13 x 11 java lecture 6 cs 1311x self-referential structures building a queue 13 x 11

Java Lecture 6

CS 1311X

Self-Referential Structures

Building a Queue

Self-Referential?

• Simply means that a class has a reference to an object of that class

• Common applications– Linked list nodes– Binary tree nodes

Linked Lists Nodes in Java

• Amazingly similar to a cons cell

• Stripped down version:

class Node

Object data;

Node next;

Node ref

Some Object Some Object

A Node ObjectA Node Object

More useful?class Node{

public Object data;public Node next;

public Node(Object data){

this(data, null); }

public Node(Object data, Node next){

this.data = data;this.next = next;

Node (continued)public String toString(){

return "Node: " + data;}

Pop Quizpublic String toString(){

return "Node: " + data + " Next:\n" + next;}

What does it do?What does it do?

Node ref

Some Object Some Object

Test Mainpublic static void main(String args[]){

Node n1 = new Node("Hello ");Node n2 = new Node("World!")'n1.next = n2;

System.out.println("Test1\n" + n1);System.out.println("Test2\n" + n2);

Node nz = new Node("and Nod.");Node ny = new Node("Blynken ", nz);Node nx = new Node("Wynken, ", ny);System.out.println("Test3\n" + nx);ny = null;nz = null;System.out.println("Test4\n" + nx);

Test Main OutputTest1Node: Hello Next:Node: World! Next:nullTest2Node: World! Next:nullTest3Node: Wynken, Next:Node: Blynken Next:Node: and Nod. Next:nullTest4Node: Wynken, Next:Node: Blynken Next:Node: and Nod. Next:null

Node n1 = new Node("Hello ");Node n2 = new Node("World!")'n1.next = n2;

System.out.println("Test1\n" + n1);System.out.println("Test2\n" + n2);

Node nz = new Node("and Nod.");Node ny = new Node("Blynken ", nz);Node nx = new Node("Wynken, ", ny);System.out.println("Test3\n" + nx);ny = null;nz = null;System.out.println("Test4\n" + nx);

Test Main

Node head = new Node("Bob,", new Node("Carol, ",

new Node("Ted, ",new Node("and Alice."))));

System.out.println("Test5\n" + head);

Scheme like construction

Test Main OutputTest5Node: Bob, Next:Node: Carol, Next:Node: Ted, Next:Node: and Alice. Next:null

Test Main

Node head = new Node("Bob,",new Node("Carol, ",

new Node("Ted, ",new Node("and Alice."))));

System.out.println("Test5\n" + head);

Scheme like construction

Test Main

Node list = new Node("Larry,", new Node(null,

new Node("Moe, ",new Node("and Curly."))));

System.out.println("Test6\n" + list);

} // main

} // Node

Another Scheme like construction

Test Main Output

Test6Node: Larry, Next:Node: *null* Next:Node: Moe, Next:Node: and Curly. Next:null

Test Main

Node list = new Node("Larry,",new Node(null,

new Node("Moe, ",new Node("and Curly."))));

System.out.println("Test6\n" + list);

} // main

} // Node

Another Scheme like construction

Questions?

So What's a Queue?

• First-In First-Out Data Structure• British word for line (Queue up for a pint.)• French word for tail (Like a horse's tail).• Multiple ways to implement

– Common to use Linked list– etc.

• Typical behaviors– isEmpty – enqueue– dequeue– head (or front or top or peek)

Linked List Implementation

• Can use our Node class

• Will need another class with a catchy name like Queue

• What's in the Queue class?– head pointer (reference)– tail pointer (reference)– Note: If head == null then tail == null (and vice versa)

and the Queue isEmpty!– implementation of behaviors

isEmpty

• Returns a boolean

• Something like:

return (head == null);

Let's write some code!class Queue{

private Node head;private Node tail;

public Queue(){

head = null;tail = null;

public boolean isEmpty(){

return (head == null);}

Enqueue(Object newData)• Case: isEmpty()

