What is Java?

Chapter 1: Introduction to Java Programming

Welcome to the first chapter of your Java programming journey. In this chapter, we'll explore the fundamentals of Java, its purpose, architecture, and how to get started writing your first program.

---

What is Java?

Java is a platform-independent, object-oriented programming language designed to build secure, scalable, and portable applications.

---

Why Was Java Invented?

Java was invented to:

• Achieve platform independence
• Develop internet-based business applications
• Build both:
• Stand-alone Applications (SA) using Core Java
• Internet Applications (IA) using Core + Advanced Java

---

Who Invented Java?

Java was invented by James Gosling at Sun Microsystems in June 1991.
The goal was to create secure, network-based business applications.

---

Types of Applications

Java helps in building two main types of applications:

1. Stand-alone Applications (SA):
• Installed on a single computer
• Examples: Calculator, MS Office, Antivirus, Games
2. Internet Applications (IA):
• Installed on a server, accessed via internet
• Examples: Amazon, Facebook, Gmail, YouTube

---

What is a Platform?

A platform is the environment where a program is loaded, executed, and produces output.

• It includes both hardware (RAM, processor) and software (OS, JVM, etc.)
• Examples of platforms:
• Java → JVM (Java Virtual Machine)
• Python → PVM (Python Virtual Machine)
• .NET → CLR (Common Language Runtime)
• HTML → Browser

---

Platform Dependency vs. Platform Independence

Type	Meaning
Platform Dependent	Program runs only on the OS it was compiled for (e.g., C, C++)
Platform Independent	Program can run on any OS using intermediate software (e.g., Java, Python)

Java achieves platform independence using JVM, allowing the same compiled code (bytecode) to run on any OS — a principle called WORA (Write Once, Run Anywhere).

Why C and C++ Are Platform Dependent?

Why C/C++ program cannot execute in different OS?

• The C and C++ program is compiled targeting one particular OS in which we are compiling this program.

• Hence the C and C++ program compiled code contains directly Machine Language of a particular OS whose format is not understandable to different OS.

• Hence C and C++ program cannot be executed in different OS. Then C and C++ programs are called PD (Platform Dependent).

---

How Java Achieved Platform Independence?

• To achieve PI (Platform Independence), unlike in C and C++, in Java language, the program is not compiled targeting to OS and in source code also we do not write any statement specific to one OS.

• In Java, compiled code does not contain ML (Machine Language), instead it contains an intermediate code called byte code. It is JVM-understandable code, not understandable to humans or the OS.

• While execution, JVM converts the Java compiled code (bytecode) into current OS-specific ML and executes it in a Platform Independent way.

• To achieve PI, compared to C, in Java:

  • ML generation is moved from compilation phase to execution phase for generating ML for the client OS in which Java program is being executed, by introducing new compiler software, bytecode, and JVM.

  • The new compiler software converts Java source code into byte code.

  • The byte code is the Java compiled code which is not native to any OS or processor. This is native code to JVM.

  • JVM is the Java platform. It is a translation software responsible to execute Java byte code by converting it into current OS ML.

  • To run a Java program we must have JVM installed in our system.

Because we have JVM for every OS to run the same compiled code in all OS, Java program becomes platform independent.

Means Java program supports WORA (Write Once Run Anywhere).

 

• In the Java programming language, all source code is first written in plain text files ending with the “.java” extension.

• Those source files are then compiled into “.class” files by the javac compiler.

• A “.class” file does not contain code that is native to your processor; it instead contains bytecodes — the native language of the Java Virtual Machine. 

• The java launcher tool then runs your application with an instance of the Java Virtual Machine.

---

Important facts on Java program and Java software

1. Java Slogan: WORA – Write Once Run Anywhere

2. Compiler is responsible to take Java source code, verify syntaxes, if not errors, convert source into bytecode and save this bytecode in a separate new file with ".class" extension with class name as file name.

3. JVM is responsible to take Java bytecode, execute this bytecode by converting it into current OS ML. After execution, generated ML is destroyed.

4. Java supports only two extension files: .java and .class

5. In Java, we do not have a file with machine language code. Java does not support .exe file because it is platform dependent, not safe, easily affected by viruses, and mostly compiled code is downloaded from the server. Hence ML code saved in client system is useless.

6. Java compiler is OS independent because both input and output files (source code and bytecode) are Java-related files.

