Java Made Easy

Calling a Method in Java Program

Calling a method in Java mainly involves of two basic steps:

1. You have to create an object to the class to which the method belongs.

Syntax for the above step : Classname obj = new Classname();

Example: First f = new First();

Here "First" is the classname and 'f' is the object created to that class.

2.Now after creating an object you can call any method using the notation "object.methodname();"

Therefore for the above example the calling goes like this : 'f.method'.

Here 'f' is the object we created.

First Java Program

Lets run our first Java Program which displays "Welcome To Java".

import java.lang.*;

class First
{
//First is the name of the Class

//"What ever we type after "//" are known as 'comments' and they wont be executed while running your Java application."

public static void main(String args[])
{
System.out.println("Welcome To Java");
}
}

Note: 'void' never returns anything.

Installing Java Software

First of all you have to install the Java software . You can download the Java software from www.java.com. After you finish the installation you may need to adjust your PATH and CLASSPATH environment variables. Instructions are included with the JDK.I put all of my stuff into a directory called java (off of my root, C:\, in Windows and off of my home directory in unix). On my windows machine, my PATH includes

"C:\j2sdk1.4\BIN;C:\j2sdk1.4\JRE\BIN".

and my CLASSPATH is set to

".;C:\JAVA" (notice the dot before the first semicolon)

Get Java Software

Important Interview Questions In Java For Freshers

-->What is the difference between '#include' we find in C language and 'import' that we find in Java?

A: '#include' makes the compiler to copy the entire header file code into a C or C++ program.So the program size increases and it wastes the memory and also the processor time

Import statement makes 'JVM' to go to java library,execute the code there and substitute the result into the Java program or application. In this case no code is copied.Therefore Import is more efficient than #include, as both memory along with processor time is saved.

-->What is Java Runtime Environment?(JRE)

A: JRE = JVM + Java Library.

-->What is uni-code system?

A:Uni-code is a standard to include the alphabet of all human languages into the character set of Java.
N:Uni-code uses two bytes to represent a single character.

-->What is the difference between Float and Double data types in Java?

A: Float can represent upto 7-digits after decimal point accurately, while double can represent upto 15-digits accurately after decimal point.

Block Diagram Of Java Virtual Machine

Class loader sub system in Java Virtual Machine(JVM) will do the following tasks.

1.It loads the dot class files into memory.

2.Then it verifies byte code instructions.

3.Finally it will allot memory needed for the program.

Note:Class loader is a piece of software that is part of JVM and it is used to read the information available in the "dot class" file and put this information in the method area.

Note: We get dot class files(these files contain 'byte code') when we compile our source code.

The memory is divided into five parts called run-time data areas.

Method Area: Method area is used for storing the information about the java classes.

Stack Space: When the JVM runs a method it stores the parameters and the local variables of the method in the memory area called as stack space.

Heap Area : Objects are created on "Heap Area".

Java Stacks: Java stacks are the places where Java methods are executed. Each method is executed on a separate frame in Java Stacks.

Program Counter Register's: These registers show memory address of the next instruction to be executed by the micro processor.

Native Method Stacks:Native method stacks are executed on native method stacks.

Execution Engine: This contains interpreter and Just in Time Compiler(JIT) which converts the byte code into the machine code so that the micro processor will execute that machine code and display the results.

Introduction To Java

Let's start our discussion from the features that Java provides.

Features of Java :

1.Simple : Java is a simple programming language. Pointers are not available in Java.
Java resembles with C,C++ languages.
In Java some features(pointers) of C language are deleted.
Reason: Pointer's lead to confusion for a programmer.
->Pointers may crash a program easily.
->Using pointers harmful programs like virus and hacking programs can be developed.

2. Object Oriented: Java is an object oriented programming language. Using java we can create objects and classes.

Definition of an Object: Any thing that exits in this real world is an Object.
An object will have ‘properties’ and perform some ‘Actions’.
Properties are represented by ‘Variables’ and ‘Actions’ are represented by ‘methods’ (nothing but the functions in C language).

Definition of a Class: A class is a group name that specifies properties and actions for a group of Objects.
A class is a ‘model’ for creating the objects.
Objects are created from a class.
Object does not exist without a class, but a class can exist without any object.
Note: In Java we must use atleast one Class during a program

3. Distributed: Information is distributed on a various computers on a network.

4. Robust: Java programs will not terminate in the middle.

5. Secure: Java enable the construction of virus-free and tamper free systems.

6.Architecture Neutral : Java’s byte code is not machine dependent.

7. Portable : Java programs give same results on all machine unlike C,C++ program’s. This is the major reason for C and C++ programs being not used on the internet as INTERNET contains different machines with different configurations.

So...what is the difference between C and Java?
Difference: C is a platform dependent while Java is an platform independent. Now you might be thinking what is platform dependent and independent?Try to compile a C program that has been developed on your computer and also try to recompile it on your friends computer. You cannot do that because, C is a platform dependent language but a Java program developed on your machine can be run on your friends machine because Java is platform independent and this is the reason that Java applications are being used on the internet.
In the next post we will see some more features of Java. Thank You!

Introduction

Hi all...this is my first post in this blog and I would like to share what I have learnt in Java! In the coming posts I'll explain Java from the basics in detail!! This may sound silly for the non-beginner's, but I want the beginner's to gain knowledge on Java from my Blog.So,we will start learning Java from my next post.Your comments are most welcome..

Thank You,

JR

Java Rocker ;)