Java Overview

• Developer: James Gosling at Sun Microsystems
• Released: 1995
• Philosophy: "Write Once, Run Anywhere" (WORA)
• Current Owner: Oracle Corporation

Brief History & Evolution

Key Java Features

Platform Independence

Object-Oriented

Robust and Secure

Other Important Features

Java Platform Architecture

JDK (Java Development Kit)

Includes:

• javac - Java compiler
• javadoc - Documentation generator
• jar - Archive tool
• jdb - Debugger
• JRE - Runtime environment

JRE (Java Runtime Environment)

Components:

• JVM - Java Virtual Machine
• Java Class Libraries - Core APIs
• java - Runtime launcher
• Supporting files - Configuration and resources

JVM (Java Virtual Machine)

Key Components:

• Class Loader Subsystem - Loads bytecode
• Execution Engine - Interprets/compiles bytecode
• Memory Management - Heap, stack, method area
• Garbage Collector - Automatic memory cleanup

Setting up Java Development Environment

Step 1: Download JDK

• Visit Oracle's official website or adopt OpenJDK
• Choose appropriate version for your OS
• Download the installer package

Step 2: Install JDK

• Run the downloaded installer
• Follow installation wizard
• Note the installation directory

Default locations:

• Windows: C:\Program Files\Java\jdk-version
• macOS: /Library/Java/JavaVirtualMachines/
• Linux: /usr/lib/jvm/

Step 3: Set Environment Variables

JAVA_HOME

# Windows
set JAVA_HOME=C:\Program Files\Java\jdk-11

# Linux/macOS
export JAVA_HOME=/usr/lib/jvm/java-11-openjdk
                    

PATH

# Windows
set PATH=%JAVA_HOME%\bin;%PATH%

# Linux/macOS
export PATH=$JAVA_HOME/bin:$PATH
                    

Step 4: Verify Installation

# Check Java version
java -version

# Check compiler version
javac -version
                    

Expected output:

java version "11.0.12" 2021-07-20 LTS
Java(TM) SE Runtime Environment (build 11.0.12+8-LTS-237)
Java HotSpot(TM) 64-Bit Server VM (build 11.0.12+8-LTS-237)
                    

Structure of a Java Program

Basic Java Program

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

Class Declaration

public class MyFirstProgram {
    // Class body
}
                    

• public - Access modifier
• class - Keyword to define a class
• MyFirstProgram - Class name (must match filename)
• Class body enclosed in curly braces { }

Main Method

public static void main(String[] args) {
    // Program logic goes here
}
                    

• public - Accessible from anywhere
• static - No object creation needed
• void - Returns nothing
• main - Special method name
• String[] args - Command line arguments

Output Methods

println() method

System.out.println("Hello, World!");
System.out.println(42);
System.out.println(3.14);
                    

print() method

System.out.print("Hello ");
System.out.print("World!");
// Output: Hello World!
                    

Comments in Java

Single-line comments

// This is a single-line comment
System.out.println("Hello World"); // Comment at end of line
                    

Multi-line comments

/* This is a multi-line comment
   that spans multiple lines
   and explains complex code */
System.out.println("Hello World");
                    

Compilation and Execution Process

Step 1: Write Java Code

Create a file with .java extension

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

Step 2: Compile the Code

javac HelloWorld.java
                    

• Compiler (javac) converts source code to bytecode
• Creates HelloWorld.class file
• Bytecode is platform-independent
• If compilation errors occur, fix them first

Step 3: Execute the Program

java HelloWorld
                    

• Use class name (without .class extension)
• JVM interprets and executes bytecode
• Output: Hello, World!

Complete Workflow

Source Code (.java)
        ↓
    javac compiler
        ↓
   Bytecode (.class)
        ↓
       JVM
        ↓
   Machine Code
        ↓
      Output
                        

Detailed Compilation & Execution Process

Importance of Bytecode

Key Benefits:

• Platform Independence - Same bytecode runs on any OS
• Security - Bytecode verification before execution
• Optimization - JIT compilation for better performance
• Portability - "Write Once, Run Anywhere"

Garbage Collection

How it works:

• Automatic - No manual memory deallocation
• Background Process - Runs when needed
• Unreferenced Objects - Collects unused objects
• Memory Efficiency - Prevents memory leaks

Benefits:

• Prevents memory leaks
• Improves program stability
• Simplifies memory management