Chapter 7. The 'Random arcs' application

Chapter 7. The 'Random arcs' application
	Part II. Basic concepts and concepts for more reliable code

Table of Contents

Step 1: Create a new application
Step 2: Creating objects
Step 3: Unit testing
Advantages (so far)
Step 4: Making data more reliable and secure
Step 5: Object conversions and error handling
Step 6: File input/output
Step 7: Creating the 'random arcs' picture
Step 8: Testing the application

We are now going to write an application that creates a picture based on random data provided by people. The picture will be composed of adjacent 90 degrees arcs with different radii and colors. Here is an example of the final result:

Figure 7.1. Obix programming language random arcs tutorial example

The arcs' radii will be based on data provided by users of the application. A single user input for one arc is called a contribution and is composed of:

a numerical identifier (autoincrement)
the first name and last name of the contributor
a random value used to calculate a radius, ranging from 1 to 15
the date and time the contribution was submitted

The application will be able to collect contributions directly submitted by users (e.g. via a web site) and/or by other applications (e.g. via a HTTP request). All contributions will be saved in a JSON text file.

The arcs' colors will be defined by a random generator.

The final picture will be created in the Scalable Vector Graphics (SVG) format and stored in a file.

Note

This application clearly doesn't belong to the category 'life saving applications'. Despite this fact we will nevertheless write reliable code. User input data can be invalid or malicious, and resources like files can fail at runtime. Therefore, invalid data protection and resource error handling will play a very important role in the following chapters. The objective is to see how to write professional, reliable applications.

To develop this application you will do the following:

Create a new application
Write code to create contribution objects
Write unit tests (to detect bugs)
Make data more reliable and secure (!)
Convert objects to (and re-create them from) JSON
Use files to store/restore data
Create the 'random arcs' picture and save it in a SVG file
Run the application with test data (and hopefully enjoy beautil drawings)

	Note
	The full project with source code is included in the Obix download. It is located in subdirectory `projects/pr_ex_21_random_arcs/.`

Step 1: Create a new application

To create the 'random arcs' application proceed as follows:

Open a system console (a text terminal in Linux, or a command line window in Windows).
Go to the root directory of your Obix installation.
Create a new project named random_arcs by entering the following command:
```
on Linux type:
./obix.sh create_project project_id:random_arcs kind:java

on Windows type:
obix create_project project_id:random_arcs kind:java
```
Obix creates a directory named pr_random_arcs in subdirectory projects of your Obix installation (e.g. obix-0.7.5/projects/pr_random_arcs)

If you need more information please look at the README.TXT file in the project's root directory or refer to the previous Chapter 4, How to develop a standard PC application?.


Part II. Basic concepts and concepts for more reliable code		Step 2: Creating objects