Linked List

• A linked list consists of a number of links, each of which has a reference to the next link.

• Adding and removing elements in the middle of a linked list is efficient.

• Visiting the elements of a linked list in sequential order is efficient

• Random access is not efficient

linked list

Java's LinkedList class

http://java.sun.com/j2se/1.4.2/docs/api/index.html

Some of the simpler methods:

o void addFirst(Object obj) o void addLast(Object obj)o Object getFirst() o Object getSecond() o Object removeFirst() o Object removeLast()

listIterator provides a way to access all links: ListIterator listIterator(int index)

• listIterator method provides an object of type ListIterator ListIterator is an interface, so the return object from this method provides all the methods in this interface

ListIterator Interface: void add(Object o) boolean hasNext(), boolean hasPrev() Object next(), int nextIndex() Object previous(), int prevIndex() void remove(), void set(Object)

* provide a way to move forward thru list elements

A ListIterator object

hasNext() method ..

• hasNext returns true if there is a next element

LinkedList list = new LinkedList();

// other calls

ListIterator iterator = list.listIterator(0); if (iterator.hasNext())

………

next() method ..

• The next method moves the iterator

ListIterator iterator = list.listIterator(0); if (iterator.hasNext())

iterator.next();

• next throws a NoSuchElementException if you are already past the end of the list

• The next method returns the object of the link that it is passing

while (iterator.hasNext()) {

Object obj = iterator.next(); Dog mydog = (Dog)obj;

mydog.draw();

} •

other iterator methods.. • To move the list position backwards, use:

o hasPrevious o Previous

• The add method: o Adds an object after the iterator o Moves the iterator position past the new element iterator.add("Juliet");

• The remove method: o Removes ando Returns the object that was returned by the last call to next or

previous

Adding and Removing from a LinkedList

• This loop removes all objects that fulfill a certain condition

while (iterator.hasNext()) {

Object obj = iterator.next(); if (obj fulfills condition) iterator.remove();

}

File ListTest.java

• ListTest is a sample program that

o inserts elements into a list

o iterates through the list, adding and removing elements

o prints the list

File ListTest.java01: import java.util.LinkedList;

02: import java.util.ListIterator;

03:

04: /**

05: A program that demonstrates the LinkedList class

06: */

07: public class ListTest

08: {

09: public static void main(String[] args)

10: {

11: LinkedList staff = new LinkedList();

12: staff.addLast("Dick");

13: staff.addLast("Harry");

14: staff.addLast("Romeo");

15: staff.addLast("Tom");

16:

17: // | in the comments indicates the iterator position

18:

19: ListIterator iterator = staff.listIterator(); // |DHRT

20: iterator.next(); // D|HRT

21: iterator.next(); // DH|RT

22:

23: // add more elements after second element

24:

25: iterator.add("Juliet"); // DHJ|RT

26: iterator.add("Nina"); // DHJN|RT

27:

28: iterator.next(); // DHJNR|T

29:

30: // remove last traversed element

31:

32: iterator.remove(); // DHJN|T

33:

34: // print all elements

35:

36: iterator = staff.listIterator();

37: while (iterator.hasNext())

38: System.out.println(iterator.next());

39: }

40: }

Implementing Linked Lists • LinkedList class has a private inner class Link

class LinkedList {

private class Link {

public Object data; public Link next;

} }

Implementing Linked Lists • LinkedList class

o Holds a reference first to the first link

o Has a method to get the first element

Implementing Linked Lists class LinkedList {

public LinkedList() {

first = null; }

public Object getFirst() {

if (first == null) throw new

NoSuchElementException(); return first.data;

} . . . private Link first;

}

Adding a New First Element

• When a new link is added to the list o It becomes the head of the list o The old first link becomes the next

link

Adding a New First Element • The addFirst method

class LinkedList {

. . . public void addFirst(Object obj) {

Link newLink = new Link(); newLink.data = obj;

newLink.next = first; first = newLink;

} ...

}

Adding a Link to the Head of a Linked List

Removing the First Element

• When the first element is removed o The data of the first link are saved and later

returned as the method result

o The successor of the first link becomes the first link of the shorter list

o The link will be garbage collected when there are no further references to it

Removing the First Element • The removeFirst method

class LinkedList { . . .

public Object removeFirst() {

if (first == null) throw new NoSuchElementException(); Object obj = first.data; first = first.next; return obj;

} . . .

}

Removing the First Link from a Linked List

LinkedListIterator

• Private inner class of LinkedList

• Implements a simplified ListIterator interface

• Has access to the first field and private Link class

LinkedListIteratorThe LinkListIterator class

class LinkedList { . . .

public ListIterator listIterator() { return new LinkedListIterator(); } private class LinkedListIterator implements ListIterator { public LinkedListIterator() {

position = null; previous = null;

} . . . private Link position; private Link previous;

} . . .

}

LinkListIterator's next Method • position reference is advances to position.next

• Old position is remembered as previous

• If the iterator points before the first element of the list, then the old position is

null and position must be set to first

LinkListIterator's next Method private class LinkedListIterator implements ListIterator { . . .

public Object next() { if (!hasNext())

throw new NoSuchElementException(); previous = position; // remember for remove if (position == null)

position = first; else

position = position.next; return position.data; } . . .

}

LinkListIterator's hasnext Method

• The next method should only be called when the iterator is not at the end of the list

• The iterator is at the end

o if the list is empty (first == null)

o if there is no element after the current position (position.next == null)

LinkListIterator's hasnext Method

private class LinkedListIterator implements ListIterator {

. . . public boolean hasNext() {

if (position == null) return first != null;

else return position.next != null;

}

}

LinkListIterator's remove Method

• If the element to be removed is the first element, call removeFirst

• Otherwise, the link proceeding the element to be removed needs to have its next reference updated to skip the removed element

• If the previous reference is null:

o this call does not immediately follow a call to next

o throw an IllegalArgumentException

• Set previous reference to null

LinkListIterator's remove Method private class LinkedListIterator implements ListIterator { . . .

public void remove() { if (position == first)

{ removeFirst(); position = first; }

else {

if (previous == null) throw new IllegalStateException();

previous.next = position.next; position = previous;

} previous = null; }

. . . }

Removing a Link From the Middle of a Linked List

LinkListIterator's set Method • Changes the data stored in the previously visited

element • The set method

private class LinkedListIterator implements ListIterator{ . . .

public void set(Object obj) { if (position == null) throw new NoSuchElementException(); position.data = obj; } . . .

}

LinkListIterator's add Method

• Inserts the new link after the current position

• Sets the successor of the new link to the successor of the current position

LinkListIterator's add Method private class LinkedListIterator implements ListIterator{ . . . public void add(Object obj) { if (position == null) { addFirst(obj); position = first; } else { Link newLink = new Link(); newLink.data = obj; newLink.next = position.next; position.next = newLink; position = newLink; } previous = null; } . . .}

Adding a Link to the Middle of a Linked List