public class Water{
    public static void main(String[] args){
        System.out.println("H20");
    }
    
}

"javac" is the command to compile a human text file to a Java bytecode (.class) file to be executed by a Java Virtual Machine

javac Water.java

It should output .class files

"java" command executes the Java bytecode without typing the (.java) or (.class) after the file's name. It actually execute the .class file.

java Water

H20