1. Object Orientated Programming
An Introduction to Java

2. For the most accurate and up-to-date tutorials, please access the latest version from Sun's official website for the Java SE Tutorials (Last Updated 9/23/2009), which can be found at:

3. Features of OOP Classes Objects Instantiation Inheritance
Membership Functions

4. More OOP Features Data Encapsulation Polymorphism Operator Overloading
Message Passing
Software Re-Use

5. History of Java Java Java, first released by Sun Microsystems in 1994
Originally for intelligent consumer-electronic devices
Then used for creating Web pages with dynamic content
Now also used for:
Develop large-scale enterprise applications
Enhance WWW server functionality
Provide applications for consumer devices (cell phones, etc.)

6. The Emergence of Java
In the past decade Java has emerged in wide use partially because of its similarity to C and to C++
Perhaps more important is its implementation using a virtual machine that is intended to run code unchanged on many different platforms.
This last feature has made it very attractive to larger development shops with heterogeneous environments.

7. Key Benefits of Java Java is “write once, run anywhere”
architecture neutral
portable across different platforms
Due to Java Virtual Machine (JVM)
Security features
highly configurable security levels prevent any piece of Java code doing harm to the host system
Network-centric platform
easy to work with resources across a network and to create network based applications
Object Oriented
an interacting collection of independent software components
dynamic extensible programs

8. Key Benefits of Java Internationalisation
uses 16 bit Unicode characters that represents the phonetic and ideographic character sets of the entire world
Performance
although an interpreted language Java programs run almost as fast as native C, C++ programs
Simple and easy to develop
powerful & well designed set of APIs

9. Java Class Libraries Classes Include methods that perform tasks
Return information after task completion
Used to build Java programs
Java contains class libraries
Known as Java APIs (Application Programming Interfaces)

10. Basics of a Typical Java Environment
Java programs normally undergo five phases
Edit
Programmer writes program (and stores program on disk)
Compile
Compiler creates bytecodes from program
Load
Class loader stores bytecodes in memory
Verify
Verifier ensures bytecodes do not violate security requirements
Execute
Interpreter translates bytecodes into machine language

11. Typical Java environment.
Primary
Memory .
Disk
Editor
Compiler
Class Loader
Program is created in an editor and stored on disk in a file ending with .java.
Compiler creates bytecodes and stores them on disk in a file ending with .class.
Class loader reads .class files containing bytecodes from disk and puts those bytecodes in memory.
Phase 1
Phase 2
Phase 3
Bytecode Verifier
Bytecode verifier confirms that all bytecodes are valid and do not violate Java’s security restrictions.
Phase 4
Interpreter
Interpreter reads bytecodes and translates them into a language that the computer can understand, possibly storing data values as the program executes.
Phase 5
Typical Java environment.

12. Java In discussing java it is important to distinguish between :
The Java Programming Language
The Java Virtual Machine
The Java Platform

13. Java
The Java Programming Language is the language in which Java applications, applets , servlets and components are written. It is an object orientated language like C++.
The Java Virtual Machine, (or java interpreter) is the crucial piece of every Java installation. Java applications are portable, but they are only portable to platforms to which a java virtual machine ( or java interpreter ) has been ported.
3. The Java Platform . All programs written in Java rely on the set of predefined classes that comprise the java platform. These classes are organised into related groups known as packages. The java platform defines packages for functionality such as input/output, networking, graphics, user interface creation.

14. JVM Compiled by Java compiler Interpreted by JVM myCode.class Bytecode …
myCode.class
Bytecode
Compiled by
Java compiler
class myCode { …
… }
myCode.java
Interpreted by JVM
Source Code
Application runs

15. JVM
JVM provides the run time environment for the bytecode (Java Runtime Environment JRE)
executes the bytecode and causes native machine code instructions to execute on the CPU that the JVM is on
 each target platform needs an implementation of the JVM

16. Java Compiler myCode.java Source Code Errors Byte Code Compiler
class myCode { …
… }
myCode.java
Byte Code
Compiler
Source Code
Interpreter for Computer A (JVM)
Interpreter for Computer B (JVM)
Interpreter for Computer C (JVM)

17. Operation of the JRE (Java Runtime Environment)
Compile
Class Loader
java
______
________
myCode.java
Bytecode verifier
Load from hard disk, network or other source
javac
Interpreter
______
________
myCode.class
RunTime
Hardware

18. JRE
At runtime the java bytecode that makes up a java software program are
Loaded, checked and run in an interpreter
In the case of applets you can download the bytecode and they are then interpreted by the JVM built into the browser.
Bytecode verifier checks
Attributes correctly typed
Body of the methods check
Type checking
References to other types checked

19. Java Applications vs Java Applets
There are two categories of Java Programs
A Java Application –
These are stand alone programs in the traditional sense
Run independently
Run under the java interpreter
Java applets
These are programs designed to run from browsers such as Netscape or Sun’s HotJava.
They don’t run independently
Programs that can be included in web documents.

20. First Java Program (Application)
// First Program HelloWorld
public class HelloWorld {
public static void main(String args []){
System.out.println(“Hello world...”): } }
A class definition is the basic block of a Java program
Each program must have a main method to tell it where to start executing

21. Notes // First Program HelloWorld Comments begin with //
Can also use C style comments
/* First Program HelloWorld */

22. Notes 2 public class HelloWorld
This begins a class definition for class HelloWorld
These classes are known as Programmer defined classes or User defined classes
The public and class terms are java keywords which are always lowercase
HelloWorld is the class identifier which always start with an uppercase letter

23. Notes 3
When you save your class definition in a file the filename must be the same as the class identifier with a .java extension
So the HelloWorld class would be stored in a HelloWorld.java file

24. Notes 4 public static void main(String args [])
Java applications automatically begin executing at main
Parentheses () after main indicate a program piece called a method which define some operational component of the class.
Void indicates that the method will perform a task without returning any information when it completes
String args [] are used as input arguments to the method. In this case we don’t use input but java requires it anyway

25. Notes 5 System.out.println(“Hello world...”):
System.out is called the standard output object
This will display a line of text in the command window
In Java , any source and destination for I/O is considered a stream of bytes or characters.
To perform output we insert bytes or characters into a stream. To perform input we extract bytes or characters from a stream.
java.lang.System class contains three predefined streams
System.out
System.err for errors
System.in

26. A First Java Program
This is a Java application not a Java applet – indicated by presence of main
Source code exists in a file with same name as the class and an extension of .java. So for the previous example it is HelloWorld.java
Java is case sensitive (like C.)

27. Using the JDK Create source files for each class in your program
The name of source file should be the same as the name of class
public class myCode { …
… }
myCode.java
Source File

28. Compiling your source code
Compile each class source file into bytecode (class files)
To compile a java source file
javac myCode.java
This creates a classfile called myCode.class …
myCode.class
Class File

29. To run your program
To start your program running you run the bytecode of the program control class
The program control class has the main method
To run bytecode – pass it to the JVM
java classFileName
e.g. java myProg
note no .class included