5.13. Free Response Question (FRQ) for Classes

As of 2019, the AP exam’s second free response question (FRQ) is on classes, where students design and implement a described class. This question involves creating a class with private instance variables and public methods. The College Board will provide a description of the class and the methods. This question does not involve more complex topics such as arrays.

To succeed on the FRQ Question 2 on Classes, you must know how to:

5.13.1. 2019 StepTracker Class FRQ

The 2019 FRQ 2 for the class StepTracker is a good example of what to expect. It is available as question 2 on page 7 of https://apstudents.collegeboard.org/sites/default/files/2019-05/ap19-frq-computer-science-a.pdf , reproduced below.

Question 2.

This question involves the implementation of a fitness tracking system that is represented by the StepTracker class. A StepTracker object is created with a parameter that defines the minimum number of steps that must be taken for a day to be considered active. The StepTracker class provides a constructor and the following methods.

  • addDailySteps, which accumulates information about steps, in readings taken once per day

  • activeDays, which returns the number of active days

  • averageSteps, which returns the average number of steps per day, calculated by dividing the total number of steps taken by the number of days tracked

The following table contains a sample code execution sequence and the corresponding results.

Statements and ExpressionsValue Returned (blank if no value)Comment
StepTracker tr = new StepTracker(10000); Days with at least 10,000 steps are considered active. Assume that the parameter is positive.
tr.activeDays(); 0No data have been recorded yet.
tr.averageSteps(); 0.0When no step data have been recorded, the averageSteps method returns 0.0.
tr.addDailySteps(9000); This is too few steps for the day to be considered active.
tr.addDailySteps(5000); This is too few steps for the day to be considered active.
tr.activeDays(); 0No day had at least 10,000 steps.
tr.averageSteps(); 7000.0The average number of steps per day is (14000 / 2).
tr.addDailySteps(13000); This represents an active day.
tr.activeDays(); 1Of the three days for which step data were entered, one day had at least 10,000 steps.
tr.averageSteps(); 9000.0The average number of steps per day is (27000 / 3).
tr.addDailySteps(23000); This represents an active day.
tr.addDailySteps(1111); This is too few steps for the day to be considered active.
tr.activeDays(); 2Of the five days for which step data were entered, two days had at least 10,000 steps.
tr.averageSteps(); 10222.2The average number of steps per day is (51111 / 5).

This question asks you to write the complete StepTracker class, including the constructor and any required instance variables and methods. Your implementation must meet all specifications and conform to the example.

5.13.2. groupwork Determining the Instance Variables

Work in pairs or groups to read through the problem statement and determine the instance variables required for this class. During the exam, it helps to circle the words that are important and may describe the instance variables. Different groups may come up with different variables that will still work. Groups should report back and compare answers in class to determine the best variables before writing the class.

It may help to first identify the variables that are needed for the constructor and the accessor and mutator methods.

Click to reveal multiple choice questions that will help you to determine the instance variables through the constructor parameters and accessor and mutator methods.

5-13-5: What are the instance variables (at least 4!) that you need for the StepTracker class? What are the data types for each instance variable?

5.13.3. Writing the Class Header and Constructor

You will receive at least 1 point if you write the class header and a constructor that has the same name as the class and no return type. You will receive another point for creating private instance variables inside the class. Complete the class definition below with the class name, the instance variables you determined above, and the constructor. Remember that for this problem, a StepTracker object is created (with a constructor) with a parameter that defines the minimum number of steps that must be taken for a day to be considered active. The constructor will often have a parameter which it should assign to an instance variable. It should also assign default values to the other instance variables.

Write the first draft of the class StepTracker below with the class name, the instance variables, and the constructor with a parameter for the minimum number of steps threshold for active days. Make sure it compiles.

Here is the rubric for the instance variables and the constructor for this problem. Did you receive all 3 points? In class, your teacher may have you grade each others’ code.

Rubric for instance variables and constructor

Figure 1: Rubric for instance variables and constructor

5.13.4. Writing the Accessor Method activeDays

Each method in the FRQ is worth 1 - 3 points. The method header is usually worth 1 point and the code in the method body is usually worth another point or two depending on how complex it is.

This problem asks you to write a simple accessor method called activeDays which returns the number of active days (which should be an instance variable) for 1 point.

Remember that accessor methods usually look like the following:

class ExampleClass
{
  //Instance variable declaration
  private typeOfVar varName;

  // Accessor method template
  public typeOfVar getVarName()
  {
     return varName;
  }
}

Click to reveal a multiple choice problem to help you determine the accessor method header.

Copy the code from your first draft of the class StepTracker above with the instance variables and constructor. Write the accessor methods activeDays which returns the number of active days.

Here is the rubric for the accessor method activeDays() for this problem. The second column is small mistakes that will still earn the point but the third column is larger mistakes that will not earn the point. Did you receive the point for this method? In class, your teacher may have you grade each others’ code.

Rubric for acccessor methods

Figure 2: Rubric for accessor method activeDays()

5.13.5. Writing the Mutator Method addDailySteps

This problem asks you to write a more complex mutator method called addDailySteps worth 3 points.

Remember that mutator methods often look like the following:

class Example
{
    //Instance variable declaration
    private typeOfVar varName;

    // Mutator method template
    public void changeVarName(typeOfVar newValue)
    {
       // an instance variable is changed through = or an operator like +=, -=, ++, etc.
       varName = newValue;
    }
}

Click to reveal a multiple choice problem to help you determine the mutator method header.

The code for this mutator method is a little more complex than the template above, because it needs to change more than 1 instance variable. Notice the comments in the sample code execution:

Statements and ExpressionsValue Returned (blank if no value)Comment
tr.addDailySteps(5000); This is too few steps for the day to be considered active.
tr.activeDays(); 0No day had at least 10,000 steps.
tr.addDailySteps(13000); This represents an active day.
tr.activeDays(); 1Of the three days for which step data were entered, one day had at least 10,000 steps.

Consider each of your instance variables and whether this method should change them. The problem which you can reveal below may help you.

Copy the code from your draft of the class StepTracker above with the class name, the instance variables, constructor, and accessory method. Write the mutator method addDailySteps which takes a parameter and adds it to the appropriate instance variable and changes other instance variables appropriately.

Here is the rubric for the mutator method for this problem. The second column is small mistakes that will still earn the point but the third column is larger mistakes that will not earn the point. Did you receive all the points? In class, your teacher may have you grade each others’ code.

Rubric for mutator method

Figure 3: Rubric for mutator method

5.13.6. Writing the Accessor Method averageSteps

This problem asks you to write a more complex accessor method which uses the instance variables to calculate and return the averageSteps for 2 points. This method returns the average number of steps per day, calculated by dividing the total number of steps taken by the number of days tracked.

Click to reveal a multiple choice problem to help you determine the accessor method header.

The complex accessor method averageSteps() must calculate the average number of steps from your instance variables. Notice that the first time it is called in the sample code execution, it returns 0.0 since there are no steps recorded. This avoids a divide by 0 error.

Statements and ExpressionsValue Returned (blank if no value)Comment
tr.averageSteps(); 0.0When no step data have been recorded, the averageSteps method returns 0.0.

Copy the code from your draft of the class StepTracker above with the instance variables, constructor, accessor and mutator methods. Write the accessor method averageSteps which returns the average number of steps per day, calculated by dividing the total number of steps taken by the number of days tracked.

Here is the rubric for the averageSteps method for this problem. Did you receive all the points? In class, your teacher may have you grade each others’ code.

Rubric for acccessor methods

Figure 4: Rubric for averageSteps method

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