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Lecture 1
Concepts of Java programming,
Data types,
Variables and Expressions
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Java’s lineage
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Java is related to C++, which is direct
descendent of C
From C, Java derives its syntax
Many of Java’s 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
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The creation of JAVA
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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
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The creation of JAVA (Contd.)
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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
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Why Java is Important to the Internet
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In a network, two very broad categories of objects are
transmitted between server and your PC
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passive information (i.e. email, program’s 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
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Security
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Whenever you download a program, you are risking a
viral infection
So most users don’t 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
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Java’s Magic: The Bytecode
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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
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Basics of a Typical Java Environment
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Java systems contain
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Environment
Language
APIs
Class libraries
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Basics of a Typical Java Environment (cont.)
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Java programs normally undergo five phases
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Edit
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Compile
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Verifier ensures bytecodes do not violate security
requirements
Execute
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Class loader stores bytecodes in memory
Verify
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Compiler creates bytecodes from program
Load
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Programmer writes program (and stores program on disk)
Interpreter translates bytecodes into machine language
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Fig: A typical Java environment.
Pha se 1
Edit or
Pha se 2
Com piler
Disk
Disk
Prog ram is c reat ed in
the edit or and sto red
on d isk.
Com piler c rea te s
byt ec odes and st ores
the m on disk.
Prim ary
Mem ory
Pha se 3
Class Lo ad er
Class loa de r p ut s
byt ec o des in m em ory.
Disk
.
.
.
.
.
.
Prim ary
Mem ory
Pha se 4
Byt ec ode Ve rifier
.
.
.
.
.
.
Prim ary
Mem ory
Pha se 5
Int erp ret er
.
.
.
.
.
.
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By te c ode ve rif ier
c onf irms t hat a ll
byt ec odes are v alid
and do not violat e
Ja va ’ s sec urit y
rest ric t ions.
Int erpret er read s
byt ec odes and
translat es them into a
la nguag e t hat t he
c om put er c an
und ersta nd, possib ly
st oring d at a va lues a s
the program exe c ut es.
10
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Benefits of bytecode
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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
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An overview of JAVA
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Object Oriented Programming
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Two paradigms of programming
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Process-oriented model: characterizes a
program as a series of linear steps (i.e. C)
What is happening?
 Code acting on data
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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
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Essential elements of OOP
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Abstraction (the way humans manage
complexity)
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i.e. people don’t think of a car as a set of tens of
thousands individual parts. They think of it as a welldefined 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
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Encapsulation
Inheritance
Polymorphism
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Inheritance
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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.");
}
}
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Entering the program
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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
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Compiling the Program
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C:>\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
C:>\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
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A closer look at the first program
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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 don’t 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
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Control statements
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if (condition) statement
for(initialization;condition;iteration)
statement
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Lexical issues
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White space between tokens
Identifiers
Literals (the way constant value is created)
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Integer
Floating point
Character
String
Comments
Separators
Keywords
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Data types
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Integers
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Floating-pointer numbers
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long (64 bit)
int (32 bit)
short (16 bit)
byte (8 bit)
double (64 bit)
float (32 bit)
Characters (char, 8 bit)
Booleans (boolean, true or false)
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Arrays
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Arrays in Java work differently than they do in other
languages
 First declare the array
int month_days[];
 Then allocate memory
month_days=new int[12];
Arrays can be initialized during declaration
Multidimensional arrays
int twoD[][]=new int[4][5];
Alternative array declaration
int [] a2=new int[3];
String type object
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