CIS3023: Programming Fundamentals for CIS Majors II Summer 2010

CIS3023: Programming Fundamentals for CIS Majors IISummer 2010

Ganesh Viswanathan

Objects and Classes (contd.)

Course Lecture Slides 19 May 2010

Objects and Classes

Credits: Adapted from CIS3023 lecture slides (Spring 2010) by Dr Seema Bandyopadhyay,

University of Florida, Gainesville.

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Object InstantiationDeclaring and Creating object in two steps

Declaring and Creating object in single step

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// <ClassName> <objectRefVar>;

Circle myCircle;myCircle = new Circle();

// <ClassName> <objectRefVar> = new <ClassName>();

Circle myCircle = new Circle();

Trace Code

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Circle myCircle = new Circle(5.0);

Circle yourCircle = new Circle();

yourCircle.radius = 100;

Declare myCircle

no valuemyCircle

animation

Trace Code, cont.

: Circle radius: 5.0

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no valuemyCircle

Create a circle

animation

Trace Code, cont.


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reference valuemyCircle

Assign object reference to myCircle

animation

Trace Code, cont.


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no valueyourCircle

Declare yourCircle

animation

Trace Code, cont.


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no valueyourCircle


Create a new Circle object

animation

Trace Code, cont.


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Circle yourCircle = new Circle( );



reference valueyourCircle


Assign object reference to yourCircle

animation

Accessing ObjectsReferencing the object’s data:

Invoking the object’s method:

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// objectRefVar.datamyCircle.radius

//objectRefVar.methodName(arguments)myCircle.getArea()

Trace Code, cont.


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Circle yourCircle = new Circle( );





Assign object reference to yourCircle

animation

Trace Code, cont.


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Change radius in yourCircle

animation

Value Type vs. Reference Type

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1 Value type int i = 1 i

Reference type Circle c c reference

Created using new Circle()

c: Circle

radius = 1

Copying Variables

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i

Primitive type assignment i = j Before:

1

j

2

i

After: 2

j

2

c1

Object type assignment c1 = c2

Before:

c2

c1

After:

c2

c1: Circle radius = 5

C2: Circle radius = 9

c1: Circle radius = 5

C2: Circle radius = 9

Information Hiding

In a well designed OO application, a class publicizes what it can do i.e. its method signatures

but hides the internal details both of how it performs these services (method bodies)

and the data (attributes) that it maintains in order to

support these services

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Access Modifiers

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class Circle { private double radius;

public Circle() { this(1.0);

}

public Circle(double newRadius) { radius = newRadius; }

public double getArea() { return radius * radius * 3.14159; } }

Typically:

Attributes are declared private

Methods are declared public

Visibility Modifiers

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public– The class, data, or method is visible to any class

private – The data or methods can be accessed only by the declaring class.

Accessing private attributespublic class Driver {

public static void main(String[] args){

Circle s1, s2; s1 = new Circle(14);

s2 = new Circle(7);

System.out.println(“Radius = “, s1.radius);outcome = s2.getArea(); //ok!

}}

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Illegal: because attribute radius is hidden or private!

Accessing private attributes

How can code in any other class access them?

Programmer can provide methods to get and set them.

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Get/Set MethodsGet method or Accessor

A method that returns the value of an attributee.g., getUFID( ) { …

return studentUFID; }

Set method or MutatorA method that changes the value of an attributee.g., getUFID( ) { …

return studentUFID; }

Typically, have one get method and one set method per attribute.

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Example Class Code

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public class Circle { private double radius;

public double getRadius() {return radius;

} public void setRadius(double radius){

this.radius = radius; }

public Circle() { this(1.0); } public Circle(double r) { setRadius(r); } public getArea() { return 3.14*radius*radius; } }

Example Driver Code

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public class Driver { Circle c1 = new Circle(14); Circle c2 = new Circle(7);

System.out.println(c1.getRadius()); //ok! c1.setRadius(5); //ok! boolean outcome = s2.getArea(); //ok! }

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public class Circle { private double radius;

public double getRadius() {return radius;

} public void setRadius(double r){

if (r>0) radius = r; }

public Circle() { this(1.0); } public Circle(double r) { setRadius(r); } public getArea() { return 3.14*radius*radius; } }

Some more code

public class Student {private String name; private String ssn;private float gpa;

public Student(int i, String n) {setSsn(i); setName(n); setGpa(0.0f);

}

// other constructors and methods, not shown here

public String getName () { return name; }public void setName(String newName) { name = newName;}

public String getSsn () { return ssn; }public void setSsn(String s) { ssn = s;}

public float getGpa () { return gpa; }public void setGpa(float newGpa) { gpa = newGpa;}

}

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Still more code

public class Student {private String name; private String ssn;private float gpa;

private int numDsFs;

public Student(int i, String n) {setSsn(i); setName(n); setGpa(0.0f); setNumDsFs(0);

}

// other methods, not shown here

public boolean isOnProbation() { if(numDsFs > 3) return true;else return false; }

public String getName () { return name; }public void setName(String newName) { name = newName;}public String getSsn () { return ssn; }

public float getGpa () { return gpa; }public void setGpa(float newGpa) { gpa = newGpa;}

public void setNumDsFs(int n) { numDsFs = n; }}

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Lots more code

Benefits of Information Hiding

• Allows Data Validation

• Allows control over the level of access given for an attribute

• Simplifies Code Maintenance

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Class attributes

Each instance of a class (called an object) has a copy of the attributes

Changing an attribute in one object doesn’t affect the attribute of another object

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Class Attributes, cont.

Sometimes you may want some data to be shared among all instances.

Example: we want all Student objects to have a shared access to the total student enrollment count at the university.

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Static Attributespublic class Student {

private String name; private String ssn;private float gpa;

private static int totalNumStudents = 0;

public Student(int i, String n) {setSsn(i); setName(n); setGpa(0.0f);totalNumStudents++;

}

// other constructors, accessors/mutators, and methods, not shown here

public int getTotalNumStudents() { return totalNumStudents;

} }

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Static Attributes, cont.A static attribute is one whose value is shared by all

instances of that class.

It in essence belongs to the class as a whole.

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Static Attributes, cont.

// Client code:

Student s1 = new Student();Student s2 = new Student();Student s3 = new Student();

System.out.println(s1.getTotalNumStudents());System.out.println(s2.getTotalNumStudents());System.out.println(s3.getTotalNumStudents());

All of these println statements will print the value 3.

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Static Methods

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public class Student { private String name; private static int totalNumStudents = 0;

// other attribute details omitted ...

public static int getTotalNumStudents() { return totalNumStudents; }}

// Client codeStudent s1 = new Student();Student s2 = new Student();Student s3 = new Student(); System.out.println(Student.getTotalNumStudents());System.out.println(s1.getTotalNumStudents());

Static Methods

may only access static attributes

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class Student { private String name; // NOT static private static int totalStudents; public static void print() {

System.out.println(name + " is one of " + totalStudents + " students."); // ILLEGAL! }}

Get more info!

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• Value Types and Reference Types: http://java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html

• Creating and initializing objects: http://docstore.mik.ua/orelly/java-ent/jnut/ch03_02.htm

• (Online) Search keywords:• JAVA classes and objects• JAVA pass by reference or value

http://java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html

http://docstore.mik.ua/orelly/java-ent/jnut/ch03_02.htm

CIS3023: Programming Fundamentals for CIS Majors II Summer 2010

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