Java101

Hello World

As traditional a start as you can get.

Do

Create the following file named HelloWorld.java:

public class HelloWorld {

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

}

Then, in the terminal run the following:

javac HelloWorld.java
java HelloWorld

If all goes well, you should see Hello, world! printed on the command line. If you don’t see that or get some other kind of error, make sure you did the setup correctly in 00_setup.

Discuss

Line by line

We’ll go more in-depth into some of these keywords in later lessons, but here’s a brief overview.

1. public class HelloWorld { - This is the opening line of the file. It defines a class named HelloWorld. It
2. public static void main(String[] args) { - This is the method signature of the main method. It defines a static method named main which returns no value and has a single parameter named args which is of a String array type. All those terms will make sense in due course!
3. System.out.println("Hello, world!"); - This is our first and only instruction. System.out.println will print to the console the arguments given. In this case, the argument given is “Hello, world!”. All instructions in Java must end in a semi-colon (;). If you miss out a semi-colon then you’ll get an error when running javac saying “error: ‘;’ expected”.
4. } (x2) - Notice how the class and method signature end in an opening parenthesis ({). These must be matched up with closing parentheses. If you leave one of these out, you’ll get an error running javac that says “error: reached end of file while parsing”.

Explain the funny words above, magic man

What is a class?

Every Java file is a class (kinda, see how this is not entirely true in future lessons!). One file has one top level class. This class must have the same name as the file. A class contains other Java constructs that we’ll come across.

What is the main method?

Java contains methods. These are the functions and operations that are run in your program to make stuff happen. This class has the special main method. When you run java HelloWorld, you’re running the java program which opens the HelloWorld.class file and looks for the main method. Whatever is in the main method is executed, aka run.

What is public or static or void or String[] args!?

Find out soon! For the curious (and to have something to latch on to later):

• public is the visiblity modifier. This method being public means it can be called from any other class
• static means the method belongs to the class and is shared, rather than belonging to a single an instantiation of the class
• void is the return type. A void method returns nothing.
• String[] args sets a String array parameter named args that is require to call this method.

All of the above will make sense in time!

What does javac actually do?

This compiles the human readable .java file to a machine readable .class file. Human readable means a human can read it (should’ve seen that one coming) If you open the .class file, you’ll just see what’s called byte code, a bunch of gibberish looking 4 character strings.

It’s not important to know about byte code now, but remember that your .class file is what is executed, not the .java file. So to make a .class, we need to run javac TheJavaFile.java which makes a .class file in the same location. If you make changes to the .java file, you must recompile the .class file (i.e. run javac HelloWorld.java)

Try it now! Change “world” to your name. Recompile the code and run java HelloWorld. What happens?