Online Java Code Editor for Interviews

Run a Java interview in a shared editor: the candidate writes, compiles, and runs the code, and you see every edit and result in real time. Free, no sign-up, no JDK install.

Open the Java editor
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Code sample

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Map<Character, Integer> counts = new TreeMap<>();
        for (char c : "interview".toCharArray()) {
            counts.merge(c, 1, Integer::sum);
        }
        System.out.println(counts);
    }
}

Output

{e=2, i=2, n=1, r=1, t=1, v=1, w=1}

What the editor does

  • Java 15: the code compiles and runs on the server in an isolated sandbox, and every participant sees compile errors and output.
  • Real-time collaborative editing: up to 5 participants, with edits and cursors visible instantly.
  • The problem statement is written in Markdown right in the session, next to the code.
  • Autotests: if the task has unit tests, you can run them from the session and immediately see how many passed.
  • The entry point is the main method in class Main, as in the example above; declare helper classes in the same file below it.
  • The task bank is open without an account. After a free sign-in: your own tasks, an interview mode with timers, candidate scoring, and interview history.

How to run the interview

  1. Click "Run an interview" — the session is created with a sample program, which you can replace with your own task.
  2. Describe the problem in Markdown and send the link to the candidate: they need no sign-up.
  3. Have the candidate verify the solution with calls from main and printed output — you will see any compile errors together with them.

Typical interview tasks

  • How HashMap works, and why equals and hashCode must be overridden consistently.
  • Group and count data using the Stream API and Collectors.groupingBy.
  • Implement an LRU cache based on LinkedHashMap.
  • Validate a sequence of brackets using a Deque.
  • A thread-safe counter: synchronized versus AtomicInteger.
  • Design an immutable class and explain why final fields are needed.

Limitations

  • The version is Java 15: switch expressions and text blocks are available, but records and sealed classes remain preview features in this version and do not compile without a special flag.
  • A single run is limited to 3 seconds and 64 KB of output, and JVM startup eats into that time, so heavy computations may not fit.

FAQ

Which Java version is used?
Java 15. The code compiles and runs on the server, so the version is the same for every participant.
What should I name the class with the main method?
Main, as in the example on this page, and it must come first in the file. Declare helper classes below it.
Are there autotests for Java?
Yes. A task's tests are written as test(...) calls with assertEqual(...); the Check button runs them and shows how many passed.
Do I need to sign up?
No. Create a session, send the link, and start. The task bank is open without an account too. Signing in is needed for your own tasks, interview mode, and history.
Is the standard library available?
Yes, the full Java 15 standard library: collections, the Stream API, java.time, and java.util.concurrent. There are no third-party libraries like Guava — interview tasks usually do not need them anyway.

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