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If you've already been programming for a year, I guess I'll skip the really basic stuff. This defines a class called HelloWorld. Everything defined in Java has to live within a class. This source file, when compiled, will become a file called HelloWorld.class. There can be directives that exist outside a class (e.g. package and import). These are directives to the compiler. They influence how the compiler behaves while compiling that one file, but they do not "define" anything new, so they don't get placed inside the class. This class has a single method, main. The method has two modifiers: public - the method is available to callers that exist outside this class. static - the method can be called without first creating an instance of the class. The signature of a main method has to look like this. It has to be public and static, it has to return void, and it has to take a String array as a parameter. If you tweak that signature and then try to run the class, it won't work. The JVM won't be able to find a valid main method. System is a class in java.lang . But the package java.lang is automatically imported. That's why you don't need to write java.lang.System.out.println nor do you need an explicit import of java.lang. This is the only package that works this way. System has a static final field called out with type PrintStream . It's rare to expose fields in this way, but I suspect this was done for performance reasons. Like a static method, a static field can be accessed without creating an instance of the class. Because the field is final, its value cannot be swapped to reference a different PrintStream. And println is just a regular instance method on PrintStream. Dunno if that is what you wanted. More on reddit.com

Untested, but I think deserves extra points for: sounding Transylvanian; and preserving the semantic intent of the message; whilst keeping to the terms of the challenge :-) echo hello vorld | tr [u-x] [v-y] More on reddit.com

type - "java HelloWorld", to execute your file. More on reddit.com