Hello world in Java

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

What does each keyword mean?

public - Should be visible to JVM
class - Everything in java is a an object which is an instance of class
HelloWorld - class name, same as file name if class is public. Only one public class per .java file
static - static means the method is part of class and not it’s instance. If it’s non-static, then instance needs to be created which is not realistic.
void - return type
main() - program entrance
String[] - array of strings that can be sent to help with program initialization

Bytecode from javac is not readable

javap -c gives disassembled code

e.g.: javap -classpath . -c HelloWorld

-verbose instead of -c gives more details

First steps: Load, link, initialize - before executing main method

Load:

Load binaries from class\/interface into JVM

Link:

Incorporate binary type data into run-time state of JVM.

Three steps:

Initialization:

Assign class variables with proper initial values

Loading:

done by Java classloaders. JVM loads three class loaders when it starts up

Hello World in Java