In our lessons, we will learn Java as a computer language
not as a development environment. This means that we will primarily build
applications that show how this language functions, not how to create graphical
applications. The types of programs we will create on this site are called
console applications. By default, a console application displays its result(s)
in a black window. To make it a little easier, we will use an environment that
displays a window but with a different background color.
As you may be aware, the computer doesn't speak or
understand human languages. Still, to communicate your instructions to the
computer, you can write them in a human language, using characters, letters,
symbols, and words you are familiar with. When doing this, you prepare text that
would be communicated to the computer. For the computer to understand these
instructions, they must be "translated" into the computer's own
language, also called the machine language. This intermediary
"translator" is also called a compiler. There are various compilers on
the market. One of them, or a few of them, are provided by Sun and they are
free. Although we will create only console applications, we will use NetBeans
IDE.
To start, connect to the Sun
Microsystems web site, click the Downloads link or do a search on the word
"Java". Download the latest Java 2 Platform, Standard Edition or J2SE development kit. At the time of this
writing, it is J2SE 5.0. You may be prompted to download the NetBeans IDE + JDK 5.0 Update 2.
This is fine.
Imagine you want to create a list of books of your personal
library. To characterize each book, you can create a list of words that describe
it. This list would be made of general characteristics of each book. Here is an
example:
In the computer world, the work Book in the above example is
referred to as a class: A class is a list of criteria used to describe
something. Each word or group of words, in bold in the above example, is
called a property of the class. In our example, Book Title is a property of the
Book class. In the same way, Author is another property of the of the Book
class. In Java, there is never space between two words that represent the same
entity. This means that the above list is better represented as follows:
One class can also be a property of another class
(unfortunately, we need to mention these issues in our introduction but don't
make too much effort to understand them: everything will be clearer in future
lessons). For example, imagine you create another class that can be used to
describe each author of a book. The list of properties can be created as
follows:
You can then use this class to define a criterion in the
Book class, exactly as done above: only the Author word is necessary in the Book
class, its Name and Nationality criteria are directly represented.
Every property that is created in the class is referred to
as its member. For example, in the above lists, Nationality is a member of the
Author class while Publisher is a member of the Book class.
To describe a particular book of your collection, you can
provide information for each characteristic of the class used to describe it.
Here is an example:
In the same way, you can describe each author of the
collection by filling out its characteristics using the above criteria. When
describing an object as done in this example, you must first give a name to the
object (this is not always necessary as we will see in future lessons). In the
above example, Modjang is the name of the object. Author is (still) the name of
the class. This name is necessary because you would need it in a class where
Author must be used as a member. Based on this, you can use the name of the
above class when describing a Book object. Here is an example:
It is important to understand the difference between a class
and an object. An object is the result of describing something using the preset
criteria of a (known) class. In the strict sense, you must first create a class
by specifying each criterion you want to use to eventually describe some
objects. On the other hand, you can create an object only that is based on an
existing class.
Everything statement in Java must end with a semi-colon.
This results in the following:
In many cases, only characteristics are necessary to have a
class on which complete objects can be described. Still, an object may be
expected to perform one or more assignments or actions. An assignment that an
object can perform is called a method.
Imagine you have books that that can open or close
themselves when asked to do so (Sci-Fi anyone?). The action used to open may be called Open. To
differentiate an assignment, or method, from a property, the name of a method
must be followed with parentheses. Here are two examples:
Although this list shows the methods last, this is not a rule, the
methods can be the first, the last, or included between, as long as they belong
to the class. In the same way, a class that is made a property (member) of a class can
also have its own methods.
Every item that is created in a class is referred to as
its member.
A computer language is developed from scratch. Then other
languages can be developed using its existing rules. This is also a rule that
people who developed Java followed. One of the assignments they performed was to
create many classes that you can directly use in your program to lay a solid
foundation and work on top of that. The classes that already exist with Java are
referred to as built-in classes. To make it easy to "catalog" the
various built-in classes, they were created in libraries called packages and
there are various of them.
One of the classes that ship with Java is called System.
One of the jobs of the System class is to display something on the screen. To
perform this job, the System class is equipped with a property called out.
Actually, out itself is a class as we mentioned earlier that one class
can be used as a property of another class. To actually display something on the
screen, the out class is equipped with a method called println (read
Print Line). To display something, you can write it between the parentheses of
this method. If the item to display is a word, you must include it between
double-quotes.
We will see that another job it can handle is to get a value
from the user.
A Java application is primarily created from a file located
in a folder. As we will see in the next section, this file contains a class (we will
learn more about classes starting in Lesson 4). You must
use a folder in which you will save the file:
Instead of working manually, you can use NetBeans.
Java code consists of instructions written in plain
English. The simplest document of a Java code contains a class and
something called main. The formula we will use and accept at this time is:
public class Exercise {
public static void main(String[] args) {
}
}
Every single word or symbol on the above code will be
explained in later lessons. For now, accept it "as is".
After typing the code, save it in the desired folder. Here
is an example:
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Type the name of the folder and press Enter
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Practical
Learning: Starting NetBeans
After writing code and saving the file, you can prepare it to
see the result. You must first compile it. Of course you must have downloaded
the Java compiler or NetBeans. If you are working from the Command Prompt, it
may be a good idea to modify the System Variable. Before doing this, locate the
installation of Java on your computer and show the contents of the bin
folder:
Select and copy the path in the Address bar.
To prepare the path, open the Control
Panel and access the Administrative Tools followed by System. Click Advanced
In the Startup and Recovery section, click Environment
Variables. In the section, System Variables section, double-click Path. In
the Variable Value, press End, type ; and type the path to the bin folder of
Java. Here is an example:
Click OK.
To compile the project at the Command Prompt, switch
to the folder where you saved the file:
To actually compile, you use the program named javac
followed by the name of the file and the extension .java. An example is:
javac Exercise.java 
To see the result of an application, you must
execute it. To do this at the Command Prompt, the program to use is
called java. Therefore, to execute an application at the Command
Prompt, type java followed by the name of the file with the
extension and press Enter.
Java code consists of instructions written in plain
English. Many of these instructions can be written on one line of code.
Here is an example:
public class Exercise{public static void main(String[] args){}}
This code works fine because Java doesn't care where
the empty spaces among the words and expressions of code are positioned.
For example, the above code could have been written as follows:
public
class
Exercise
{
public
static
void
main
(
String
[
]
args
)
{
}
}
It would still work fine. If you write code like any
of the above two sections, it can be difficult to read. The alternative is
to write each statement on its own line. A statement is a line that code
that ends with a semi-colon.
The exception to this rule is when you use the for
loop as we will learn in future lesson. Besides the semi-colon statements,
you can use indentation to make your code easy to read. Indentation
consists of aligning sections of code that belong to the same block. For
example, if you start a section of code with an opening curly bracket
"{", you must end that section with a closing curly bracket
"}". Here is an example:
public class Exercise
{
public static void main(String[] args)
{
}
}
A comment is text that is not part of your code but
serves as guide when trying to figure out what the lines of code of your
program are. This means that a comment is not taken into consideration
when your code is built. For this reason, when writing a comment, you use
plain language as you wish.
There are two usual ways of
including comments in your program. To comment the contents of one line,
you start it with double forward slashes like this //
Here is an example: // Here is a simple sentence
You can include many lines of comments in your
program. To do that, comment each line with the double slashes. An
alternative is to start the beginning of the commented line or paragraph with /*
and end the commented section with */
Here is another commented version of our program:
// A simple exercise
/* Here is a simple sentence that I want to display
when the program starts. It doesn't do much.
I am planning to do more stuff in the future. */
// The end of my program
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