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Java Programming
A Brief Intro
Overview of Java
 Java
Features
 How Java Works
 Program-Driven vs Event Driven
 Graphical User Interfaces (GUI)
Java Features
 Simple,
Object-Oriented, Familiar
 Robust and Secure
 Architecture Neutral and Portable
 High Performance
 Interpreted, Threaded, and Dynamic
Simple, OO, Familiar

Its simplicity comes from the fact that there are no
explicit pointers in Java.
 The programmer does not have to manage pointers and
the resultant problems resulting from their use

All programs in Java are based on objects
 Java uses the familiar syntax and the same
fundamental control structures of C/C++
 Complex, nuts and bolts Unix operations are
encapsulated in objects that provide easy to use
interfaces
 e.g. Sockets
Robust and Secure

Robust programs run without crashing due to
programming errors, erroneous input, or failure of
external devices. Java has many checks at
compile-time and provides run-time exception
handling to deal with unexpected events.
 Security, especially across the internet, requires
careful measures, which are implemented in Java
Architecture Neutral and
Portable
 Java
programs run on a variety of
processors using various operating systems
 Portability depends not only on architecture
but also on implementation. Java specifies
the language carefully to reduce
implementation dependencies.
Not perfectly independent, but better than most
Java API includes just about everything
High Performance
 Java
versions continually increase
performance capabilities.
 In network applications, communication
delays usually far exceed performance
delays.
Interpreted, Threaded, and
Dynamic
 Interpreted, not
compiled.
 Threaded – capable of multi_tasking and
concurrent processing, even across the
internet
 Dynamic linking to library code as it needs
it.
 Java is ideally suited for general,
interactive, and network programming
How Java Works
Java
Source
Code
Java
Compiler
Java
Byte
Code
Java Interpreter
For Processor 2
Java Interpreter
For Processor 1
Setting up Linux/Unix for Java

Make a directory called java somewhere.
 Put this java directory in your CLASSPATH in your
environment (.cshrc on Unix)
setenv CLASSPATH
“.:$home/java:whatever_is_there_already“
 this allows packages starting from your java subdirectory
Put your JDK’s bin directory in your path variable (in
the .login file?)
 Give commands to make the changes effective.

 These changes should be applied automatically on subsequent
logins
Testing Changes
prompt > java -version
java version "1.5.0_01"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_01-b08)
Java HotSpot(TM) Client VM (build 1.5.0_01-b08, mixed mode, sharing)
prompt >
Building an Application

Edit the source file either in your favorite editor,
or IDE, and save it as <file>.java
 The file name must be the same as the one and
only public class name in the file
 Compile by giving the command
javac <file>.java
at the command line of a DOS or Unix window.
 Run the program by giving the command
java <file>
hello.java
//
//
//
//
//
//
//
//
File:
Compiler:
Compile:
Executable:
Execute:
hello.java
Sun javac
javac hello.java
hello.class
java hello
Purpose:
Prints usual first
program
hello.java
public class hello
{
public static void main(String[] args)
{
int x = 3;
System.out.println(“Hello, World!” +
“\nx = “ + (x) );
}
}
Application Programs
 The
file name must be the same as the
one and only public class in the file.
 This public class must have a static
method called main (Why static??)
public static void main( String[] args)
{
local declarations and statements
}
Exercise
 Setup
your machine to compile and run a
Java Application program. Test it with
the HelloWorld program hello.java.
The example is in the java/hello subdirectory
on the instructor’s website
Basic Java Programming

The C/C++ component
 Comments – C/C++ style
 Identifiers and keywords
 Types, variables, expressions
 Control structures
 Functions

System output
 Console input
 Java is not accommodating to non-GUI input
 read() method only for characters

Packages
Identifiers and Keywords

A Java identifier must start with a letter, followed by 0 or
more letters and/or digits. Java is case-sensitive.
 Keywords cannot be used as user-identifiers. See text for a
list of keywords.
 Style – recommended & preferred; consistency is a must!





Class names begin with a capital letter
Variable names begin with a lower case letter
Function names begin with a verb which is lowercased.
Constants are all upper case.
Multiple word names are lower case except for the beginning of each
word component.
Examples
 Request
would be a class name
 myRequest would be a variable name
 getRequest() would be a function
(method) name
 THE_REQUEST would be a constant.
 The Java standard style convention
should be followed in our programming.
Standard Types
 char –
ascii or unicode
 boolean – true or false
 Numerical types – various sizes of
numbers
Numerical Types

Standard numerical types in Java are
type
size least value greatest value
________________________________________
byte
8
-128
127
short
16
-32768
32767
int
32
-2147483648
2147483647
long
64
-263
263-1
float*
32
~ -3.4 x 1038
~ 3.4 x 1038
double*
64
~ -1.7 x 10308
~ 1.7 x 10308
* 7 and 15 digit accuracy respectively
Variables and Expressions
 Java
follows the syntax of C/C++ for
expressions and assignment.
 The operators for the standard types are
the same as those for C/C++
 Remember that = is assignment and
== is equal relational operator.
 You should NOT use = in a cascading
manner.
Control Structures
The control structures are the same as C/C++
 if
 switch
 for
 while
 do – while
Note: unlike C/C++, where a test expression can
evaluate to int, a test expression MUST be of type
boolean
Functions (Methods)

In Java there are no independent functions
 A function (method) is always a member function of
some class.

The syntax is very similar.
modifier(s) resulttype name( <params>)
{
local declarations
and statements
}
// the modifier is public, private, or protected, and can also
be prefaced static
Methods: Parameters
 Different
rules than C++
technically, all pass is by value
objects not passed, their address is
passing address means in method, there are two
references to same object
 effect is same as pass by reference, as change to object
inside method changes caller’s (same) object
simple types can’t be passed by reference
 must be wrapped in an object
• problem of immutability in java.lang objects
System Output

Output is generated by using streams. The
stream classes are defined in the standard Java
package java.io.
 The class System in the package java.lang
contains three different streams for use in Java
programs:
System.in
the keyboard
System.out
the screen
System.err
the screen
 System.out.println( any string)
Examples of Output
 To
print an object, the object should have
overloaded the method toString that is
inherited from the Class Object.
 Standard types have this method.
 System.out.println(“The value of x = “ + x );
 The + is the concatenation operator for strings.
System Input

System input is quite complicated, so many
authors provide a package of IO functions for
the standard types.
 Dependence on anything proprietary defeats
the purpose of Java. A class InputReader is in
the public java directory for text input.
 it uses only pure Java to perform input

More fundamental IO will be discussed later
 Java is made for GUIs, particularly components
such as TextFields, Menus. etc;
Examples of input
 helloYou.java
 helloInt.java
 java/hello
directory on web
Things that are different

String concatenation is the operator +
 It takes two operands which are “stringable”,
that is any operand that is a string or has the
toString method overloaded for that type.
 System.out.println(76 + “ trombones”);
 Beware:
 System.out.println(23+45)
 What is output? 2345, or 68?
Packages
 Java
organizes code into packages which
correspond to directories in the file system.
 Each Java class is contained in a package.
 The default package is . (the current
directory)
Is . In your classpath????
 The
System class is found in java.lang
 The Applet class is found in java.applet
Comments
 When
calling methods of the same class, we
do not need to use the class name as a
prefix.
 When calling methods of another class, we
use the class name or an object of that class
as a prefix.
Objects
 Still
declared as a class
 No separate sections
Each declaration specified with access
public, private, protected
 static
declarations part of class, but not
object
 non-static declarations part of instantiated
objects only