You can see we have the package name first. Notice how the line ends with a semicolon. If you miss the semicolon out, the programme won't compile:
package firstproject;
The class name comes next:
public class FirstProject {
}
You can think of a class as a code segment. But you have to tell Java where code segments start and end. You do this with curly brackets. The start of a code segment is done with a left curly bracket { and is ended with a right curly bracket }. Anything inside of the left and right curly brackets belong to that code segment.
What's inside of the left and right curly brackets for the class is another code segment. This one:
public static void main( String[ ] args ) {
}
The word "main" is the important part here. Whenever a Java programme starts, it looks for a method called main. (A method is just a chunk of code. You'll learn more about these later.) It then executes any code within the curly brackets for main. You'll get error messages if you don't have a main method in your Java programmes. But as its name suggest, it is the main entry point for your programmes.
The blue parts before the word "main" can be ignored for now.
(If you're curious, however, public means that the method can be seen outside of this class; static means that you don't have to create a new object; and void means it doesn't return a value - it just gets on with it. The parts between the round brackets of main are something called command line arguments. Don't worry if you don't understand any of that, though.)
The important point to remember is that we have a class called FirstProject. This class contains a method called main. The two have their own sets of curly brackets. But the main chunk of code belongs to the class FirstProject.
In the next part, you'll learn how to run your Java programmes.
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Running your Java Programmes
When you run a programme in NetBeans, it will run in the Output window at the bottom of your screen, just underneath your code. This is so that you don't have to start a terminal or console window - the Output window IS the console.
There are various ways to run your programme in NetBeans. The easiest way is to press F6 on your Keyboard. You can also run programmes using the menus as the top of NetBeans. Locate the Run menu, then select Run Main Programme:
The Run Menu in NetBeans
You can also click the green arrow on the NetBeans toolbar:
The Run icon on the toolbar
Another way to run your programmes is from the Projects window. This will ensure that the right source code is being run. Simply right click your java source file in the projects window and you'll see a menu appear. Select Run File.
Running the programme from the Projects window
Using one of the above methods, run your programme. You should see something happening in the Output window:
The Java Output window
The second line in the Output window above is our code: My First Project. You can quickly run it again by clicking the two green arrows in the top left of the Output window.
Writing you own Java Methods
You have been using methods in the previous section, and have seen how useful the inbuilt ones can be. In this section, you'll learn to write your own methods.
The Structure of a Method
A method is just a chunk of code that does a particular job. But methods are set out in a certain way. You have a method header, and a method body. The header is where you tell Java what value type, if any, the method will return (an int value, a double value, a string value, etc). As well as the return type, you need a name for your method, which also goes in the header. You can pass values over to your methods, and these go between a pair of round brackets. The method body is where you code goes.
The Java method header
The method's return type goes first, which is an int type in the code above. After the method type, you need a space followed by the name of your method. We've called the one above total. In between a pair of round brackets we've told Java that we will be handing the method a variable called aNumber, and that it will be an integer.
To separate this method from any other code, you need a pair of curly brackets. Your code for the method goes between the curly brackets. Note the word return in the method above. This is obviously the value that you want to return from your method, after your code has been executed. But it must be of the same type as the return type in the method header. So the return value can't be a string if you started the method with int total.
Sometimes you don't want Java to return anything at all. Think of Trim in the previous section. You may only want the Trim method to get on with its job, and not return anything to you. A method that doesn't return any value at all can be set up with the word void. In which case, it doesn't need the return keyword. Here's a method that doesn't return a value:
A void method in Java
All the method above does is to print some text. It can just get on with its job, so we've set it as a void method. There's no return value.
Methods don't need to have values passed to them. You can just execute some code. Here's a void method without any values being passed over:
A void method with no parameters
And here's an int method that has no values being passed:
A Java int method
As you can see, the round brackets are empty in both methods. But they are still needed. Miss the round brackets out and you'll get an error message.
In the next lesson, you'll learn how to call your methods into action.
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