Java programming basics

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Java Programming Basic
Concepts

Learning Facilitator:
Date:

© Copyright GlobalLogic 2009 1


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Agenda

• What JAVA is for?
– Software Portability
– Why Portability Matters
– Language and Libraries

• Basic Concepts
– JVM, Classpath
– Packages
– Data Types
– JavaDoc/Comments
– Garbage Collection

• Example Java Application

3


What JAVA does for you?

• Hardware Systems
– Processor + memory + I/O devices
(Platform to execute softwares)

• Operating Systems
– An operating system (OS) is a set of computer programs that manage
the hardware and software resources of a computer.
– Runs on top of hardware, so tightly attached with the hardware

• Application Softwares
– Runs on top of operating system, a subclass of computer software
which address a particular objective e.g. Word-processors ..
– Application made for one OS/hardware system may not run on
another.
4


Why Portability?

• Softwares Portability

“Applications that are independent of all hardware
and operating systems”

You can compile a Java program on any system and run the
resulting binary executable file on the same or any other system

• “Software portability” refers to adapting a software to a new
environment, without re-developing it. This reduces the re-
development efforts.

5


Why Portability?

• There are different hardware and operating systems
• Even for one OS, there are numerous releases

• Software portability is all about “Future-proofing” your
Software investment
“Future proofing” is a process of trying to anticipate future
developments, so that appropriate actions can be taken to minimize
negative consequences.
• With application programs written in Java, you can
change/upgrade OS and applications independently

6


• Example C program
void main()
{
int arr[2];
printf(“%dn”, arr*3+);
}

• Run this on windows (using turbo c)
• Run this on Unix (using gcc)
• Compare the results

7


Language & Libraries

• A “programming language” is about
– Describing DATA
– Describing STATEMENTS that work on DATA
– Describing the ways the two can be put together
• How expressions are formed
• What statements look like
• Java is a Object-oriented, strongly typed language.
– Strongly typed languages specify one or more restrictions on how
operations involving values having different data types can be
intermixed.
• Library (package)
– Frequently reused code
– Not an executable program, but contains executable code
– Linking with program's address space
8


JVM

• A virtual machine (VM) is an abstract computer architecture
• Software on top of a real hardware
• Can run the same application on different machines where the VM
is available
Java program C program v1 C program v2

Compiler
C compiler and executor 1

JVM 1 JVM 2 JVM 3 C compiler and executor 2

Platform 1 Platform 2 Platform 3

9


• Java source code is compiled to byte-codes whose target architecture is
the Java Virtual Machine (JVM)
• Just-in-time (JIT) compiler provides compilation of byte-code to
machine code

10


11


Exercise

• Write a Java program to print “Hello World!!!” on the command
prompt
(Use notepad)

– Compile the program
– Run the program

12


Classpath

• The Classpath is an argument (Environment variable) that tells the
Java Virtual Machine where to look for user-defined classes and
packages in Java programs.

• Setting CLASSPATH from command-line
SET CLASSPATH=Dir;

• Setting CLASSPATH in Windows via Control-panel

13


Exercise

• Revisit the hello world program

• Compiling and running the program from any other directory

14


Java Packages

• A Java package is a mechanism for organizing Java classes into
namespaces.
• A package provides a unique namespace for the types it contains.
• Classes in the same package can access each other's protected
members.
• Packages are usually defined using a hierarchical naming pattern
• Example – java.lang.*, com.globallogic.*

15


Exercise

• Move the hello world program in a package
com.globallogic.training.java

• Now compile and execute the program

16


Data Types

• A data type is a set of values and the operations on those values
– For example, the Java "int" type is the set of 32-bit integers together
with the operations "+", "*", "%", etc that operate over integers
• A data type describes representation, interpretation and structure
of values manipulated by algorithms or objects stored in computer
memory or other storage device
• Java has two groups of data types, primitive data types and object
references.
– primitive data types store actual data,
– Java object references are variables which hold references to objects

17


Data Types

• Arrays
int[] anArray;
anArray = new int[10];
OR
int[] anArray = new int[10];
• String
String greeting = "Hello world!";

character array
char[] helloArray = { 'h', 'e', 'l', 'l', 'o', '.'};
String helloString = new String(helloArray);
Basic operations
length
concat
substr
• int, float
18


Basic I/O

• I/O streams
• An I/O Stream represents an input source or an output destination. A
stream can represent many different kinds of sources and destinations,
including disk files, devices, other programs, and memory arrays.
• Input stream
• Output stream

• Writing on console
System.out.println(“...”);

• Reading from console
BufferedReader in = new BufferedReader(new
InputStreamReader(System.in));
in.readLine();

19


Exercise

• To determine whether a given string is a palindrome.
– Input : a String to be read from console
– Ouptput: true/false

20


Garbage Collection

• Garbage collection (GC) is a form of automatic memory
management.

• The garbage collector or collector attempts to reclaim garbage, or
memory used by objects that will never be accessed the application

• The basic principle of how a garbage collector works is:
– Determine what data objects in a program will not be accessed in the
future
– Reclaim the resources used by those objects

• A key feature of Java is its garbage-collected heap, which takes care
of freeing dynamically allocated memory that is no longer
referenced.
21


JavaDoc/Comments

Java comments are of three types
– Line Comment //line comment
– Block Comment /* block comment */
– JavaDoc Comment
/**
* ...
*/

• Javadoc is a tool for generating API documentation in HTML format
from doc comments in source code

22


Q&A



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Java programming basics

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