Using Java at Computer Science

Below it will be described how you write a Java program using a text editor and execute the program using a command prompt.

Start of editor and command prompt

Start the text editor Notepad. It is available from the menu Start via the menus All Programs and Accessories. Adjust the window of the editor such that it approximately occupies the upper 2/3 of the computer screen. Select Command Prompt from the menu Start via the menus All Programs and Accessories. Thereby a window with a command prompt will appear where commands to the computer can be typed. To begin with, the window of the command prompt contains the following.

 

...
 
C:\Documents and Settings\torben>_
 

Of course, your own user name will occur in the window instead of torben. Adjust the window of the command prompt such that it approximately occupies the lower 1/3 of the computer screen.

Writing the program

Using the command prompt, generate a new folder with the name my_programs. This is done by typing the command md my_programs in the window of the command prompt and then pressing Enter at the keyboard. After the command md my_programs is executed, the window of the command prompt contains the following.

 

...
 
C:\Documents and Settings\torben>md my_programs
 
C:\Documents and Settings\torben>_
 

The command prompt is still operating on the folder Documents and Settings\torben at the C drive of the computer, but it has to operate directly on the folder my_programs. This is accomplished by typing the command cd my_programs and then pressing Enter at the keyboard. After this command is executed, the window contains the following.

 

...
 
C:\Documents and Settings\torben>md my_programs
 
C:\Documents and Settings\torben>cd my_programs
 
C:\Documents and Settings\torben\my_programs>_
 

Now, type the following Java program in the window of the text editor.

 

public class Hello 
{
   public static void main(String[] args
   {
      System.out.println("Hello, World");
   }
}
 

Save in the following way the typed program as a file in the folder my_programs: Select Save as from the File menu left-most at the top of the editor. Thereby a dialogue window pops up where the name Hello.java is typed and where the folder my_programs is selected (you go upwards in the file system using the Save in: menu at the top of the dialogue box and you go down by clicking at the desired folder in the dialogue window). Select All Files in the Save as type: menu at the bottom of the dialogue window. Finally, click at the button Save in the dialogue window whereby the program is saved as a file with the name Hello.java, and the dialogue window disappears. Note that what is immediately before .java in the name of the file is identical to the name of the class in the program (this MUST be the case).

Compiling and executing the program

Type the command javac Hello.java in the window of the command prompt and press  Enter at the keyboard. Thereby the program in the file Hello.java is compiled and the compiler generates a file with the resulting bytecode. The generated file has the name Hello.class and is located in the folder my_programs (the same folder as where the file Hello.java is located). After execution of the command javac Hello.java, the window contains the following.

 

...
 
C:\Documents and Settings\torben>md my_programs
 
C:\Documents and Settings\torben>cd my_programs
 
C:\Documents and Settings\torben\my_programs>javac Hello.java
 
C:\Documents and Settings\torben\my_programs>_
 

Now, type the command java Hello in the window of the command prompt and press Enter at the keyboard; note that you have to type Hello, not Hello.java. Thereby the program is executed, that is, the bytecode in the file Hello.class is interpreted. After execution of the command java Hello, the window contains the following.

 

...
 
C:\Documents and Settings\torben>md my_programs
 
C:\Documents and Settings\torben>cd my_programs
 
C:\Documents and Settings\torben\my_programs>javac Hello.java
 
C:\Documents and Settings\torben\my_programs>java Hello
Hello, World
 
C:\Documents and Settings\torben\my_programs>_
 

As it can be seen above, the result of executing the program is that the text Hello, World is printed at the screen.

Some remarks

You can move around in the file system using the command prompt as follows: You go a step up in the file system by typing the command cd .. in the window of the command prompt and thereafter pressing  Enter at the keyboard. You go a step down in the file system by using the command cd as described above. Using the command dir you can see which folders and files that are located the actual place. Another remark: Instead of using an editor and a command prompt, an integrated development environment (abbreviated IDE) can be used to write and run Java programs, an example is JCreator which is supported by Computer Science at Roskilde University.