Introduction to Java

Module Information

Module 1: Introduction to Java
Author: Srinivas Dande

Overview

Java is Simple, High Level, Platform independent, Architecture-Neutral, Secure, Robust, Multithreaded, Distributed, and Object Oriented programming language.

Java Characteristics

Simple

Java is easy to learn because most of the complex or confusing features of C and C++ like pointers, operator overloading etc are not provided in Java.

Secure

Java programs run within the JVM which protects from unauthorized or illegal access to system resources.

Platform Independent / Portable

Java program can be executed on any kind of machine containing any CPU or any operating system.

Architecture-neutral

Since Java applications can run on any kind of CPU so it is architecture-neutral.

Robust

Java is robust because of following:

• Strong memory management
• No Pointers
• Exception handling and Type checking
• Platform Independent

Multithreaded

Java supports multithreaded programming, which allows us to write programs that do many things simultaneously.

Distributed

Using RMI and EJB we can create distributed applications in java.

Object-oriented

Java follows object oriented programming model that helps to break the complex code into easy to understand objects.

Programming Languages

Language

It is the medium of communication.

Program

It is a set of instructions for a device to perform some specific task.

Programming Language

It contains set of grammatical rules or syntax to write a program for instructing machine to perform specific tasks.

Types of Programming Languages

Types

1. Low Level Programming Language
2. Middle Level Programming Language
3. High Level Programming Language

Low Level Programming Languages

• Binary digits (0,1) will be used to write the instructions in Low Level Language.
• Instructions written in binary language or native language will be understood by the machine directly.
• No translator program is required.

Problem

• It is very complex to write, modify and debug the instructions written in Binary Language.
• Binary code or Native code is platform dependent.

Middle Level Programming Languages

• Assembly codes like ADD, SUB, MUL, DIV, MOV etc. will be used to write the instructions in Middle Level Language.
• Instructions written in assembly language will not be understood by the machine directly.
• Assembler is a program that should be used to convert Assembly language to Machine language.

Problem

The code written in Assembly language will be processor dependent.

High Level Programming Languages

• Human understandable codes like English words will be used to write the instructions in High Level Language.
• Instructions written in High Level Language will not be understood by the machine directly.
• Compiler or Interpreter is a program that should be used to convert High Level Language to Machine language.

Advantages

It is easy to write, modify and debug the instructions written using High Level Language.

Program Development Models

C Language Program Development

Platform Dependent

C/C++ compiled code is platform-specific and cannot run on different operating systems.

Java Language Program Development

Platform Independent

Java bytecode can run on any platform with JVM - Write Once Run Anywhere (WORA)

Key Concepts

What is Source Code?

Human understandable code written using High Level Programming language is called as Source Code.

What is Native Code or Binary Code or Machine Code?

Machine understandable code written using Low Level Programming language is called as Native Code.

What is Byte Code?

JVM understandable code generated by Java Compiler is called as Byte Code. Byte code is also called as Magic Value.

Why C and C++ are Platform Dependent?

When you compile C or C++ program on one Operating System then compiler generates that Operating System understandable native code. This native code generated on one OS will not run on other OS directly.

Why Java is Platform Independent?

When you compile Java program on one Operating System then Java compiler generates Byte Code. You can run that Byte code on any OS which has the JVM. JVM is understanding the Byte code and generating native code for the corresponding OS. Because of Byte code and JVM, Java is Platform Independent.

WORA

Write Once Run Anywhere - Java's key feature

Performance Comparison

Which is faster: C/C++ or Java?

C or C++ program execution is faster than Java program.

Reason: Compiled code of C or C++ program contains binary code and that binary code will be executed directly whereas compiled code of Java program contains byte code and that byte code will be converted to native code first and then native code will be executed.

Java Components

Java Compiler (javac)

Java Compiler is a program developed in C or C++ programming language with the name "javac".

Responsibilities:

• Check syntactical or grammatical errors of the programs
• Convert source code to byte code

Java Interpreter (java)

Java Interpreter is a program developed in C or C++ programming language with the name "java".

Responsibilities:

• Convert byte code to native code line by line
• Execute that native code

Performance Note

Because of converting and executing line by line, Java program execution was slower in initial versions of java.

JIT Compiler

JIT (Just-In-Time) compiler is a component of the Java Runtime Environment.

JIT Compiler compiles or translates or converts the necessary part of the bytecode into machine code instead of converting line by line. Because of this, performance of Java program has improved.

