Punarnava — purane Java ko naya Java banao, aur samjho bhi

Punarnava (पुनर्नव, "jo phir se naya ho jaye") is a free VS Code extension that turns old-style Java into modern Java 25 — side-by-side, with a plain-English explanation and a mini quiz for every single change.

Tools like OpenRewrite and IntelliJ can migrate code. They don't teach you why. Punarnava is built for students: you see your old code on the left, the modern code on the right, and a card for each change explaining what changed, which Java version added it, the mistake students usually make, and a question to check you understood.

// Before (college style)                     // After (Java 25)
import java.util.Scanner;                     void main() {
                                                  int a = Integer.parseInt(IO.readln("Enter a: ").trim());
public class Calculator {                         int b = Integer.parseInt(IO.readln("Enter b: ").trim());
  public static void main(String[] args) {        IO.println("Sum = " + (a + b));
    Scanner sc = new Scanner(System.in);      }
    System.out.print("Enter a: ");
    int a = sc.nextInt();
    System.out.print("Enter b: ");
    int b = sc.nextInt();
    System.out.println("Sum = " + (a + b));
    sc.close();
  }
}

What it modernizes

safe rules never change what your program prints. review rules can change behaviour slightly (e.g. IO.readln() reads a whole line, Scanner.nextInt() reads one token), so they are off by default — tick them in the panel once you understand the difference.

What it deliberately leaves alone

Punarnava would rather skip a change than break your code. It does not touch: code inside comments or strings · println(char[]) (IO would print [C@…) · var with the diamond <> or an interface type (the type would change) · fields · switches with real fall-through, early break, or variables shared between cases · files with a package line, extends, or no no-arg constructor (for compact files / instance main) · Scanner loops using hasNextInt().

How to use

• Open any .java file. A ✦ Punarnava line appears above main() — click it.
• Or press Ctrl+Alt+J (Cmd+Alt+J on macOS), or click the ✨ icon in the editor title bar.
• In the panel: Apply to file (undo with Ctrl+Z) · Open in new tab (your file stays untouched) · Check with javac · ▶ Run old / ▶ Run new in the terminal to compare.
• Choose a Target Java (25 / 21 / 17 / 11) to match the JDK you have installed.
• Punarnava: Open an old-Java example gives you four ready programs to try.

Needs JDK 25 or newer to run Java 25 code (void main(), IO). Pick Target Java 21/17 if your college lab has an older JDK. Punarnava: Check my JDK version tells you what you have.

How we know the output is correct

Every release runs the test suite on Windows, Linux and macOS with JDK 25: the original and the modernized program are both compiled with javac 25, run with the same input, and their output must match byte for byte. Plus unit tests for every "leave this alone" case above. Run it yourself: node test/run-tests.js.

Settings

Also works from a terminal: node cli.js OldCode.java --explain.

Made by Kanak Prabhakar · Cooding with Kanak · MIT License