Classes and Objects

Classes and ObjectsClasses and Objects

Class: describes the form of an object, a template or blueprint or mold specifies data representation, behavior, and inheritance (via variables, methods and parents)

Object: an instance of a class — has unique copy of every non-static variable (i.e., the “instance variables” but not the class variables).

Naming Conventions:Classes: Identifiers begin with cap letters for each word in the Identifier, e.g., class NeuralNetworkObjects: Identifiers begins with lower case letter, then caps for other words in identifier, e.g., thisObjectIdentifier

Difference between “a class and an object of that class” is analogous to the difference between “a type and a variable of that type”.

Key Idea

Specification of a Classclass Box { int iLength; int iWidth; int iHeight;

public void setLength (int iNewLength) { iLength = iNewLength; } // of setLength

public int getLength ( ) { return (iLength); } // of getLength public void setWidth (int iNewWidth) { iWidth = iNewWidth; } // of setWidth

public int getWidth ( ) { return (iWidth); } // of getWidth

public void setHeight (int iNewHeight) { iHeight = iNewHeight; } // of setHeight

public int getHeight ( ) { return (iHeight); } // of getHeight

public int getVolume ( ) { return ( getLength( ) * getWidth( ) * getHeight( ) ); } // of getVolume} // of class Box

Use accessors and modifiers!

Usage tip

Box sampleBox = new Box();

sampleBox.iLength = 10;

Legal?

Preferred:

sampleBox.setLength(10);

Why?

Constructors

Constructors

• Default provided unless...

• Must have same name as class

• Must not have return type (if you want a constructor)

• Multiple constructors typical– Each must have unique signature

• number, type, order of parameters

Exampleclass Box {

int iLength;

int iWidth;

int iHeight;

public Box() {

} // Default constructor

public Box(int iL, int iW, int iH) {

iLength = iL;

iWidth = iW;

iHeight = iH;

} // Box Constructor

// class definition continues...

Comments omitted for clarity?

Example class Box {

int iLength; int iWidth; int iHeight;

public Box() {this(0, 0, 0);

} // I can even do this...

public Box(int iL, int iW, int iH) {iLength = iL;iWidth = iW;iHeight = iH;

} // Box Constructor

// class definition continues...

Creating Objects or Instances

10

Creating Instances of ClassesCreating Instances of ClassesInvolves three things:

1. Creating the reference: Box thisBox ;

2. Instantiating the object: thisBox = new Box( );OR do first two steps at once, e.g.,

Box thisBox = new Box( );

3. Having constructor(s) set initial values:

public Box (int iNewLength, int iNewWidth, int iNewHeight) {

setLength (iNewLength);setWidth (iNewWidth);setHeight (iNewHeight);

} // of constructor

With an appropriate constructor, we can do all three at once:

Box thisBox = new Box (10, 5, 25);

If you just understand one thing...

• ...it should be what we mean by a reference.– Every object created in Java is created in the

dynamic area of memory (heap).– The creation operation involves the return (to

you) of the location of the object. This is known as a reference. It is the same idea as a pointer.

Box someBox; // This creates a reference

someBox = new Box(10, 20, 30);

// We create the new box and stick its

// reference in someBox

Objects and ReferencesObjects and References

Distinguish between primitives and objects.

Assignment with Primitives:

Code: Memory:

int x; x

int y;

x = 5; 5

y = x;

yx

5 5

x

x

y

y

Objects and ReferencesObjects and References

Assignment with References to Objects:

Code: Memory:

box1box2

Box box1;Box box2;

box2 = box1;// note: two references// but only one object

box1

box2

L=8, W=5, H=7

box1 = new Box(3, 9, 2); box1

box2 L=8, W=5, H=7

L=3, W=9, H=2

box1 = box2;// Old reference lost!

L=3, W=9, H=2box1

box2 L=8, W=5, H=7

box1

box2

box1 = new Box(8, 5, 7); L=8, W=5, H=7

Questions?

Parameters

Our Next Real Challenge• Java only has “in” parameters

public void swap(int a, int b){

int temp;temp = a;a = b;b = temp;

}• We can return values from functions but

this won’t be enough.• When a parameter is a reference to an

object– We can access the object’s methods– We can make persistent changes

Pop Quiz!class Pop {

public static void swap(int x, int y) {

int t;

t = x;

x = y;

y = t;

} // swap

public static void main(String args[])

{

int a = 7;

int b = 99;

swap(a, b);

System.out.println("a = " + a + " b = " + b);

} // main

} // Pop

The pseudocode for our Example

algorithm Compute_Grade// Calculates the course grade based// on the student’s assignments

prog_avg, quiz_avg, lab_avg, exam_score, grade_avg isoftype Num

Get_Data(prog_avg, quiz_avg, lab_avg, exam_score)

grade_avg <- Average(prog_avg, quiz_avg, lab_avg, exam_score)

Output_Grade(grade_avg) // Outputs letter grade

endalgorithm

The pseudocode for Get_Data

procedure Get_Data (prog, quiz, lab, exam isoftype out Num)

// Purpose: Prompts user for data & returns them// Pre-condition: none// Post-condition: passes values back to point of

call

print(“Enter your program average:”) read (prog)

print (“Enter your quiz average:”) read (quiz)

print (“Enter your lab average:”) read (lab)

print (“Enter your exam score:”) read (exam)

endprocedure //Get_Data

How do we do this in Java?

