Online Java Compiler
Write, compile, and run Java programs instantly in your browser. Featuring syntax highlighting, multiple Java versions, and professional editing features.
How Our Online Java Compiler Works
Our Java compiler provides a seamless coding experience by handling all the complexity of compilation and execution on our servers. Here's what happens when you click "Run":
Code Transfer
Your code is securely transmitted to our servers over HTTPS. We never store your code unless you explicitly save it.
Compilation
The Java compiler (javac) compiles your code into bytecode. Any syntax errors are caught and reported with line numbers.
Execution
The Java Virtual Machine (JVM) executes your compiled bytecode in a secure sandbox environment with resource limits.
Output Display
The output (or any runtime errors) is returned to your browser and displayed in the output console.
Supported Java Versions
| Version | LTS | Key Features | Best For |
|---|---|---|---|
| Java 8 | ✅ | Lambda expressions, Stream API, Optional | Legacy projects, enterprise applications |
| Java 11 | ✅ | Local variable syntax, HTTP Client, String methods | Modern development, cloud applications |
| Java 17 | ✅ | Records, Sealed classes, Pattern matching | Latest features, recommended for learning |
| Java 21 | ✅ | Virtual threads, Pattern matching enhancements | Cutting-edge development, high concurrency |
Common Java Compilation Errors & Solutions
class, interface, or enum expected
Cause: Code is written outside of a class definition.
Solution: Make sure all your code is inside a class. In Java, everything must be inside a class.
cannot find symbol
Cause: Using a variable or method that hasn't been declared.
Solution: Check for typos, ensure variables are declared before use, and verify imports.
';' expected
Cause: Missing semicolon at the end of a statement.
Solution: Add a semicolon (;) at the end of the indicated line.
incompatible types
Cause: Assigning a value of one type to a variable of another incompatible type.
Solution: Use type casting or change the variable type to match.
array required, but found
Cause: Using array indexing [] on a non-array variable.
Solution: Declare the variable as an array or use the correct syntax.
Tips for Java Beginners
- Start with Hello World: Always begin by printing a simple message to understand the basic structure.
- Use meaningful variable names: Choose descriptive names like
studentAgeinstead ofx. - Indent your code: Proper indentation makes code much easier to read and debug.
- Comment your code: Use // for single-line and /* */ for multi-line comments to explain complex logic.
- Test frequently: Compile and run after small changes to catch errors early.
- Read error messages: Errors always include line numbers - go to that line first!
- Use System.out.println for debugging: Print variable values to understand program flow.
- Learn from examples: Use the Examples dropdown to see well-written code patterns.
- Practice daily: Consistency is key - even 30 minutes daily makes a big difference.
- Don't memorize, understand: Focus on understanding concepts rather than memorizing syntax.
Frequently Asked Questions
How does the online Java compiler work?
Our compiler sends your Java code to secure servers where it gets compiled using javac and executed using the java runtime. The output is then returned to your browser instantly.
Which Java versions can I use?
We support Java 8, Java 11, Java 17, and Java 21 (all LTS versions). You can select your preferred version from the dropdown menu above the editor.
Can I provide input to my Java program?
Yes! Click the "Input" button in the toolbar to open the input panel. Enter your input data (one per line) and it will be provided to your program via stdin.
How do I save my code?
Click the "Save" button to save your code and receive a unique shareable URL. You can also download the code as a .java file using the "Download" button.
What are the execution limits?
Programs have a maximum execution time of 15 seconds and memory limit of 256MB. Network access and file system operations are restricted for security.
Is my code secure?
Yes! Your code is transmitted over HTTPS and executed in isolated containers. We do not store your code unless you explicitly save it.
Can I use external libraries?
Currently, only the standard Java library is available. Support for common libraries like Apache Commons, Gson, and Jackson is coming soon.
What keyboard shortcuts are available?
Press Ctrl+Enter (Cmd+Enter on Mac) to run your code. The editor also supports standard shortcuts like Ctrl+C/V for copy/paste, Ctrl+Z for undo, etc.