AP Computer Science A 2023 FRQ

FRQ 2

This FRQ asks you to write an entire class, given specifications of what the class should do, and including any variables and methods the class should include.

• class: a list of attributes and instructions (a blueprint) for how to create an instance of itself
  • attribute: characteristics of an object
  • behaviors: actions hat an object can perform
• object: an instance of the class
• instantiate: to all the constructor to create an object
• constructor: a block of code that has the same name as the class and tells the computer how to create a new object
• creating objects example
• parameters: values that you can pass into methods or constructors to provide input or configuration for their operations
• methods: a function

After reading FRQ, we are tasked with creating a sign with a specific number of characters per line.

• Class Description: Create a Sign class.

• Constructor: The class has a constructor with two parameters:
  • A String message to be displayed on the sign.
  • An int representing the width of each line on the sign.

• numberOfLines Method: Implement a method called numberOfLines that returns the number of lines needed to display the text on the sign.

• getLines Method: Implement a method called getLines that returns the message broken into lines separated by semicolons (;) or null if the message is empty. No semicolon should appear at the end of the returned string.

• Testing: Create instances of the Sign class with different parameters and test the numberOfLines and getLines methods to ensure correct behavior.

// STEP 1: Create class
public class Sign
{
    // STEP 2: Create variables
    private String m;
    private int w;

    // STEP 3: Create constructor with two parameters referencing information given in the table
    public Sign(String message, int width)
    {
        this.m = message;
        this.w = width;
    }

    // STEP 4: Create numberOfLines() method with no parameters
        // Method should divide message by width of sign
        // If result has a remainder, another line must be added
    public int numberOfLines()
    {
       int result = m.length() / w;
       if(m.length() % w > 0)
       {
            result += 1;
       }
       return result; 
    }

    // STEP 5: Create getLines() method with no parameters
        // If length = 0, return null
        // Write message based on message length, adding a semicolon at the end
    public String getLines()
    {
        if(m.length() == 0)
        {
            return null;
        }
        String result = "";
        int count = 0;

        for(int i = 0; i < m.length(); i++)
        {
            if(count == w)
            {
                result += ";";
                count = 0;
            }
            result += m.substring(i, i+1);
            count++;
        }

        return result;
    }
    

}

Testing the Code

Using the table provided by College Board, we test the code by instantiating class Sign and providing different parameters to see if the method call return value is expected.

String str;
int x;

Sign sign1 = new Sign ("ABC222DE", 3);
x = sign1.numberOfLines();
str = sign1.getLines();

System.out.println(str);
System.out.println(x);

ABC;222;DE
3

Sign sign2 = new Sign ("ABCD", 10);

x = sign2.numberOfLines();
str = sign2.getLines();

System.out.println(str);
System.out.println(x);

ABCD
1

Sign sign3 = new Sign("ABCDEF",2);

x = sign3.numberOfLines();
str = sign3.getLines();

System.out.println(str);
System.out.println(x);

AB;CD;EF
3

Sign sign4 = new Sign("",4);

x = sign4.numberOfLines();
str = sign4.getLines();

System.out.println(str);
System.out.println(x);

null
0

Sign sign5 = new Sign("AB_CD_EF", 2);

x = sign5.numberOfLines();
str = sign5.getLines();

System.out.println(str);
System.out.println(x);

AB;_C;D_;EF
4

Peer Grading: Sreeja

Point	Requirement	Responses to Earn Point	Responses Not to Earn Point	Points
1	Declares class header: class Sign	Declare the class as public	Declare the class as something other than public	1 point
2	Declares appropriate private instance variable(s) and constructor	Initialize instance variables using appropriate parameters	Declare the variable outside the class, or in the class within a method or constructor	1 point
3	Declares constructor header: Sign(String __, int __)	Calculate values in the constructor that are returned by other methods (correctly or incorrectly), as long as the parameter types are correct	Declare the constructor as something other than public	1/1
4	Declares method headers: - public int numberOfLines() - public String getLines()	Omit either method	Omit public or declare the method as something other than public	1/1
5	numberOfLines divides the message length by the line width	Perform the division in a method other than numberOfLines	Perform the division without using the division operator by counting line-widthsized portions of the message or by counting lines produced by the line-delimiting algorithm; Incorrectly account for the final line; Use a method name inconsistent with the examples, as long as it is recognizably equivalent	1/1
6	numberOfLines returns appropriate value (algorithm)	Perform the return value calculation in the constructor; Return a different number of lines than getLines produces, as long as the number returned is the correct number for the message; return an incorrect number of lines for the message, as long as the number returned is exactly the number of lines produced by getLines; use a method name inconsistent with the examples, as long as it is recognizably equivalent	incorrectly account for the final line	1/1
7	getLines returns null appropriately	Identify null case in a method other than getLines; Use an invalid call to length or == in guard for null return; Use a method name inconsistent with the examples, as long as it is recognizably equivalent	Guard the return with incorrect logic	1/1
8	Calls substring and length (or equivalent) on String objects	Calculate substring parameter values incorrectly; Call substring and/or length from a method other than getLines; Use a method name inconsistent with the examples, as long as it is recognizably equivalent	Fail to call substring or length on String objects; Call substring or length with an incorrect number of parameters, with a parameter of an incorrect type, or with incorrectly ordered parameters, anywhere in the class	1/1
9	getLines constructs the delimited sign output appropriately (algorithm)	Call substring and/or length incorrectly; Fail to return the constructed String (return not assessed); Handle the empty string /null case incorrectly; Construct the output in the constructor; Use a method name inconsistent with the examples, as long as it is recognizably equivalent	End the constructed output with a ; or extraneous spaces; Modify the contents of message or width after they have been initialized (no additional –1 penalty)	1/1

Score: 9/9

1/1

Major Takeaways

• Make a diagram of the class, variables, and methods
• Read descriptions of methods carefully as they tell you exactly what to do
• be careful of null outputs (make sure to include them!)
• Use sample code execution sequence to test code by mentally inputting them into your code