AP Computer Science A FRQ Walkthrough

Overview

This AP Computer Science A FRQ asks you to write the complete SignedText class. The class works with a person's name and adds a signature to a piece of text.

What the Class Needs

The SignedText class needs the following parts:

• Instance variables to store the first and last name
• A constructor
• A getSignature() method
• An addSignature(String text) method

Step 1: Understand the Signature Rules

The method getSignature() creates a signature using the first name and last name.

Rule 1: Empty First Name

If the first name is empty, the signature is only the last name.

new SignedText("", "Wong")

The signature would be:

Wong

Rule 2: Non-Empty First Name

If the first name is not empty, the signature uses the first letter of the first name, followed by a dash, followed by the last name.

new SignedText("henri", "dubois")

The signature would be:

h-dubois

Step 2: Decide the Instance Variables

Since the constructor receives a first name and a last name, the class needs to store both values.

private String firstName;
private String lastName;

These variables are called instance variables because they belong to each individual SignedText object.

Step 3: Write the Constructor

The constructor receives the first and last name as parameters and saves them in the instance variables.

public SignedText(String first, String last)
{
    firstName = first;
    lastName = last;
}

Step 4: Write getSignature()

The getSignature() method returns the signature as a String.

First, check whether the first name is empty.

firstName.equals("")

Another possible way to check this is:

firstName.length() == 0

Solution for getSignature()

public String getSignature()
{
    if (firstName.equals(""))
    {
        return lastName;
    }

    return firstName.substring(0, 1) + "-" + lastName;
}

Why Use substring(0, 1)?

In Java, substring(0, 1) gets the first character of a String as a String.

"henri".substring(0, 1)

This returns:

h

Step 5: Understand addSignature(String text)

The method addSignature(String text) receives a String and returns a possibly changed version of that String.

This method has three possible cases:

Step 6: Use getSignature() Inside addSignature()

Instead of rebuilding the signature again, call the method getSignature().

String sig = getSignature();

This is good programming because it avoids repeating code.

Step 7: Check if the Signature Is at the End

If the text already ends with the signature, return the text unchanged.

if (text.endsWith(sig))
{
    return text;
}

Step 8: Check if the Signature Is at the Beginning

If the text starts with the signature, remove it from the beginning and add it to the end.

if (text.startsWith(sig))
{
    return text.substring(sig.length()) + sig;
}

How This Works

text.substring(sig.length()) removes the signature from the front of the text.

Step 9: If the Signature Is Missing

If the signature is not at the beginning and not at the end, add it to the end.

return text + sig;

Complete Solution

public class SignedText
{
    private String firstName;
    private String lastName;

    public SignedText(String first, String last)
    {
        firstName = first;
        lastName = last;
    }

    public String getSignature()
    {
        if (firstName.equals(""))
        {
            return lastName;
        }

        return firstName.substring(0, 1) + "-" + lastName;
    }

    public String addSignature(String text)
    {
        String sig = getSignature();

        if (text.endsWith(sig))
        {
            return text;
        }

        if (text.startsWith(sig))
        {
            return text.substring(sig.length()) + sig;
        }

        return text + sig;
    }
}

Student-Friendly Algorithm

1. Store the first name and last name in instance variables.
2. In getSignature():
   • If the first name is empty, return only the last name.
   • Otherwise, return the first letter of the first name, a dash, and the last name.
3. In addSignature(String text):
   • Save the signature in a variable.
   • If the text already ends with the signature, return the text unchanged.
   • If the text starts with the signature, remove it from the front and add it to the end.
   • Otherwise, add the signature to the end.

Key Java Concepts Used

• String variables
• Instance variables
• Constructors
• substring()
• length()
• equals()
• startsWith()
• endsWith()
• if statements
• return statements

Common Mistakes to Avoid

• Forgetting to create instance variables.
• Using == instead of .equals() to compare Strings.
• Forgetting that substring(0, 1) gets only the first character.
• Adding the signature twice when it is already at the end.
• Forgetting to remove the signature from the front before moving it to the end.

Quick Practice Questions

Question 1

What would this return?

SignedText s = new SignedText("Anna", "Smith");
s.getSignature();

Answer:

A-Smith

Question 2

What would this return?

SignedText s = new SignedText("", "Wong");
s.getSignature();

Answer:

Wong

Question 3

If the signature is FOX, what does this return?

addSignature("Hello")

Answer:

HelloFOX

Question 4

If the signature is FOX, what does this return?

addSignature("FOXHello")

Answer:

HelloFOX

Question 5

If the signature is FOX, what does this return?

addSignature("HelloFOX")

Answer:

HelloFOX

Final Takeaway