How Java works?

Basic Flow

• To run Hello World, we need the JVM (Java Virtual Machine).
• OS works on hardware.
• JVM works on top of OS → allows Java to be platform independent.

Platform Independence

• Write Once, Run Anywhere (WORA):

  • Code written on one machine can run on any other, if JVM is installed.

Java Code Execution Flow

1. Write Java code (.java file).
2. Compile with javac → generates bytecode (.class file).
3. JVM executes bytecode → converts to machine code (runs on OS/hardware).

Example:

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

Commands:

javac Hello.java    # Compile → generates Hello.class
java Hello          # Run → output: Hello World

Main Method Signature

• Required entry point of every Java application:

  public static void main(String[] args)

• JVM invokes main to start execution.

• Without this exact signature, program won’t run.

Diagram 1 – Code to Execution

Java Code (.java)  --javac-->  Bytecode (.class)  --JVM-->  Machine Code

JDK, JRE, JVM

• JDK (Java Development Kit): Development tools (compiler, debugger, JRE).
• JRE (Java Runtime Environment): Libraries + JVM needed to run programs.
• JVM (Java Virtual Machine): Executes bytecode.

Diagram 2 – Inside JDK

JDK
 ├── JRE
 │    ├── Libraries
 │    └── JVM
 └── Tools (javac, debugger, etc.)

⚡ Key Points:

• .java → source code
• .class → bytecode
• JVM executes bytecode
• Java is OOP, hence class is mandatory