Understanding the Code - Java

Now that you've created a program to bid good riddance to those who laughed when you told them about your impending job with NASA, it's time to examine the code you've just written to understand how it works.

The first section we will look at is the Source Code Comments, shown below.

/*
* This application will print the words “Goodbye Cruel World” across
* your monitor. Also note that these comments are in your code for
* other programmers, and is invisible to the compiler.
*/

As you can see, the /* informs the computer that this is where you will be making comments and that it is not allowed to read these, no matter how curious it might get. These comments are for other programmers, should they ever need to fix your code or add something to it. After all, with all your brains and chutzpah, you won't be in the same position forever.

To close the Comment section and let the computer know that it is okay to look again, simply type */ to end the comment. You can open another comment anywhere within the application you are creating and in fact, it is recommended that you do so, and often.

// text

In the above example, the computer ignores everything from // to the end of the line.

Next we will look at the Class Definition section of the Code. Don't fret too much over this new phrase at the moment; I will discuss it in more depth in the second part of this tutorial.

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

In the above sample, the bold lines indicate the opening and closing parts of the Class Definition.

Finally, we come to the Main Method:

public static void main(String[] args)

Every program you write within Java must contain this Main Method. Basically, it passes information to your application that it will need to perform the function you created it to do.

And lastly, the line

System.out.println("GoodbyeCruelWorld!");

tells the program to print the phrase “GoodbyeCruelWorld!” (or any other phrase you place within it) to your screen.

Let's say we want to work with numbers. In this example, you will see something called variables. Don't worry about those just yet, as we will be touching upon them in great detail in future articles. In this sample, the program is going to multiply one number by another number and print out the result.

/*
A program to calculate numbers.
We will name this program “TheMathWizard.java”.
*/
class TheMathWizard {
  public static void main(string args[]) {
    int num; // this is how you declare a variable and name it num
    num=4000 //this places the value 4000 in our variable named num
    System.out.println(“This is your puny number!: “ + num);
    num = num*2; //this tells the program to multiply our variable num
    // by two
    System.out.print (“The total of your puny number * 2 is “);
    System.out.println(num);
  }
}

After running this mathematically complex program, you should see the following on your screen:

This is your puny number! : 4000
The total of your puny number *2 is 8000

If we wanted to complicate matters, we could have the program ask you for a number to multiply by or have the user enter a number to be multiplied (or both). But we'll save that for another day.

You are now free to take a break. Go update your resume and get ready to send it to NASA. But don't mail it off just yet. The second part of the "How to Program in Java" tutorial will be coming soon.