Adding your first line of Java code!

If you've followed this guide to creating a Java project in NetBeans, then you should now be staring at a window that is essentially fairly blank except for a line of icons at the top and a panel with a "tree" in the top-left, as pictured opposite. This tree allows you to browse the various files that make up your project. For now, we won't get too bogged down in the details of these files. The one that interests us is that labelled Main.java, as circled. For now, this is where our program will live. Later on, we'll see that a typical Java application will actually consist of various different little pieces of program. But to get us started, the single "main" file will do just fine.

If you double-click on Main.java, the right-hand side of the screen will change to show the actual program. Instead of providing us with a file that is literally blank, NetBeans actually helps us by providing a "skeleton" program that we can add to. Initially, it should look something like this:

This may look slightly complicated, until you consider that the parts in grey are just comments (basically, notes that the programmer adds to help him/herself and fellow programmers) and are actually ignored by Java. So our skeleton program is really just three lines and a bit of formatting (the curly braces { and } ). For now, we're going to ignore the meaning of these lines and just add a line of program to what's already there.

Locate the line starting with // TODO (indicated by the red arrow in the screenshot above). We're going to insert a line of Java code at this point. You can either add the line instead of the // TODO line, or else add the line below. Remember that the line is just a comment, so if you leave it in, Java will ignore it. But equally, Java won't care if you remove it. In the illustration below, we'll actually remove the comment.

In traditional fashion, we're going to add a line to the program so that when run, it prints the message Hello, world!. The line we're going to place instead of the comment is the following:

System.out.println("Hello, world!");

If you're new to programming, a key thing to bear in mind is that punctuation details matter. So the dots, brackets, quotes and semicolon must be in the right place. Spaces don't generally matter, but for reasons we'll see as we go along, it makes the program easier to read if you indent the line by a couple of spaces. You should end up with something looking something as follows (to save space, we omit parts of the program above and below in this next screenshot):

Notice how, as you type, NetBeans adds colouring automatically to the program. This colouring is called syntax highlighting. As you learn more about Java, you'll see that syntax highlighting helps you to read and spot mistakes in your program.

Running the program

If you've typed the line exactly as shown, then you're ready to run the program. Click the green arrow at the top of the window to run the program.

If you've typed everything correctly, then you should see something like the following appear at the bottom of the screen:

You'll see that our Hello, World! message appears in and amongst various "administration" messages. Of course, this wouldn't be a very user-friendly way of displaying a message to the user, and as you progress with Java, you'll learn about how to build "proper" user interfaces. But for now, be happy that you've entered a line of Java program, run it, and seen the result, however uninspiring that result might seem for now!

Next: learning to program in Java

Now you've run your first Java program, you're ready to actually start learning Java and hopefully do some more interesting things than just printing Hello World to the screen.

 Beginner's Java tutorial