7. JVM is OS dependent because it must generate OS-dependent ML from the given bytecode.

8. Java software contains:

   • Compiler → PI

   • JVM → PD

   • API → PI / PD

9. Java software is platform dependent because JVM is platform dependent. Therefore, separate Java software is needed for every OS. Based on our system OS and processor, we must download and install Java software.

10. Java program is platform independent and Java software is platform dependent, both because of JVM.

    • Java program is platform independent, because JVM is available for every OS, so Java program can be executed in all OS.

    • Java software is platform dependent, because we have separate JVM for every OS, so the same Java software cannot be installed on all OS.

11. For all OS, JVM is developed by SUN (Oracle), not by OS vendors.

    • JVM will not be installed inbuilt along with OS installation.

    • We must install JVM explicitly in our computer after OS installation.

    • Programmer is responsible for developing, compiling, and executing a Java program, whereas

    • Customer/Client is responsible for only executing Java program.

    • Based on programmer and customer responsibilities, Java software is divided into two types.

Java Software Types

Java consists of multiple components. Most importantly:

• JDK (Java Development Kit) – includes compiler and JVM
• JRE (Java Runtime Environment) – runtime only (removed in Java 11)
• JVM (Java Virtual Machine) – executes bytecode
• JIT (Just-In-Time Compiler) – improves runtime performance

From Java 11 onwards, JRE is removed — JDK includes all required tools.

---

How to Install JDK?

Steps:

1. Search for “Download Latest JDK” on your browser.
2. Go to Java Downloads and select the Windows tab.
3. Download jdk.exe.
4. Install in C:\jdk-22\ (or your preferred directory).
5. Set this as JAVA_HOME.
6. Test installation using command prompt:
• javac -version → shows compiler version
• java -version → shows JVM version

---

Software Required for Java Development

Purpose	Software
Writing code	Notepad, VS Code, Eclipse, IntelliJ, etc.
Compiling and Running	JDK (includes Compiler and JVM)
Terminal	Command Prompt

---

Java Programming Elements

Java supports 8 core programming elements:

Element	Purpose
Module	Grouping packages
Package	Grouping classes
Class	Representing real-world objects
Variables	Storing data
Methods	Defining behavior or logic
Constructors	Initializing values
Blocks	Defining execution blocks
Inner Classes	Creating nested object behavior

---

Essential Java Statements

To write and run a Java program, you must include:

1. Class declaration
2. Main method
3. System.out.println(); for printing output

---

Java Program Structure

class ClassName {

    public static void main(String[] args) {

        System.out.println("Message");

    }

}

---

Steps to Create and Run a Java Program

1. Write the code:

Use Notepad or any editor to write Java code and save as Example.java.

2. Compile using Command Prompt:

javac Example.java

3. Run the Program:

java ClassName

---

Example Program 1: Print a Message

class FirstProgram {

    public static void main(String[] args) {

        System.out.println("QAWorld!");

    }

}

Command Line Steps:

D:

cd D:\FSJD\01CJ\01JB

javac Example.java

java FirstProgram

Output:

QAWorld!

---

Compilation and Execution Workflow

• Compilation (javac): Converts .java to .class (bytecode)
• Execution (java): JVM runs the bytecode from .class file

javac Example.java    → FirstProgram.class

java FirstProgram     → Output on console

---

Example Program 2: Print Your Name

class MyName {

    public static void main(String[] args) {

        System.out.println("Your Name");

    }

}

---

Speaking Language vs Programming Languages

Speaking Language	Programming Languages
1. Letters (26)	→ 1. Programming Elements (8)
2. Words (n)	→ 2. Reserved Keywords (71)
3. Sentences (n)	→ 3. Statements (19)
4. Grammar (20)	→ 4. Syntax (19)
5. Paragraph (n)	→ 5. Logic (method) (2)
6. Page (n)	→ 6. Program (class) (n)
7. Text Book (n)	→ 7. Project (n)

---

1. Programming Elements

Java supports 8 programming elements.
They are:

1. Module – A group of packages

2. Package – A group of classes

3. Class – A group of variables and methods

4. Variables – Memory for storing values

5. Methods – An operation that contains logic

6. Constructors – A kind of method used for storing values

7. Blocks – A kind of method used for storing values

8. Inner Classes – A class created inside another class for grouping sub-variables and methods

Written by: QAWorld Team