Online compiler, debugger and visualizer

Online Java compiler

Write and run Java 21 in your browser with a real JDK. Read input with Scanner, split code into several classes and files, debug with breakpoints and watch your program run step by step. Useful for college lab programs, practice and interviews.

Open the Java compiler
  • Free
  • No sign-up
  • Real compilers
  • Works on phones
Main.java
import java.util.Scanner; public class Main {    public static void main(String[] args) {        Scanner in = new Scanner(System.in);        int n = in.nextInt();        long sum = 0;        for (int i = 1; i <= n; i++) sum += i;        System.out.println("Sum of 1.." + n + " = " + sum);    }}

What you can do

  • Real Java 21

    Your code is compiled and run by the real Java toolchain on our servers, not a simulation, with the exact errors you would see on your own computer.

  • Input while it runs

    Programs that read input (Scanner, input(), scanf, cin) ask for it in the console as they run, just like a terminal. Or prepare the input in advance.

  • Step-by-step debugger

    Set breakpoints, step over, into and out, and watch variables and expressions change, right in the browser.

  • Visualizer

    Replay your program line by line and see arrays, objects and the call stack drawn as it runs. Ideal for learning recursion and data structures.

  • Test cases

    Add inputs with the expected output and run them all with one click. See which lines differ, like on a coding judge, for any code you write.

  • Errors explained

    Compiler and runtime errors link to the exact line, with a plain-English explanation of what went wrong and how to fix it.

  • AI help that sees your code

    Ask why a test fails or what an error means. The assistant sees your files and the real output, and can fix the right line for you.

  • Code together live

    Share a link and code with friends or students in real time: shared cursors, edits, runs and test results.

How it works

  1. 1

    Click "Open the Java compiler". A project with a ready-to-run Main.java opens.

  2. 2

    Write your code. Files save automatically in your browser as you type.

  3. 3

    Press Run (Ctrl+Enter). If the program reads input, type it in the console, or press Debug (F5) to step through it.

  4. 4

    Add test cases to check your program against expected outputs, or share a live link to code with someone.

Open the Java compiler

Questions and answers

Which Java version is used?

Java 21 (Eclipse Temurin), the current long-term support release.

Can I use Scanner to read input?

Yes. Type the input in the console while the program runs, or prepare it in Program Input before you press Run.

Can my project have several classes?

Yes. Add as many .java files and folders (packages) as you need; the class with main() is found automatically.

Is it free?

Yes. Running, debugging, tests and live sessions are free, with no sign-up.

Where are my projects saved?

In your browser, on your device, automatically as you type. Nothing is lost when you close the tab. They stay until you delete them.

Is my code safe to run?

Every program runs in its own isolated sandbox with no internet access and strict limits on time, memory and processes, and is deleted when it finishes.

Compilers and tools