Hello World!

A typical introduction of a programming language to a new programmer is the simple "Hello World" program. This minimal program simply outputs "Hello World".

Let's break down Java's version of this program, in file Hello.java

package example;

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

package example

This course uses the convention that all source files belong to a package. So even for this simple program, we declare the Hello class to be part of the example package.

public class Hello

Every Java file must contain exactly one top-level class with the same name as its base name. Thus, class Hello is in file Hello.java. The class is preceded by the public access modifier, which declares the class to be accessible to all. While not necessary, it is this class's convention for classes to be public. Look to modifiers for a description on access modifiers.

public static void main(String[] args)

Memorize this in its exact form. Every Java program must have a class with a main method where the program starts its execution. public declares this method to be accessible to all, a necessity for an entry point. static says this method can be called without an object instance. We shouldn't need to create a Hello object to call the entry point, then it's not the entry point. Look to static for a description on the static keyword. void declares this method's return type to be non-existent, it does not return anything. Execution simply ends when the end of the main method is reached (at least with the single-threaded applications we'll be writing). main is the name of the method, the entry point must have a consistent name. (String[] args) is the list of parameters this function accepts. This function accepts one parameter named arg of type String[] (an array of Strings). This is the list of strings passed from the java's invocation on the command line.

System.out.println("Hello World!");

This line is what actually prints, the rest is just overhead. System is a class that contains functions that interact with the system. It has a public static variable, out, which represents standard output to the terminal console. out has a function, println, which takes the String you wish to print out. Strings in Java must be put between double quotes ("). The ln at the end of println signifies the function implicitly prints a newline. Use print when a newline is not desired.

To run

> javac -d . Hello.java
> java -cp . example.Hello
Hello World!

A description of what's going on here can be found in compiling