3.2 Impact of Program Design

1. System reliability: test with many conditions

System reliability means the program can perform its tasks as expected under stated conditions without failure 3.2.A.1. Programmers should try to maximize reliability by testing the program with a variety of conditions 3.2.A.1.

Here is a method that turns a quiz score into a percent. It works on the first test, so it looks finished.

public class QuizScore
{
    public static int percent(int correct, int total)
    {
        return correct * 100 / total;
    }
}

System.out.println(QuizScore.percent(18, 20));
System.out.println(QuizScore.percent(0, 20));
System.out.println(QuizScore.percent(5, 0));

Output:

90
0
Exception in thread "main" java.lang.ArithmeticException: / by zero

The third test, a quiz with no questions yet, makes the program fail with a run-time error. A good test plan includes:

Kind of test	Example input	Why
Typical value	18 out of 20	The normal case works.
Boundary value	0 out of 20, 20 out of 20	The smallest and largest results are correct.
Unusual value	5 out of 0	The program does not crash on input it did not expect.

The improved version states its behavior for the unusual case, so it works as expected under all stated conditions:

public class QuizScore
{
    /** Returns the percent correct, or 0 when total is 0 */
    public static int percent(int correct, int total)
    {
        if (total == 0)
        {
            return 0;
        }
        return correct * 100 / total;
    }
}

System.out.println(QuizScore.percent(18, 20));
System.out.println(QuizScore.percent(20, 20));
System.out.println(QuizScore.percent(5, 0));

Output:

90
100
0

2. Programs have beneficial and harmful impacts

Creating a program has impacts on society, the economy and culture, and these impacts can be both beneficial and harmful 3.2.A.2. A program meant to fill a need or solve a problem can have unintended harmful effects beyond its intended use 3.2.A.2.

Program	Beneficial impact	Possible harmful impact
A route planner	Drivers save time and fuel (economy).	It sends heavy traffic through quiet streets next to a school (society).
A music recommendation app	Listeners discover new artists (culture).	It keeps showing the same popular songs, so small artists are heard less (culture, economy).
An online grade portal	Students see their progress quickly (society).	Students without home internet see it later than others (society).

The route planner problem can start with one design choice in the code. This method only looks at travel time:

public class RoutePlanner
{
    public static String choose(int mainRoadMinutes, int sideStreetMinutes)
    {
        if (sideStreetMinutes < mainRoadMinutes)
        {
            return "side street";
        }
        return "main road";
    }
}

System.out.println(RoutePlanner.choose(14, 12));

Output:

side street

Every user gets the same answer, so thousands of cars go down the side street. A more careful design adds a condition the designers care about, such as avoiding a school zone during school hours:

public class RoutePlanner
{
    public static String choose(int mainRoadMinutes, int sideStreetMinutes,
                                boolean schoolHours)
    {
        if (sideStreetMinutes < mainRoadMinutes && !schoolHours)
        {
            return "side street";
        }
        return "main road";
    }
}

System.out.println(RoutePlanner.choose(14, 12, true));
System.out.println(RoutePlanner.choose(14, 12, false));

Output:

main road
side street

Watch out: on the AP® exam, an “impact” answer should name who is affected and how. “It is bad for people” is too vague; “it increases traffic danger near a school” is specific.

3. Legal issues and intellectual property

Code is intellectual property: it belongs to the person or company that wrote it. Legal issues and intellectual property concerns arise when you create programs 3.2.A.3.

• Programmers often reuse code written by others that is published as open source and free to use 3.2.A.3. Open source code comes with a license that says what you may do with it, for example that you must keep the author’s name and the license text.
• To include code that is not published as open source, the programmer must obtain permission and often purchase the code before putting it in their program 3.2.A.3.

A good habit is to record where reused code came from, right in a comment:

public class TextUtils
{
    // The method below is adapted from an open source utility library.
    // It is free to use under that library's license, which asks us
    // to keep this notice and credit the original authors.
    public static String initials(String first, String last)
    {
        return first.substring(0, 1) + last.substring(0, 1);
    }
}

System.out.println(TextUtils.initials("Sam", "Rivera"));
System.out.println(TextUtils.initials("Lee", "Chen"));

Output:

SR
LC

Situation	Can you use the code?
Open source library, free to use, you follow its license	Yes 3.2.A.3
A company’s code you found in a leaked file	No: it is not open source and you have no permission.
A paid library you have not bought or licensed	Only after you obtain permission, which often means buying it 3.2.A.3