Online editor and judge

Java coding practice, checked like a placement test

Write Java in your browser, run it on the examples and submit it to hidden test cases. 580+ coding problems and 7,200+ practice exercises, from your first if-else to DSA.

Start practising free See the MentorNxt DSA 150

How it works

Write, Run, Submit

1. Write
Open any coding problem or practice exercise and write your program in the editor. It has syntax colours, auto brackets and line numbers, and saves your code as you type, so you can continue on another device.
2. Run
Run your program on the example input and see your output next to the expected output. Tick "Use my own input" to try any input you like.
3. Submit
Submit checks your program on hidden test cases, just like a placement test. You see how many passed, a box per test, and the first test that failed with its input, the expected output and yours.

Accepted? The problem is marked solved, you earn XP, and the worked solution opens so you can compare it with yours.

Your result

What each result means

The same results online judges and placement tests use, with what to check next.

Accepted: Every test passed.
Wrong Answer: Your program printed something different. Compare your output with the expected one, fix it and submit again.
Time Limit Exceeded: Your program was too slow, or never finished. Look for an endless loop or a faster approach.
Runtime Error: Your program stopped with an exception. Read the error below for the line that failed.
Compilation Error: Your code does not compile yet. Fix the first error shown, then try again.
Memory Limit Exceeded: Your program used too much memory. Look for very large arrays or collections.
Output Limit Exceeded: Your program printed far too much. Look for a print inside an endless loop.

Where to practise

Must-do problems for every topic

Each problem-solving module ends with a set of must-do problems, from easy to hard. Every practice exercise in the lessons can be run and checked too.

Coding challenges by topic MentorNxt DSA 150

FAQ

Questions, answered

Do I need to install Java to practise?

No. Your code runs on our servers with Java 21, so a browser is enough, on a laptop or a phone. For bigger projects the Getting Started lessons also set up the JDK and an editor on your computer.

Is it free?

Yes. Create a free account and every coding problem and practice exercise opens, with Run and Submit.

What are hidden test cases?

Inputs your program has not seen, with the output it must print. They check the edge cases a quick test misses: zero, negative numbers, very large values, the smallest and largest input. Placement tests and coding interviews judge your code the same way.

Why does my program pass the example but fail on Submit?

Usually an edge case (negative numbers, a single item, the largest limit) or the exact output format: an extra space inside a line, a missing line, or a different number of decimal places. Spaces at the end of a line and blank lines at the end are ignored. The first failed test shows you the input, the expected output and yours.

Can I see the solution?

Yes, once you have earned it: the worked solution opens when you solve the problem, or after three submissions. Hints open one at a time whenever you are stuck.

Does it work on a phone?

Yes. On a phone the editor adds a row of the symbols phone keyboards hide, like { } ( ) ; and Tab, so you can write code comfortably.

Solve your first problem today.

No install, no setup. Open a problem, write, run, submit.

Start practising free

Write, Run, Submit

1. Write

2. Run

3. Submit

What each result means

Must-do problems for every topic

Questions, answered

Solve your first problem today.

Write, Run, Submit

1. Write

2. Run

3. Submit

What each result means

Must-do problems for every topic

Questions, answered

Solve your first problem today.