Classes and Objects in Java_Object-oriented programming pps

// GradeBook.javapublic class GradeBook { // display a welcome message to the GradeBook user public void displayMessage { System.out.println "Welcome to the Grade Book!" ; } } //

Classes and

Objects in Java

Object-oriented programming

 Classes

 Working with objects

 Attributes, methods, and access control

 Constructors

 Readings:

Java program

 A Java program is a collection of objects

 Each class is specified in one source file (file name is the same with class name)

 Every line of code you write in Java must be inside a class (not counting import directives)

 Increase modularity

 Easier to modified code, shorter compile time

// GradeBook.java

public class GradeBook {

// display a welcome message to the GradeBook user

public void displayMessage() {

System.out.println( "Welcome to the Grade Book!" );

} } // end class GradeBook

// GradeBookTest.java

public class GradeBookTest {

// main method begins program execution

public static void main( String args[] ) {

// create a GradeBook object and assign it to myGradeBook

GradeBook myGradeBook = new GradeBook();

// call myGradeBook's displayMessage method

myGradeBook.displayMessage();

} } // end class GradeBookTest

Welcome to the Grade Book!

class declared as public must be

stored in a file with the same name class declaration

begins / ends

main() is called automatically by

the Java Virtual Machine when the program is executed

02 classes

in 02 files

creates an instant / object of the class myGradeBook is the reference to it

 Objects are manipulated via references

 Object references play the roles similar to pointers

 Objects must be explicitly created by new operator

public class GradeBookTest {

// main method begins program execution

public static void main( String args[] ) {

// create a GradeBook object and assign it to myGradeBook

GradeBook myGradeBook = new GradeBook();

// call myGradeBook's displayMessage method

myGradeBook.displayMessage();

} } // end class GradeBookTest

Objects and Object references

myGradeBook

GradeBook

Heap memory

// create a GradeBook object and assign it to

myGradeBook

GradeBook myGradeBook = new GradeBook();

The object

reference the object created by

new GradeBook()

Attributes, methods,

and access control

 Access modifiers:

 Public

 Accessible anywhere by anyone

 Protected

 Accessible only to the class itself and to its subclasses or other classes in the same “package”

 Private

 Only accessible within this class

// GradeBook.java

public class GradeBook {

private String courseName; // course name for this GradeBook

// method to set the course name public void setCourseName( String name ) {

courseName = name; // store the course name

} // method to retrieve the course name

public String getCourseName() {

return courseName;

} // display a welcome message to the GradeBook user

public void displayMessage() {

System.out.println( "Welcome to the Grade Book!" );

} } // end class GradeBook

attribute

Each Gradebook object has its

own instant variable named

courseName

private:

accessed by

the class’s

methods only

access

modifiers

GradeBook

- courseName : String + setCourseName( name : String ) + getCourseName() : String

+ displayMessage()

methods

Overloading methods

 Methods can have the same name but different argument lists.

class MyDate {

…

public boolean setMonth(int m) { …}

public boolean setMonth(String s) { …}

}

…

d.setMonth(9);

d.setMonth(”September”);

 Every class has a default “method” called a Constructor

 Invoked when the object is to be “created” / “allocated” by using

“new”

 Main purposes:

 Initialise object’s attributes / data members

 A class may have multiple constructors

 Distinguished at compile time by having different arguments

 The default constructor takes no arguments and is implicit when

no other constructors are specified

// GradeBook.java

public class GradeBook

{

private String courseName; // course name for this GradeBook

// constructor initializes courseName

public GradeBook( String name )

{

courseName = name; // initializes courseName 12

}

// method to set the course name

public void setCourseName( String name )

{

courseName = name; // store the course name

}

// method to retrieve the course name

public String getCourseName()

{

return courseName;

}

// display a welcome message to the GradeBook user

public void displayMessage()

{

System.out.println( "Welcome to the Grade Book!" );

}

} // end class GradeBook

GradeBook

- courseName : String

«constructor» GradeBook( name:

String ) + setCourseName( name: String ) + getCourseName() : String

+ displayMessage()

// GradeBookTest.java

…

// create a GradeBook object and assign it to myGradeBook

GradeBook myGradeBook = new GradeBook( "CS101 Introduction to Java Programming“ );

…

Implementation vs Interface

 GradeBookTest: a “client” of GradeBook

 Implementation

 Data structures and code that implement the features (variables and methods)

 Usually more involved and may have complex inner workings

 Clients don’t need to know

 Interface

 The controls exposed to the “client” by the implementation

 The knobs on the block box

Encapsulation / information hiding

 “Don’t expose internal data structures!”

 Objects hold data and code

 Neither is exposed to the end user

 Objects expose an interface

 Anthropomorphic nature of objects

 Think of objects and people who have specialized roles!

 Lawyer, Mechanic, Doctor

 Complexity is hidden inside the object

 Make life easier for clients

 More modular approach

 Implementation changes in one component doesn’t affect others

 Less error-prone