JRE (Java Runtime Environment)

JRE stands for Java Runtime Environment. It is an implementation of JVM. It contains class libraries, Interpreter, JIT Compiler etc.

TIP

Only JRE is enough to run the Java program.

JVM (Java Virtual Machine)

JVM stands for Java Virtual Machine. It is a specification provided by SUN Microsystem whose implementation provides an environment to run our Java applications. JVM becomes an instance of JRE at run time.

Popular Implementations:

• SUN JRE
• IBM JRE
• Oracle JRE
• Open JDK

JDK (Java Development Kit)

It is a set of various utility programs which are required for developing and executing the java programs.

Platform Dependent

JDK is platform dependent. Various JDKs are provided for various Operating Systems.

Components:

1. Java Development Tools
   • javac
   • java
   • javap
   • jar
   • etc
2. Source Files
3. JRE

History

Java Timeline

• Early 1990s: Java was designed by Sun Microsystems to solve the problem of connecting household machines
• Initial Name: OAK
• Creator: James Gosling and team (James Gosling is the Father of Java)
• Public Release: 1995
• Current Owner: Oracle Corp (acquired Sun Microsystems)

Java Editions

Java Standard Edition (JSE)

Used to develop standalone applications using applet and swing.

Java Enterprise Edition (JEE)

Used to develop enterprise applications using Servlets, JSP, JDBC etc.

Java Micro Edition (JME)

Used to develop applications for micro devices like Mobiles, Setup Box etc.

Java Versions

Java Version	Class Format	Code Name	Released
JDK Alpha and Beta			1995
JDK 1.0		Oak	1996
JDK 1.1			1997
J2SE 1.2		Playground	1998
J2SE 1.3		Kestrel	2000
J2SE 1.4	48	Merlin	2002
J2SE 5	49	Tiger	2004
Java SE 6	50	Mustang	Dec 2006
Java SE 7	51	Dolphin	July 2011
Java SE 8 (LTS)	52		March 2014
Java SE 9	53		Sept 2017
Java SE 10	54		March 2018
Java SE 11 (LTS)	55		Sept 2018
Java SE 12	56		March 2019
Java SE 13	57		Sept 2019
Java SE 14	58		March 2020
Java SE 15	59		Sept 2020
Java SE 16	60		March 2021
Java SE 17 (LTS)	61		Sept 2021
Java SE 18	62		March 2022
Java SE 19	63		Sept 2022
Java SE 20	64		March 2023
Java SE 21 (LTS)	65		Sept 2023
Java SE 22	66		March 2024

LTS Versions

Long-Term Support versions: Java SE 8, 11, 17, 21

Installation Steps

Prerequisites

This guide covers JDK 8 installation on Windows.

Steps:

1. Run the installer file jdk-8u45-windows-x64.exe or jdk-8u66-windows-i586.exe
2. Click on NEXT button
3. Click on CHANGE button
4. Change the destination location to E:\jdk1.8.0\
5. Click on OK button
6. Click on NEXT button
7. Wait for the installation to complete
8. Select JRE destination (use defaults)
9. Click on NEXT button
10. Click on CLOSE button after completion

Directory Structure

E:
└── jdk1.8.0 (JDK Directory)
    ├── bin
    │   ├── javac.exe
    │   ├── java.exe
    │   ├── javap.exe
    │   ├── jar.exe
    │   └── etc
    ├── lib
    ├── jre (PRIVATE JRE)
    └── src.zip

C:
└── Program Files
    └── Java
        └── jre1.8.0_66 (PUBLIC JRE)

Workspace

A workspace is a directory on your hard drive where the files related to your project will be stored.

Example Workspace: D:\JLC\Core

Writing Your First Java Program

Step 1: Write Java Code

1. Open Notepad: Start → Run → notepad
2. Write the following code:

class Hello {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

3. Save as Hello.java in D:\JLC\Core

Step 2: Compile Java Program

1. Open Command Prompt: Start → Run → cmd

2. Change to workspace directory:

   cd D:\JLC\Core

3. Set PATH for JDK bin directory:

   set path=E:\jdk1.8.0\bin;%path%

4. Compile the source code:

   javac Hello.java

Result

Compiler generates Hello.class file in D:\JLC\Core

Step 3: Run Java Program

Execute the java class:

java Hello

Output

Hello World

Setting PATH Permanently

Temporary vs Permanent

Command line PATH setting is temporary and only works for that session.

