Lists and Iterators

H C P T E R 21

A

Copyright © 2011 by Maria Litvin, Gary Litvin, and Skylight Publishing. All rights reserved.

Java MethodsJava MethodsObject-Oriented Programming

and Data Structures

Maria Litvin ● Gary Litvin

2nd AP edition with GridWorld

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Objectives:

• Learn to work with ListNode objects and do-it-yourself linked lists

• Understand singly-linked list, linked list with a tail, circular list, and doubly-linked list

• Learn to implement iterators

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Singly-Linked List

• Each node holds a reference to the next node

• In the last node, next is null

• A linked list is defined by a reference to its first node (often named head or front)

value 0 value 1 value 2

value ... value n-1

head

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Singly-Linked List (cont’d)public class ListNode{ private Object value; private ListNode next;

public ListNode (Object v) { value = v; next = null; }

public ListNode (Object v, ListNode nx) { value = v; next = nx; }

public Object getValue ( ) { return value; } public ListNode getNext ( ) { return next; } public void setValue (Object v) { value = v; } public void setNext (ListNode nx) { next = nx; }}

A reference to the next node

Represents a node of a singly-linked list

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Singly-Linked List — Example 1• Append x at the head of a linked list and

return the head of the new list.

public ListNode append (ListNode head, Object x) { return new ListNode (value, head); }

value 0 value 1 value 2

value ... value n-1

x

head

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Singly-Linked List — Example 2 • Assuming the list has at least two nodes,

remove the last node.

public void removeLast (ListNode head) { ListNode node = head; while (node.getNext ().getNext () != null) node = node.getNext ( ); node.setNext (null); }

null

nullA B C D

node

head

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Singly-Linked List — Traversal

public void printList (ListNode head) { for (ListNode node = head; node != null; node = node.getNext ( )) System.out.println (node.getValue ()); }

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Do-it-Yourself Iterator

public class SinglyLinkedList implements Iterable<Object>{ private ListNode head; ... public Iterator<Object> iterator () { return new SinglyLinkedListIterator (head); } ...}

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Do-it-Yourself Iterator (cont’d)public class SinglyLinkedListIterator implements Iterator<Object>{ private ListNode nextNode;

public SinglyLinkedListIterator (ListNode head) { nextNode = head; }

public boolean hasNext ( ) { return nextNode != null; }

public Object next ( ) { if (nextNode == null) throw new NoSuchElementException ( );

Object obj = nextNode.getValue ( ); nextNode = nextNode.getNext ( ); return obj; } ...}

public void remove ( ) { throw new UnsupportedOperationException( ); }

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Singly-Linked List with a Tail

• Keeps a reference to the last node

• Suitable for implementing a queue

value 0 value 1 value 2

value ... value n-1

head tail

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Doubly-Linked List

• Each node has references to the next and previous nodes

• In the last node, next is null; in the first node, previous is null.

• Can be traversed backwards

a0 a1 a2 an-1...

head tail

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Doubly-Linked Circular List

• next in the last node points to the first node

• previous in the first node points to the last node

a0 a1 a2 an-1...

head

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Doubly-Linked Circular List with a Header Node

• That’s how java.util.LinkedList is implemented

a0 a1 a2 an-1...

private ListNode2 header;

a field in the DoublyLinkedList class

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Review:

• What does an object of the ListNode class represent?

• Which fields and methods have to be added to ListNode to make it suitable for doubly-linked lists?

• What is a circular list?

• In an empty circular list with a header node, what are the values of next and previous in header?