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Debugger

Java debugger online breakpoints, steps and watches

Stop guessing with println. Click the gutter to set a breakpoint, press Shift+F9 and step through your Java or Kotlin program line by line, with frames, threads and variables you can change while it is paused.

Start coding free Read the docs
  • Breakpoints & watches
  • Step over · into · out
  • Java and Kotlin 
public class Main {
    public static void main(String[] args) {
        int[] scores = {72, 85, 91, 64};
        int total = 0;
        for (int i = 1; i < scores.length; i++) {
            total += scores[i];
        }
        System.out.println("Average: " + total / scores.length);
    }
}

Set a breakpoint inside the loop, press Shift+F9 and watch i: the first score is never added.

Built on the tools Java and Kotlin developers already trust

Java 8 Java 11 Java 17 Java 21 Java 25 Kotlin 2 Maven Gradle Gradle Kotlin DSL Maven Central JUnit 5 Eclipse JDT google-java-format Step debugger
Debugger

What you get

Debug Java and Kotlin online: breakpoints in the gutter, step over and into code, edit variables, add watches, evaluate expressions and stop on exceptions. Free.

  • Breakpoints in the gutter

    Click next to a line number or press Ctrl+F8. Breakpoints stay with the file while you edit.

  • Step through code

    Step Over with F8, Step Into with F7, Step Out with Shift+F8 and resume with F9.

  • Frames and threads

    See the call stack of every thread and jump between frames to inspect their variables.

  • Edit variables and watch

    Change a value while the program is paused and add watches that are re-evaluated on every step.

  • Stop on exceptions

    Pause where an exception is thrown instead of reading a stack trace afterwards.

  • Input while debugging

    The console still accepts keyboard input, so interactive programs can be debugged too.

How it works

Under the hood

  1. 01

    A debugger, not a log viewer

    Debug starts the same program as Run with a debugger attached. When a breakpoint is hit the editor highlights the line, the Debug tool window shows frames, threads and variables, and you decide what runs next.

    It works for Java and Kotlin with Application and JAR Application run configurations, including programs that read from the console.

  2. 02

    Limits worth knowing

    A debug session ends after thirty minutes. Maven and Gradle goal configurations run without the debugger; debug the application configuration of the same project instead.

FAQ

Questions, answered

Is it a real step debugger?

Yes. It sets breakpoints in the running JVM, pauses threads, steps line by line and reads and changes variables.

Can I debug Kotlin?

Yes. Breakpoints and stepping work in .kt files the same way as in Java.

Can I debug a Maven or Gradle configuration?

Debugging works with Application and JAR Application configurations. Maven and Gradle projects have both, so debug the application configuration.

What are the debugger shortcuts?

Shift+F9 starts debugging, Ctrl+F8 toggles a breakpoint, F8 steps over, F7 steps into, Shift+F8 steps out and F9 resumes.

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