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Object Orientated Programming

Object Orientated Programming. An Introduction to Java. 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: http://java.sun.com/docs/books/tutorial.

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Object Orientated Programming

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  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: http://java.sun.com/docs/books/tutorial.

  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. Primary Memory Primary Memory Primary Memory Program is created in an editor and stored on disk in a file ending with .java. Editor Phase 1 Disk Compiler creates bytecodes and stores them on disk in a file ending with .class. Compiler Phase 2 Disk Class Loader Phase 3 Class loader reads .class files containing bytecodes from disk and puts those bytecodes in memory. Disk Bytecode Verifier Phase 4 Bytecode verifier confirms that all bytecodes are valid and do not violate Java’s security restrictions. Interpreter reads bytecodes and translates them into a language that the computer can understand, possibly storing data values as the program executes. Interpreter 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 Languageis 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. 1001100101001 … … Interpreted by JVM class myCode { … … … …} Bytecode myCode.class myCode.java JVM Compiled by Java compiler 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. class myCode { … … … …} myCode.java Java Compiler Errors Byte Code Compiler Source Code Interpreter for Computer A (JVM) Interpreter for Computer B (JVM) Interpreter for Computer C (JVM)

  17. ______ ________ ______ ________ myCode.class myCode.java Operation of the JRE (Java Runtime Environment) Runtime Class Loader Compile java Bytecode verifier Load from hard disk, network or other source javac Interpreter 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) • 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 // First Program HelloWorld public class HelloWorld { public static void main(String args []){ System.out.println(“Hello world...”): }}

  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 1001101001110101011 … … … … 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

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