Classes and Objects in Java_Object-oriented programming pps

Classes and
Objects in Java
Object-oriented programming
Classes and objects in
Java
2
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Outline

Classes

Working with objects

Attributes, methods, and access control

Constructors

Readings:

Java how to program, chapter 3, 8
Classes and objects in
Java
3
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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
3
Classes and objects in
Java
4
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// 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
Classes and objects in
Java
5
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5
Objects

Objects are manipulated via references

Object references play the roles similar to pointers

Objects must be explicitly created by new operator
5

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
Classes and objects in
Java
6
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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()
Classes and objects in

Java
7
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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
Classes and objects in
Java
8
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// 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 Gradeb ook 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
Classes and objects in
Java
9
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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”);
Classes and objects in
Java
10
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Constructors

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
Classes and objects in
Java
11
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// 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“ );
…
Classes and objects in
Java
12
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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
Classes and objects in
Java
13
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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