Concepts of Java programming,

1
Lecture 1

• Concepts of Java programming,
• Data types,
• Variables and Expressions

2
Javas lineage

• Java is related to C, which is direct
  descendent of C
• From C, Java derives its syntax
• Many of Javas object-oriented features were
  influenced by C
• OOP is a programming methodology that helps
  organize complex programs through the use of
  inheritance, encapsulation and polymorphism

3
The creation of JAVA

• Java was developed at Sun Microsystems at 1991,
  after an effort of 18 months
• The original impetus for Java was not Internet!
  Instead the primary motivation was the need for a
  platform-independent language that could be used
  to create software to be implemented in various
  consumer electronic devices, such as microwave
  ovens and remote controls
• The trouble with other languages is that they are
  designed to be complied for a specific target

4
The creation of JAVA (Contd.)

• We know, compilation of a language requires a
  compiler targeted for that CPU, which is
  expensive and time consuming to create (.i.e
  windows and linux C compiler)
• So effort was given to develop a
  platform-independent language that could be used
  to produce code that would run on a variety of
  CPUs under differing environments. The ultimate
  result is Java
• A second force worked behind the creation of
  Java, that is the emergence of World Wide Web or
  Internet
• The code for internet required portability and
  this realization caused the focus of Java to
  switch from consumer electronics to Internet
  programming

5
Why Java is Important to the Internet

• In a network, two very broad categories of
  objects are transmitted between server and your
  PC
• passive information (i.e. email, programs code)
  
• Dynamic, active programs (i.e. self-executing
  program). Such a program is an active agent on
  the client computer, yet is initiated by the
  server (i.e. applets)
• An applet is an application designed to be
  transmitted (dynamically downloaded) over the
  Internet and executed by a Java-compatible Web
  browser.
• Applet is an intelligent program (not just an
  animator or image file or media file) that can
  react to user input and dynamically change

6
Security

• Whenever you download a program, you are risking
  a viral infection
• So most users dont download executable programs
  or scan them prior to execution
• Another type of malicious programs gather private
  information (i.e. credit card numbers, bank
  account balances, passwords etc)
• Java provides a firewall between a networked
  application and your computer
• Also Java achieves the protection by confining a
  Java program to the Java execution environment
  and not allowing it access to other parts of the
  computer

7
Javas Magic The Bytecode

• The secret behind the security and portability of
  Java is that the output of a Java compiler is not
  executable code. Rather, it is bytecode
• Bytecode is a highly optimized set of
  instructions designed to be executed by the Java
  run-time system, which is called the Java Virtual
  Machine (JVM)
• JVM is an interpreter of bytecode

8
Basics of a Typical Java Environment

• Java systems contain
• Environment
• Language
• APIs
• Class libraries

9
Basics of a Typical Java Environment (cont.)

• 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

10
Fig A typical Java environment.
11

12
Benefits of bytecode

• Translating a Java program into bytecode helps
  makes it much easier to run a program in a wide
  variety of environments
• Only JVM needs to be implemented for each
  platform
• Once the run-time package exists for a given
  system, any java program can run on it

13

• An overview of JAVA

14
Object Oriented Programming

• Two paradigms of programming
• Process-oriented model characterizes a program
  as a series of linear steps (i.e. C)
• What is happening?
• Code acting on data
• Object-oriented programming organizes a program
  around its data (that is, objects) and a set of
  well-defined interfaces to that data
• Who is being affected?
• Data controlling access to code

15
Essential elements of OOP

• Abstraction (the way humans manage complexity)
• i.e. people dont think of a car as a set of tens
  of thousands individual parts. They think of it
  as a well-defined object with its own unique
  behavior. They are free to utilize the object as
  a whole
• A complex system can be broken into more
  manageable pieces, which are hidden by
  hierarchical abstraction
• The three OOP Principles
• Encapsulation
• Inheritance
• Polymorphism

16
Inheritance
17
A First Simple Program

• /
• This is a simple Java program. Call this file
  "Example.java".
• /
• class Example
• // Your program begins with a call to main().
• public static void main(String args)
• System.out.println("This is a simple Java
  program.")

18
Entering the program

• The name of a source file is very important
• In Java, all code must reside inside a class. By
  convention, the name of that class should match
  the name of the file that holds the program
• A source file use the .java filename extension
• Java is case-sensitive

19
Compiling the Program

• Cgt\javac Example.java
• The javac compiler creates a file called
  Example.class, that contain the bytecode version
  of the program
• To actually run program, you must use the Java
  interpreter, called java
• Cgt\java Example
• When java source code is compiled, each
  individual class is put into its own output file
  named after the class and using the .class
  extension

20
A closer look at the first program

• All Java application begin execution by calling
  main()
• A complex program will have dozens of classes,
  only one of which will need to have a main()
  method to get things started
• Java applets dont have any main() method
• Java interpreter run only the class that have a
  main method
• println() is a built-in method
• System is a predefined class that provides access
  to the system, and out is the output stream that
  is connected to the console

21
Control statements

• if (condition) statement
• for(initializationconditioniteration) statement

22
Lexical issues

• White space between tokens
• Identifiers
• Literals (the way constant value is created)
• Integer
• Floating point
• Character
• String
• Comments
• Separators
• Keywords

23
Data types

• Integers
• long (64 bit)
• int (32 bit)
• short (16 bit)
• byte (8 bit)
• Floating-pointer numbers
• double (64 bit)
• float (32 bit)
• Characters (char, 8 bit)
• Booleans (boolean, true or false)

24
Arrays

• Arrays in Java work differently than they do in
  other languages
• First declare the array
• int month_days
• Then allocate memory
• month_daysnew int12
• Arrays can be initialized during declaration
• Multidimensional arrays
• int twoDnew int45
• Alternative array declaration
• int a2new int3
• String type object