Java Online Runner

Java Online Runner is a fully browser-based Java compilation and execution tool. No local JDK or development environment is required — simply open the page and start writing Java programs immediately. It is an ideal solution for learning, practicing, and quickly validating Java code.

Core Features

• Real Java Execution: Provides full Java 8 execution capability directly in the browser. Compilation and runtime output are consistent with a local development environment, with no compatibility concerns.
• Automatic Fallback: If runtime resources are unavailable, the tool automatically switches to a built-in execution engine, ensuring most standard Java programs still run correctly without interrupting your workflow.
• Professional Code Editor: Features a fully equipped code editor with Java syntax highlighting, bracket matching, code folding, auto-completion, undo/redo, and find-and-replace — an experience close to a local IDE.
• Multi-Tab Support: Open and edit multiple files simultaneously, rename tabs with a double-click, and switch between them freely to support multi-file projects.
• Standard Input Support: A dedicated Input panel lets you pre-fill stdin content. The program reads it line by line via Scanner or BufferedReader, fully simulating command-line interaction.
• File Upload: Upload .java, .jar, .txt, .csv, and .json files. Java source files open automatically in a new tab; other files are available for your program to read at runtime.
• Code Sharing: Generate an encoded share link with one click. Anyone who opens the link can view and run your code immediately — great for collaboration and communication.
• Built-in Snippets: Includes classic examples such as Hello World, Fibonacci, Bubble Sort, OOP, Collections, and Multithreading. Load any template instantly to get started quickly.
• Quick API Reference: A built-in sidebar covers common Java API methods with signatures and descriptions. Click any entry to insert it directly into the editor, saving time on documentation lookups.
• Execution Statistics: After each run, view elapsed time, line count, class count, and method count to get a quick overview of your program.

Who Is It For?

• Java Beginners: Skip complex environment setup and focus entirely on learning syntax and writing code.
• Students: Complete assignments and verify algorithm logic anytime, from any device.
• Interview Candidates: Practice coding problems and test logic quickly without opening a local IDE.
• Teachers and Instructors: Demonstrate code live in class without installing a development environment on the presentation device.
• Developers: Quickly validate a snippet of Java logic without creating a new project or switching tools.

How to Use

1. Type or paste your Java code into the editor (a Hello World example is loaded by default).
2. If your program requires input, switch to the Input tab and fill in the stdin content.
3. Click the Run button in the top-right corner (or press Ctrl+Enter) to execute.
4. View the output in the Console panel. Click Stop to terminate a running program at any time.
5. Use the Share button to generate a link and share your code with others.

Frequently Asked Questions

• Q: Why is the first run slow?
  A: Runtime and compiler resources need to be downloaded on the first run. Once cached locally, subsequent runs are significantly faster.
• Q: Which Java version is supported?
  A: Java 8 syntax is fully supported, covering the vast majority of beginner and intermediate learning scenarios.
• Q: Can I use third-party libraries?
  A: Yes. Upload a .jar file and it will be automatically added to the classpath when your program runs.
• Q: Is my code saved?
  A: Code persists within the current browser session. Use the share link to permanently embed and share your code via URL.
• Q: Can I use multiple classes?
  A: Yes. Write multiple Java files across separate tabs, or upload .java files to open them automatically in new tabs.