Online Code Compiler — Run Python, Java, C++ and JavaScript Free
An online code compiler lets you write, run, and test code directly in your browser without installing a development environment. The free compiler on PublicSoftTools supports JavaScript, Python 3, Java, and C++ — with stdin support and separate stdout and stderr output panels.
Supported Languages
| Language | Runtime | Startup | Read stdin with | Notes |
|---|---|---|---|---|
| JavaScript | Browser Web Worker | Instant | readline() | No network request; 10 s timeout |
| Python 3 | Piston API | 1-3 s | input() | Standard library available |
| Java | Piston API | 2-4 s | Scanner | Class must be named Main |
| C++ | Piston API | 2-4 s | cin | #include headers work normally |
How to Use the Online Code Compiler
- Open the online code compiler.
- Select a language from the tabs at the top: JavaScript, Python, Java, or C++.
- Write or paste your code into the editor. Press Tab to insert 4 spaces.
- If your program reads input, click Show stdin and type your input values (one per line).
- Click Run. Stdout appears in green; stderr (errors) appears in red below it.
Use Cases for an Online Compiler
| Use case | How the online compiler helps |
|---|---|
| Homework / assignments | Write and test code without setting up a local environment — ideal for lab computers or shared devices |
| Algorithm practice | Test solutions to coding problems instantly; check for off-by-one errors with custom stdin |
| Teaching and demos | Share a snippet and let students run it immediately without environment issues |
| Quick syntax checks | Verify whether a specific language construct compiles and runs as expected before adding it to a larger project |
| Comparing languages | Run the same algorithm in Python, Java, and C++ side by side and compare output and execution time |
Advanced Workflows
Testing with multiple inputs
Open the stdin panel and enter all your inputs before running. Each line is a separate input. For a program that reads three integers, enter them on three separate lines. The program receives them in order, exactly as if a user typed them at a terminal.
Comparing algorithm performance
The execution time shown in the output header (in milliseconds) can be used to compare algorithm efficiency. Run a bubble sort on 1000 elements, note the time, then run a merge sort on the same input. The difference makes time complexity concrete — not just theoretical.
Debugging with stderr
Compilation errors and runtime exceptions appear in the red stderr panel, separate from program output. For Python, the full traceback is shown. For Java, the compilation error message points to the exact line. For C++, compiler errors from GCC are displayed verbatim.
JavaScript vs other languages
JavaScript runs in a Web Worker directly in the browser — there is no network request and startup is instant. Python, Java, and C++ are sent to the Piston execution service, which adds 1-4 seconds for network round-trip and runtime startup. For quick iteration, write logic in JavaScript first, then port to a compiled language.
Common Questions
Why does my Java code fail to compile?
The Java class must be named Main. The compiler looks for public class Main as the entry point. If your class has a different name (e.g. Solution or HelloWorld), rename it to Main and the compilation will succeed.
Can I import external libraries?
Only the standard library is available. Python can use math, collections, itertools, json, and other built-in modules. Java can use java.util, java.io, etc. Third-party packages (NumPy, Gson, Boost) are not available in the execution sandbox.
Is my code private?
JavaScript runs entirely in your browser and never leaves your machine. Python, Java, and C++ code is sent to the Piston open-source execution API for execution only — it is not stored, logged, or indexed.
Run Code Now
Write Python, Java, C++, or JavaScript and run it instantly. No signup, no install — just code.
Open Code Compiler