Concepts of OOP

Lecture 3

Object Oriented ProgrammingEastern University, Dhaka

Md. Raihan Kibria

Difference between a Class and an Object

A class A class is a template Definition of a class is only given during design/coding

time A class can be compared to a struct in c

An object An object is an instance of a class An object is made only during run time An object is made from the definition of a class

An example

Class definition:

public class A{

String code; String name;

}

Code to make a new instance:A a = new A();

Object/instance

So far we have defined a class and written code to instantiate it. But we still have not created the object.

Compile: javac A.java Run: java A.class

Only now an object or instance of class A is created.

JVM creates the instance or object based on class definition found in the byte-code

Some rules for creating class in java

A class can be defined in any file. However, a public class must be in a file name that matches the class name.

Example of a public class:public class Student{ String code; String name;}file name: Student.java

Some rules for creating class in java

Example of non-public class:class A{ String name; String code;}

Class B{ int age; String program;}

Fliename could be anything, such as Example.java

Some rules for creating class in java

Please note that many non-public classes may reside in one file.

But a file can have only one public class. A file can have one public class and many

non-public classes.

Packages

Packages are folders, roughly speaking. Typically, “package” is the first line in any

java program. Many java files can be placed under a

package. Package is a way to organize codes.

Example

package com.abc;public class A{ String ...}

This means the file A.java is under folder com/abc

Example

public class B{ String ...}

This means the file B.java is not under any folder

Compiling packaged classes

Suppoer A.java is under com folder. The first line of A.java is: package com;

To compile issue commandjavac com/A.java The output A.class file will be produced

under com folder To run issue command: java com.A

Compiling non-packaged classes

Suppose B.java has no package definition To compile issue command:javac B.javaTo run issue command java B

Compiling a package

To compile all files under package issue command:

javac com/*.java

To compile multiple packages issue command:

javac com/*.java com/abc/*.java