Online compiler, debugger and visualizer
Online debugger for Java and Python
Find bugs the way professionals do, without installing an IDE. Click in the margin to set a breakpoint, press Debug, and step through your program while its variables update next to the code.
- Free
- No sign-up
- Real compilers
- Works on phones
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
Breakpoints
Click the margin or press F9. The program pauses on that line and the editor shows where you are.
Step over, into, out
F10, F11 and Shift+F11 move one step at a time, into functions and back out.
Variables and watches
See every local variable, expand arrays, lists and objects, and add watch expressions that update at each step.
Values inline
The current value of each variable appears right next to the line that uses it.
Input while debugging
Programs that read input pause for it, so you can debug interactive programs too.
Uncaught exceptions
The debugger stops where an exception is thrown, with the explanation of what went wrong.
How it works
- 1
Open the debugger and write or paste your program.
- 2
Click in the margin next to a line to add a breakpoint.
- 3
Press Debug (F5).
- 4
Step with F10 / F11 and watch the variables change.
Questions and answers
Which languages can I debug?
Java and Python. C, C++, JavaScript and TypeScript can be run and tested.
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.