System: Built-in class from java.lang package. out: Static member (PrintStream object) of System. println(): Method that prints to console and moves to the next line.

Published May 12, 2026

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r/learnprogramming on Reddit: Describe hello world program in java

August 1, 2024 -

So I'm a newbie developer (1 yr exp), trying to get an understanding of core java. I've been coding for my company for a year now, but writing business logic doesn't seem to be helping much when it comes to improving my understanding of core java. Thought of doing an exercise of taking the simplest programs and going into detail about everything written in it.

Here's a simple hello world program in java:

class HelloWorld
{
    public static void main(String []args)
    {
        System.out.println("Hello world");
    }
};

Can I get as many details about what's happening in this file as possible? Like really go into the details here. Explain what each keyword does, why the print statement doesn't require an import, etc.

Top answer

1 of 2

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.

2 of 2

I don't do Java, but it looks close enough to C++ ... class HelloWorld is creating a new class, and the { and } at the beginning and end is the block. In that class, "public" is creating a public function called main() which returns a void, meaning it doesn't return anything when that function exits. Any public function can be called from outside of the class, whereas if it were private it would only be called from inside the class. String []args is probably defining an array of strings for the args that are being provided to the program from the command line, such as if you ran "program 1 2 3" then it would have three array elements, "1", "2", and "3". In C you'd also have an "int argc" before that which would tell you how many array elements there were. Again you have { and } which are telling you that its a block of code that all goes under main() Then you have what is essentially a printf( ) (in C) that outputs the string "Hello world", I assume to the screen. I don't know what a Java "import" is, .. but the reason that function doesn't require a type is probably because it is similar to C++ where the method is defined for multiple types and the Java interpreter knows which one to call based on whatever the type of the arg is. A java import that you mentioned might also be the C equivalent #include, which, if it is missing, I would assume that Java automatically imports the System class/object and doesn't make you declare it, but that's just a guess based on what you wrote. I don't know Java, so I don't know what "static" means in this context, because I don't think it means the same thing in Java that it means in C++

Discussions

How to use of print("HelloWorld") instead of System.out. ...

I read about import static which helps me to write direct print.. but why is it not advisable to use import static..as a first time learner I am finding it difficult why not to use import static ? ... There's nothing wrong with using import static. As BorisTheSpider points out in a comment, though, wildcard static imports (like import static java... More on stackoverflow.com

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