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Comment by whartung | Hacker News Reader

I like the clever use of the "java prog.java" style.

I don't know how many folks know, although its been there for a bit, that you no longer have to specifically compile single file java programs.

You can just run them with with "java prog.java".

(Honestly, I haven't used the technique myself, still old school -- even with silly small x.java tests and such. Muscle memory and all that.)

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I do it a lot. These single-file programs can be called Java scripts when run without compilation.

I wonder what could be done with Java script…

Sounds like a name that could have some success. Given that the logo for Java is a cup of coffee, maybe we could also call it "Coffee script".

Can you add a shebang line and have it truly be a script? I do self executing JARs with a bash stub/header all the time, but haven’t tried with this.

Sadly you can't, Java doesn't support comments starting with #

    $ ./shebang.java
    ./shebang.java:1: error: illegal character: '#'
    #!/usr/bin/java
    ^
    ./shebang.java:1: error: class, interface, enum, or record expected
    #!/usr/bin/java
      ^
    2 errors
    error: compilation failed

Self-executable jars though, that's an interesting idea. No reason why it can't work, zip archives are read from the end, you can put anything in the beginning, including "#!/usr/bin/java -jar"

this has worked since the beginning but i don't think many people know about it

I believe you can use shebang since java 11 but you can't have the filename extension be .java.

It turns out you indeed can: https://news.ycombinator.com/item?id=40912444

For the self executing JAR, I normally add a bash/shell stub for the header that ends with

    java -jar $0

The rest of it sets up the environment (JAVA_OPTS, -Xmx, etc).

How do you run a Java program without compilation?

It technically compiles it to memory before running it, but the point is that you don't have to invoke javac first. It's a relatively new feature.

Do you know for which Java version this first became available?

11: https://openjdk.org/jeps/330

Oh so actually, you can use a shebang, just without the .java extension and with the "--source" flag, it's specifically written out in that spec. I was doing my experiment wrong!

Awesome, thanks for sharing!

though, in theory java has jshell for java-based shell scripts specifically

You can run multi-file source-code programs as of JDK 22: https://openjdk.org/jeps/458

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