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1. Write a file `HelloWorld.java`.

2. Compile it to bytecode (.class file): `javac HelloWorld.java`.

3. Run it: `java HelloWorld.class`.

If you want to use other libraries, add `-cp` parameter ("cp" stands for "classpath").

If you want to package your code as a library, run `jar HelloWorld.class` (.jar is basically .zip renamed).

Hell, you don’t even have to compile it anymore — java HelloWorld.java just does the thing you wrote.

Also note that `java HelloWorld` looks for classes, so if you are in the same directory as the classfile, .class is not required.

And if you want a webapp, use SpringBoot.