To set PATH permanently:

1. Right Click on This PC → Properties
2. Click Advanced system settings
3. Click Environment Variables button
4. Select PATH → Click Edit
5. Add: E:\JDK1.8.0\bin;
6. Click OK in all windows

Comments in Java

Single Line Comments

// This is a single line comment

Multi Line Comments

/* This is a
   multi-line
   comment */

Documentation Comments

/**
 * This is a documentation comment
 * Used for generating JavaDoc
 */

Practice Questions

True/False Questions

#	Question	Answer
A	C program is platform independent	❌ False
B	Java program is platform independent	✅ True
C	Java Compiler is platform independent	❌ False
D	Java Interpreter is platform dependent	✅ True
E	JVM is platform dependent	✅ True
F	JRE is platform independent	❌ False
G	We can use Byte code in C Language	❌ False
H	Native code will be understood by Machine directly	✅ True
I	Byte code will be understood by Machine directly	❌ False
J	You can use "WORA" for C and C++	❌ False

Version Compatibility

Q: Can I run Java 9 compiled program with JRE 8?

❌ No. You will get UnsupportedClassVersionError.

Q: Can I run Java 8 compiled program with JRE 9?

✅ Yes. Backward compatibility is supported.

Version Commands

Check Java compiler version:

javac -version

Check JRE version:

java -version

Common Errors

javac is not recognized

Cause: javac.exe is not in current folder or PATH.

Solution: Set PATH to JDK bin directory.

Cannot find file

Cause: Source file not in current directory.

Solution: Navigate to correct directory or provide full path.

NoClassDefFoundError

Cause: .class file not in current folder or CLASSPATH.

Solution: Check file location and CLASSPATH settings.

Incompatible magic value

Cause: Byte code has been manipulated or corrupted.

Solution: Recompile the source file.

UnsupportedClassVersionError

Cause: Running higher version compiled bytecode with lower JRE.

Solution: Use matching or higher JRE version.

PATH and CLASSPATH

What is PATH used for?

PATH is used to access executable (.exe) files from any location.

What is CLASSPATH used for?

CLASSPATH is used to access Java .class files from any location.

Ways to set PATH/CLASSPATH

1. Temporary: At command line (current session only)
2. Permanent: In System Environment Variables

JRE Types

Private JRE vs Public JRE

• Private JRE: Installed under JDK directory
• Public JRE: Installed under Program Files

For Compilation: Private JRE is used first, then Public JRE

For Execution: Public JRE is used

Advanced Questions

Can we execute Java without setting PATH?

✅ Yes, but not recommended. You need to provide full path to executables.

JVM vs JRE difference?

JRE is an implementation of the JVM specification.

Examples: Oracle JRE, IBM JRE, Open JDK

JIT Compiler vs Java Interpreter?

• Interpreter: Converts bytecode line-by-line (slower)
• JIT Compiler: Converts chunks of bytecode at once (faster)

JIT replaced Interpreter after Java 2 for better performance.

Can I install multiple JDK/JRE versions?

✅ Yes, you can have multiple versions installed simultaneously.

Can I install only JRE without JDK?

✅ Yes, JRE alone is sufficient to run Java programs.

Can file name differ from class name?

✅ Yes, but not recommended for public classes.

Can I have multiple classes in one file?

✅ Yes, but only one can be public.

Valid main method signature?

public static void main(String[] args)

The parameter must be a String array. String args alone is invalid.

Java vs C++ Comparison

Feature	Java	C++
Pointers	❌ Not supported	✅ Supported
Multiple Inheritance	❌ Not with classes	✅ Supported
Global Variables	❌ Not supported	✅ Supported
Header Files	❌ Not used	✅ Required
Keywords (const, goto)	❌ Cannot use	✅ Can use
Destructors	❌ Not supported	✅ Supported
Virtual Functions	❌ Not supported	✅ Supported
Operator Overloading	❌ Not supported	✅ Supported
Scope Resolution	❌ No :: operator	✅ Has ::
Multithreading API	✅ Built-in	❌ Not built-in
Network Programming	✅ Built-in	❌ Not built-in
Garbage Collection	✅ Automatic	❌ Manual memory mgmt

Key Takeaway

Java prioritizes simplicity and safety by removing complex features like pointers and multiple inheritance, while providing built-in support for modern needs like multithreading and networking.