Java is a 100% POO language, so to starts a Java software, our main function need to be a class method.

The filename need to be the same of the class that was declared in this file.

So the basic structure of Java file is:

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

Where Program is the class name of this file and main the method name, i.e., the main function that will be executed.

• public means that our main() method can be used by all files that imports Program
• static means that our main() method can be called without instantiate the class Program
• void means that our main() method doesn't return anything
• args is the argument that our main() method receives. In this case, an array of String objects. This is the way that Java receives the arguments passed by the command line.
• System.out.println() is the way that Java print something in the console.
  • System is a class that Java provides to us native methods to interact with the system
  • out is a static object of System class that represents the output stream
  • println() is a method of out object that prints a string and a new line (Alternative: print(), that prints only the string without a new line)

And, the filename need to be Program.java because our class is named Program.