• Create a new Node– data points to newData

• Make head and tail point to the new Node

• Case: ! isEmpty()

• Make the old tail Node point to the new Node

• Make the tail pointer point to the new Node

Enqueue(Object newData)• Create a new Node

– data points to newData

• Case: isEmpty();

• Make head and tail point to the new Node

• Case: ! isEmpty()

• Make the old tail Node point to the new Node

• Make the tail pointer point to the new Node

Enqueue(newData): isEmpty()

Queue Object

Nota Bene

In the case of a Queue we will always make new Nodes with the next

reference set equal to null

In the case of a Queue we will always make new Nodes with the next

reference set equal to null

More useful?class Node{

public Object data;public Node next;

public Node(Object data){

this(data, null);}

public Node(Object data, Node next){

this.data = data;this.next = next;

Queue Object

newData

Queue Object

newData

Queue Object

newData

Queue Object

newData

Questions?

Enqueue(newData): ! isEmpty()

Queue Object

newData

Queue Object

newData

Queue Object

newData

Queue Object

newData

Questions?

Queue Object

newData

Queue Object

newData

Queue Object

newData

Queue Object

newData

Questions?

Let's write some more code!public void enqueue(Object o){

Node temp;temp = new Node(o);if(isEmpty()){

head = temp;tail = temp;

}else // Queue is not empty...{

tail.next = temp;tail = temp;

Almost done! Now Dequeue• Assume ! isEmpty

– we'll check

• Save value that head data reference is pointing to– (return value)

• Make head pointer point to whatever first node's next is pointing to...

• Case: head is not null

• Assume ! isEmpty– we'll check

• Save value that head data reference is pointing to– (return value)

• Make head pointer point to whatever first node's next is pointing to...

• Case: head is null• Set tail to null

Dequeue()

Queue Object

Dequeue()

Queue Object

retval

Dequeue()

Queue Object

retval

Dequeue()

Queue Object

retval

Dequeue()

Queue Object

retval

Return

Dequeue()

Queue Object

Questions?

Dequeue()

Queue Object

Dequeue()

Queue Object

retval

Dequeue()

Queue Object

retval

Dequeue()

Queue ObjectGreen

retval

Return

Dequeue()

Queue Object

Questions?

Dequeue()

Queue Object

Dequeue()

Queue Object

retval

Dequeue()

Queue Object

retval

Dequeue()

Queue Object

retval

isEmpty???

Dequeue()

Queue Object

retval

Dequeue()

Queue Object

retval

Dequeue()

Queue Object

retval

Return

Dequeue()

Queue Object

Dequeue Codepublic Object dequeue() {

Object retval;if(isEmpty()){

retval = null;}else{

retval = head.data;head = head.next;if(isEmpty()){

tail = null;}

}return retval;

Queue q;q = new Queue();q.enqueue("yada1");q.enqueue("yada2");q.enqueue("yada3");

while(! q.isEmpty()){

System.out.println(q.dequeue());}

} // main

} // Queue

Questions?

Over break...

• Rewrite class Node with– private head and tail– getHead and setHead methods– getNext and setNext methods

• Rewrite class Queue using your new Node

• Build a working scale model of the Three-Mile Island Power Plant

• Have a Merry Christmas!

What you should know about now• Syntax

– Operators– Operator overloading– Assignment statements– Control structures

• if else

• case

– Iterative structures• while

• do while

• for

• Data Types– Primitives– References

• class

• attribute– access modifiers

• public/private

– static– final/constants– initialization

What you should know about now• constructors

– access modifiers– default– chaining– overloading

• methods– access modifiers

• public/private

– static– return type/void– main method– accessors– modifiers– overloading

What you should know about now• object/instance

• Inheritance– Redefinition (Overriding)– Extension– super class– subclass– abstract

• Polymorphism

• Compilation– reference type checking– method checking– Type mismatch checking

• Run Time– interpreting– dynamic binding– Java virtual machine

13 x 11 java lecture 6 cs 1311x self-referential structures building a queue 13 x 11

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