Vizualizare sursă pentru „Writing and Executing a Java Program”

In order to develop a Java program, there are two operations that need to be performed: the compilation of a source code and the execution of the program inside the virtual machine. The source code can be written in any text editor. For Microsoft Windows, a good suggestion is [http://notepad-plus-plus.org/ Notepad++]. For Linux, depending on the X interface, you can use either ''Kate'' for KDE or ''gedit'' for Gnome/ Unity/ Cinnamon. During this lab, we will be using ''Kate''. You can run it by clicking Applications -> Accessories -> Kate.

== Command Line Tutorial ==

Before compiling and executing a Java program, we will need a short introduction for using the console in both MS Windows and Linux.

=== MS Windows cmd ===

In Windows, the console is stated by pressing "winkey + r", then typing ''cmd''. It looks like this:

[[Fișier:cmd.png|Consola sistemului de operare Windows]]

A few useful commands:

* if the files are on another partition, like D, or E, the command that changes the path to the D partition, for example, is:
<syntaxhighlight lang="dos">
c:\Users\Echo> d:
d:\>
</syntaxhighlight>
* for changing the current directory, the ''cd'' command is used (change directory):
<syntaxhighlight lang="dos">
c:\Users\Echo> cd Documents
c:\Users\Echo\Documents> cd work
c:\Users\Echo\Documents\work> cd ..
c:\Users\Echo\Documents>
</syntaxhighlight>
* for displaying the content of the current directory, you can use the ''dir'' command:
<syntaxhighlight lang="dos">
C:\Users\Echo\Documents\work\poo> dir
 Volume in drive C has no label.
 Volume Serial Number is 9C96-4AC6

 Directory of C:\Users\Echo\Documents\work\poo

18-Jul-12  10:42    <DIR>          .
18-Jul-12  10:42    <DIR>          ..
18-Jul-12  10:43               117 MainClass.java
               1 File(s)            117 bytes
               2 Dir(s)  22,145,564,672 bytes free

C:\Users\Echo\Documents\work\poo>
</syntaxhighlight>
* for creating a new directory, use ''md'' (make directory):
<syntaxhighlight lang="dos">
C:\Users\Echo\Documents\work\poo> md test

C:\Users\Echo\Documents\work\poo> dir
 Volume in drive C has no label.
 Volume Serial Number is 9C96-4AC6

 Directory of C:\Users\Echo\Documents\work\poo

18-Jul-12  10:45    <DIR>          .
18-Jul-12  10:45    <DIR>          ..
18-Jul-12  10:43               117 MainClass.java
18-Jul-12  10:45    <DIR>          test
               1 File(s)            117 bytes
               3 Dir(s)  22,145,433,600 bytes free

C:\Users\Echo\Documents\work\poo>
</syntaxhighlight>

=== Linux Bash ===

In Linux, depending on the distribution and interface, you can start the terminal by either pressing "Alt + Ctrl + T", or by selecting '''Terminal''' from Applications -> Accessories: 

[[Fișier:linux_console.png|Consola din Linux]]

A few useful commands:
* to display the current directory, use ''pwd'' (print working directory):
<syntaxhighlight lang="bash">
student@olympus07:~$ pwd 
/home/student
student@olympus07:~$
</syntaxhighlight>
* for changing the current directory, the ''cd'' command is used (change directory):
<syntaxhighlight lang="bash">
student@olympus07:~$ cd work
student@olympus07:~/work$ pwd 
/home/student/work
student@olympus07:~/work$ cd ..
student@olympus07:~$ pwd 
/home/student
student@olympus07:~$
</syntaxhighlight>
* for displaying the content of the current directory, you can use the ''ls -l'' command (list structure, long listing format):
<syntaxhighlight lang="bash">
student@olympus07:~/work/poo$ ls -l
total 4
-rw-rw-r-- 1 student student 110 Jul 18 11:08 MainClass.java
student@olympus07:~/work/poo$
</syntaxhighlight>
* to create a new directory, use ''mkdir'' (make directory):
<syntaxhighlight lang="bash">
student@olympus07:~/work/poo$ mkdir test
student@olympus07:~/work/poo$ ls -l
total 8
-rw-rw-r-- 1 student student  110 Jul 18 11:08 MainClass.java
drwxrwxr-x 2 student student 4096 Jul 18 11:10 test
student@olympus07:~/work/poo$
</syntaxhighlight>

== Writing, Compiling and Executing a Java Program == 

=== Editing the Source Code ===

The first step in writing a Java program, is editing the source code. For this, use your favorite editor and save the file with a .java extension.

<div class="regula"><font color="#ff0000">Rule:</font> A Java source code will '''always''' have a .java extension.</div>


<div class="regula"><font color="#ff0000">Rule:</font> The name of a class contained in a source file must '''always''' have the same identical name with the file. I.e.: The class MainClass '''must''' be saved in a file named MainClass.java. '''Attention''': Java is case-sensitive!!!</div>

For example, we will write the following class:
<syntaxhighlight lang="java">
public class MainClass{

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

}
</syntaxhighlight>

This class must be saved in a file called MainClass.java.

=== Compiling the Source ===

Open a console, navigate to the directory where the source file has been saved. Then, use the ''javac'' executable (Java Compiler) to compile the code:
<syntaxhighlight lang="bash">
student@olympus07:~/work/poo$ javac MainClass.java
student@olympus07:~/work/poo$ ls -l
total 8
-rw-rw-r-- 1 student student 424 Jul 18 11:22 MainClass.class
-rw-rw-r-- 1 student student 110 Jul 18 11:08 MainClass.java
</syntaxhighlight>


<div class="regula"><font color="#ff0000">Rule:</font> The Java compiler will '''always''' take as argument the '''file name''' (extension included) which needs to be compiled. </div>

If the compilation did not display any message, then it has completed successfully and the executable file for the virtual machine has been created (.class extension).

If the compiler could not be found, then check the PATH environment variable as per the JDK installation instructions.

If the compiler displayed error messages, then read them carefully and fix the Java source code problems.

=== Program Execution Inside the Virtual Machine ===

In the directory where the source code has been compiled, using the ''java'' executable (the virtual machine), the program is executed:

<syntaxhighlight lang="bash">
student@olympus07:~/work/poo$ java MainClass
Hello world!
student@olympus07:~/work/poo$ 
</syntaxhighlight>


<div class="regula"><font color="#ff0000">Rule:</font> The JVM will '''always''' take as an argument the '''name of the class''' (without the extension) which needs to be executed.div>


<div class="regula"><font color="#ff0000">Rule:</font> A Java class can only be ran in the virtual machine if it contains, as a member, the '''main''' method, exactly how it is defined in the specification: ''public static void main(String[])''</div>