLast(16);

		list.printList();

		System.out.println("List after calling addAfter():");

		list.addAfter(13,9);

		list.printList();

		System.out.println("List after calling remove():");

		list.remove(15);

		list.printList();

		System.out.println("List after calling removeAll():");

		list.removeAll(14);

		//list.append(list2);

		list.printList();

		list.append(list2);

		System.out.println("The List after calling append() : ");

		list.printList();

		System.out.println("List after printListRev() : ");

		list.printListRev();

		list.makeEmpty();

		list.printList();

		}// main function tests data

	}// class LinkedList

public class ListNode

	{

	public Object element;

	public ListNode next;


	public ListNode(Object theElement)

		{

		element = theElement;

		}//ListNode constructor


	public ListNode getNext()

		{

		return next;

		}//getNext()


	public Object getData()

		{

		return element;

		}//getData()

	}// class ListNode

Watch these videos to understand nodes

the head and tail pointers point out its a double linked list - (deque I believe) which means you can add items to each side of the list and pull from both sides of the list

I believe the example my teacher gave me was lets say you have a stack of plates - basically with a deque you can add things to either side (the bottom or the top) and take a plate from either side.

Starting at the top:

you have two private "ListNodes" head and tail
you have a method to tell if the list is empty or not
then you have a method to actually make the list empty
you have a method to add to the list
- you do a question to see if its the first add or not
the next method is add last to add to the other side of the list
- does the same thing
addafter basically makes it so you can put a plate anywhere in the deque instead of the top of bottom of the stack
remove helps you remove it
append just appends two lists lol
removeall takes all the nodes off the list (start new)
printlist just shows you all the nodes you have
printlistrev just does the same thing but backwards
the long and ugly things now just adds things prints things and deletes things (shows you how it works)
then you actually have the listnode class which is usually put at the top