Java Introduction

Java is a general-purpose, object-oriented, statically-typed, compiled-to-bytecode programming language created by James Gosling at Sun Microsystems in 1995 (now owned by Oracle). It runs on the JVM (Java Virtual Machine) — so the same compiled .class file runs on Windows, Linux, macOS, and Android.

In simple words:

• Java is C++ with a safety net — adds garbage collection, no raw pointers, and a huge standard library.
• It compiles to bytecode (not native code) and the JVM JIT-compiles it to machine code at runtime.
• Famous slogan: "Write Once, Run Anywhere" (WORA) — one binary, every platform with a JVM.
• Powers everything from Android apps and banking systems to Netflix, LinkedIn, Spotify, and Minecraft.

• Job market — #1 enterprise language. Banks, fintech, telecom, and government all run on Java.
• Android — native Android apps are Java/Kotlin (Kotlin compiles to JVM bytecode too).
• Beginner-friendly — strong types catch bugs at compile time, not in production.
• Massive ecosystem — Spring, Hibernate, Maven, Gradle, JUnit, IntelliJ.
• Performance — JIT-compiled, often within 10% of C++ for server workloads.
• Career stability — Java skills are in demand for the next 20 years (banks won't rewrite COBOL/Java overnight).

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

Hello, Codekilla!

• public class HelloWorld { ... } — every Java program lives inside a class. The file name must match the public class name (HelloWorld.java).
• public static void main(String[] args) — the entry point. The JVM looks for this exact signature.
  • public — accessible from outside the class.
  • static — belongs to the class, no instance needed.
  • void — returns nothing.
  • String[] args — command-line arguments as an array.
• System.out.println("..."); — prints a line (println = print + newline) to standard output.
• Every statement ends with a semicolon ; — Java is strict about it.

bash
javac HelloWorld.java   # produces HelloWorld.class (bytecode)
java HelloWorld         # runs the class on the JVM (no .class extension!)

• File name mismatch — public class HelloWorld must live in HelloWorld.java. Capital matters.
• Forgetting ; — every statement ends with a semicolon. The compiler points at the next line.
• Wrong case — Public or Static won't compile. Java is case-sensitive.
• Running java HelloWorld.class — drop the .class. Pass the class name, not the file.
• No main method — Error: Main method not found in class ....

1. Hello on three lines — Print your name, age, and city on three separate lines using three println calls. Hint: three statements, each ending in ;.
2. Filename check — Save a file as Hello.java containing public class HelloWorld. Compile it — note the error. Hint: rename the file to match the class.
3. Compile + run — Use javac and java on your machine and confirm output matches. Hint: install JDK 17+ first.
4. No static — Remove static from main and try to run. Read the error. Hint: JVM can't call non-static main without an instance.

💡 Think Like a Programmer: Compile errors are your friend. They catch typos and type mismatches before users do. Read them top-to-bottom — the first error often causes the rest.