Make a Grade class which contains a single grade value and some accessor/modifier methods.

Figure out how to do Get_Data:Pass in references to grade objects.

Get necessary values from user

Use grade objects methods to set values

public class Grade {String name;double value;

public Grade(String newName) { name = newName;

} // Constructorpublic void setValue(double v) {

value = v;}public double getValue() {

return value;}public void setName(String n) {

name = n;}public String getName() {

return name;}

} // Grade

Java code for Grades ClassJava code for Grades Class

Java code for Main Algorithmclass Compute_Grade {

<other stuff>

/** * Calc course grade based on the assignments*/public static void main(String[] args){

double grade_avg;Grade prog = new Grade(“Program”);Grade quiz = new Grade(“Quiz”);Grade lab = new Grade(“Lab”);Grade exam = new Grade(“Exam”);

get_Data(prog, quiz, lab, exam);

grade_avg = average( prog, quiz, lab, exam );

output_Grade( grade_avg );}

} // end of class Compute_Grade

Java code for Get_Data// Still inside the class Compute_Grade

/*** Purpose: Prompts user for data & returns them* Pre-condition: none* Post-condition: passes values back to point of call*/public static void get_Data(Grade g1, Grade g2, Grade g3, Grade g4){ g1.setValue( IOGadget.readDouble("Enter “ + g1.getName() ); g2.setValue( IOGadget.readDouble("Enter “ + g2.getName() ); g3.setValue( IOGadget.readDouble("Enter “ + g3.getName() ); g4.setValue( IOGadget.readDouble("Enter “ + g4.getName() );

} //Get_Data

Questions?

Generic Classes

All Container Classes you build should be Generic

• Generic classes are [almost] trivial to build– Every class is derived from the class Object

– Make this the type of the contained data

– An object of any type can then be put into the container

– You have to cast the object on output to its specific class

The “almost”:• Primitive data types [int, char, double] don’t work like this

– your first, trivial example programs need a little TLC

– each atomic data element has a corresponding wrapper class for this purpose

• e.g. the Integer class is an Object-type container for an int.

The Object Class provides Basic Methods

• clone() – Creates a clone of the object.

– Used to make a deep copy

• equals(Object) – Compares two Objects for equality.

• getClass() – Returns the Class of this Object.

• hashCode() – Returns a hashcode for this Object.

– Used for indexing this object with others

• toString() – Returns a String that represents the value of this Object.

– Used implicitly by print() to “stringify” the object

• And some others we’ll deal with later, perhaps.

Why?

Overloading Object’s Methods

• Each of these methods does something generic.• However, with the exception of getClass(), they

should all be overloaded when you write a specific class– consider them as virtual methods

• should be implemented specifically in derived classes

– generic behavior is not usually what you want• e.g. hashCode() will be unique to every instance of class Object

[some representation of its memory address].

• However, you probably want objects with identical content to return the same hashCode.

Example of a Linked-List Node

Pseudocode:

LL_Node isoftype record

data isoftype <generic type>next isoftype Ptr toa LL_Node

endrecord // LL_Nodedata

next

First a Nodepublic class Node {

private Object data;

public Node( Object d ) {setData( d );

} // Constructorpublic void setData( Object d ) { data = d; }public Object getData() { return data; }public String toString() {

return ( data.toString() );}public Object clone() {

return new Node( data );}public int hashCode() {

return data.hashCode();}

Ok?

Subtle Bug// public class Node (continued)

public static void main(String args[]){

Node n = new Node("Hello!");System.out.println("Should be Hello: " + n);n = new Node(null);System.out.println("Should be null: " + n);

} // main

}// Node

Should be Hello: Hello!Exception in thread "main" java.lang.NullPointerException: at Node.toString(Node.java:10) at java.lang.String.valueOf(String.java:1911) at java.lang.StringBuffer.append(StringBuffer.java:365) at Node.main(Node.java:25)

First a Nodepublic class Node {

private Object data;public Node( Object d ) {

setData( d );} // Constructorpublic void setData( Object d ) { data = d; }public Object getData() { return data; }public String toString() {

if(data == null)return "null";

elsereturn data.toString();

}public Object clone() {

return new Node( data );}public int hashCode() {

// need to deal with potential null here tooreturn data.hashCode();

}

Now the Linked-List Node

class LLNode extends Node {private LLNode next;

public LLNode( Object d, LLNode n ) {super( d );set_next( n );

}public void setNext( LLNode n ) { next = n; };public LLNode getNext() { return next; };public String toString() {return ("{ data = " + super.toString()

+ "; next = " + next + " } " );}

} // LLNode

Questions?