Java 25 makes running a single .java file easier than ever. Thanks to JEP 330 (Launch Single-File Source-Code Programs), combined with the new JEP 512 (Compact Source Files and Instance Main Methods), you can go from source to running program with a single command — no explicit javac step required.

Let’s walk through the options step by step.

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1. Prerequisites

• JDK 25 installed and on your PATH
• Verify your setup:

java --version
javac --version

Both should report version 25.

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2. Write a Single-File Program

Create a file called Hello.java. In Java 25, the simplest possible form is:

void main() {
    IO.println("Hello, Java 25!");
}

No class, no static, no String[] args, no imports. The IO class is auto-imported in compact source files.

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3. Run It Directly (No Compilation Step)

This is the recommended approach for scripts, prototypes, and small utilities:

java Hello.java

What happens under the hood:

1. The java launcher detects that the argument ends in .java.
2. It invokes the compiler in memory (nothing is written to disk).
3. The resulting bytecode is executed immediately.

Expected output:

Hello, Java 25!

Passing Arguments to Your Program

Anything after the source file name is passed as an argument to main:

java Hello.java arg1 arg2 arg3

To receive them, declare the parameter:

void main(String[] args) {
    for (String arg : args) {
        IO.println("Got: " + arg);
    }
}

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4. Compile and Run Separately (Traditional Way)

If you want a reusable .class file (for example, to distribute or run repeatedly without recompiling), do it in two steps.

Step 1 — Compile

javac Hello.java

This produces Hello.class in the current directory.

Note: For a compact source file without a class declaration, the compiler generates a synthetic class name based on the file name.

Step 2 — Run

java Hello

Note there is no .java extension here — you pass the class name, not the file name.

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5. Choosing a Specific Source Version

If your default javac isn’t Java 25, you can request the level explicitly:

javac --release 25 Hello.java

Or, when running a single-file program:

java --source 25 Hello.java

This is handy if multiple JDKs coexist on your system.

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6. Handling Files with a Non-Matching Name

With single-file source-code execution, the file name does not have to match any class defined inside — even if you declare public class Foo inside Bar.java, the following still works:

java Bar.java

However, if you switch to the classic two-step javac + java workflow, the standard rule applies: a public class must live in a file with the same name.

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7. Adding a Shebang for Script-Like Execution (Unix/macOS)

You can turn a .java file into an executable script by adding a shebang line as the very first line:

#!/usr/bin/env java --source 25

void main() {
    IO.println("Running as a script!");
}

Then make it executable and run:

chmod +x hello
./hello

The file must not have a .java extension when using the shebang trick — otherwise the launcher tries to compile the shebang line as Java source.

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8. Quick Reference

Goal	Command
Run a single source file	java Hello.java
Run with program arguments	java Hello.java foo bar
Force a specific source level	java --source 25 Hello.java
Compile only	javac Hello.java
Compile with an explicit release	javac --release 25 Hello.java
Run compiled class	java Hello

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9. Common Pitfalls

Problem	Cause	Fix
error: unknown source file extension	You passed a file without .java to java	Use java Hello.java (with extension) or java Hello (without) after compiling
error: 'class', 'interface', ...' expected	Older JDK doesn’t understand compact source files	Upgrade to JDK 25
main method not found	Wrong signature (e.g., int main())	Use void main() or void main(String[] args)
Shebang script fails to run	File has .java extension	Rename the file to remove the extension

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Summary

• For quick experiments: java Hello.java — one command, no .class file produced.
• For repeated runs or distribution: javac Hello.java then java Hello.
• Combine with Java 25’s compact source files and instance main for the shortest possible programs.

This unified workflow makes Java feel almost script-like while keeping the full power of the platform available when you need it.