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ava - programming

An Introduction

Java Short Course Day-

J

Outlines

• Packages
• Classes and Objects revisited
• Access Specifier
• Inheritance
• Borland Jubilder-

 Introduction  examples

package cont…

• Example

package my2Dgraphics;

public class Circle{

• • • • code - - - - - -

}

• Example

package bscis.semester0.xyz

public class Circle{

• • • -- - code - - - - - -

}

package cont…

• Fully qualified name

 Package name becomes part of class identity  A class name when written with its package name is termed as its fully qualified name  Example  my2Dgraphics.Circle  bscis.semester0.xyz.Circle  Directory structure reflected by two class files will be  outputpath/my2Dgraphics/Circle.class  Outputpath/bscis/semester0/xyz/Circls.class

• A package can be imported to any other program using import

statement

 Import statement should placed before start of a class  Example  import javax.swings.JOptionPane;  Import javax.swings.*;

Controlling Access to Class Member

• Class Members

 Primitive or reference variable in the class that is not part of any method in the class

• Member Access Specifier

 public  private  protected  Default access (Package access)

• Public Members

 The keyword public is written before the member to make it public  Methods or variables declared are directly accessible to other classes and objects through the object-no access restriction

• Private Members

 The keyword private is written before the member to make it private  Methods or variables declared private are not directly accessible to any other class or object

controlling access….

• Protected Members

 The keyword protected is written to make a member protected  A protected member is accessible to other classes with in the package  Outside the package only child classes can access a protected member  Not accessible to non-inherited classes outside the package

• Default or Package access

 When no access specifieris used before a class member the access of that member function or variable is default or package  Such members are accessible to other classes within the package  Such members are not accessible to other classes outside the package

controlling access….examples

Private Access

• Example:2-part

package temp;

public class Test{

private int x; public static void main (String s[]){ Test t=new Test(); t.x=90; //controversial } }

• Example:2-part package temp; public class Test2{ Test t=new Test(); public void methodx ( ){ t.x=120; //Error } }
• Example:2-part package secure; import temp.*; public class Test3{ Test t=new Test(); public void methody( ){ t.x=30; //Error } }

controlling access….examples

Protected Access

• Example:3-part package temp;

public class Test{ protected int x; public static void main (String s[]){ Test t=new Test(); t.x=90; //Accessd } }

• Example:3-part

package temp; public class Test2{ Test t=new Test(); public void methodx ( ){ t.x=120; //Accessed } }

• Example:3-part package secure; import temp.*; public class Test3{ Test t=new Test(); public void methody( ){ t.x=30; //Error non-child class } }
• Example:3-part package secure; import temp.*; public class Test3 extends Test4{ Test t=new Test(); public void methody( ){ t.x=30; //Error } }

Constructors

• Same name as that of the class
• No return type
• Primary objective is to initialize the class variables
• A constructor allocates memory to the object of a class
• Example

public class Circle{ int x,y; int radius; Circle(){ x=y=radius=0; } }

Constructor Overloading

• Same name with different parameters
• Primary objective is to provide many ways to initialize the object
• Example:

public class Circle{ int x,y; int radius; Circle(){ X=y=radius=0; } Circile(int x1,int y1){ x=x1; y=y1; } }

Inheritance

• Single Inheritance

 A class cannot have more than one immediate parents

• Parent class is also termed as super
• Inherited class is termed as child or extended class
• The keyword extends is used for inheritance
• Syntax

public class Sphere extends Cirlce{

• • • • -Code - - - - - }

Example-Inheritance

public class Point{

int x,int y; Point(){ x=y=0; } Point(int x1,int y1){ x=x1; y=y } public void showPoint(){ System.out.println(“X=“+x); System.out.println(“X=“+y); }

public class Point3D extends Point{ int z; Point(){ x=y=0; }

}

• Inherited members
  • x y show point and the two constructors

this and super

• this usually refers to the class itself
• Used to refer to variable of the class in conditions where a

parameter and class variable have same names

• super refers to the parent class
• Used to refer to variable of the parent class in conditions where a

child class and parent class have same variable or method names

• Super is also used to call the constructor of the parent class

An Introduction to JBuilder

Power on your systems and start JBuilder