FRQ 1B - Strings

Section 15.6 FRQ 1B - Strings - Part 3

From the 2025 Course and Exam Description:

Free-Response Question 1: Methods and Control Structures. Students will write two methods or one constructor and one method of a given class based on the provided specifications and examples. In Part A (4 points), the method or constructor will require students to write iterative or conditional statements, or both, as well as statements that call methods in the specified class.

🔗

In Part B (3 points), the method or constructor will require calling String methods.

🔗

FRQ 1B requires the following skills:

🔗

Complete a method or a constructor

🔗
Call String methods

🔗
Use if statements

🔗
Use a for loop or a while loop

🔗

Teacher Insights

🔗

You do not need to include import statements. You can assume that any standard Java class is already imported.

🔗
You will NEVER print. If you think you need to print, you probably need to return something.

🔗
You will NOT need to write a main.

🔗

Subsection 15.6.1 Note - Practice FRQs

There are very few examples of FRQ1B, since it has only existed since 2025-2026. You will see several examples of Strings being used in FRQs 2-3 in the following lessons.

🔗

Here are a few unofficial FRQs that are written more like an official FRQ.

🔗

Subsection 15.6.2 Practice FRQ - The EmailFilter Class

A system is being designed to "sanitize" email subject lines by removing specific "flagged" words and replacing them with a placeholder. You will write the method sanitizeSubject.

🔗

The sanitizeSubject method takes two parameters: a String subject and a String flaggedWord. The method should return a new String where every occurrence of flaggedWord in subject is replaced by the string "[REDACTED]".

🔗

/**
 * Returns a version of the subject where every instance of flaggedWord
 * is replaced by "[REDACTED]".
 * Precondition: subject and flaggedWord are not null.
 */
public String sanitizeSubject(String subject, String flaggedWord) {

    // TO BE IMPLEMENTED IN PART (B)

}

Constraints and Rules:

🔗

If the flaggedWord is not found, the original subject should be returned.

🔗
The search should be case-sensitive.

🔗

To solve this, you need a while loop to find the word, and you must "snip" the string into pieces to reassemble it.

🔗

Common AP Exam Traps to Avoid:

🔗

The Infinite Loop: If you don’t update the string you are searching (or don’t move your index), the indexOf will keep finding the same word forever.

🔗
The "Last Bit" Bug: Many students forget to add result += remaining at the very end. Without it, your code would cut off the end of the sentence (e.g., "Check out this [REDACTED]" instead of "Check out this [REDACTED] offer!").

🔗

Activity 15.6.1.

A system is being designed to "sanitize" email subject lines by removing specific "flagged" words and replacing them with a placeholder. You will write the method sanitizeSubject.

🔗

public class EmailFilter {

public static String sanitizeSubject(String subject, String flaggedWord) {}

public static void main(String[] args) {
        // Test 1: Standard case with one word
        System.out.println("Test 1: Single Flagged Word");
        System.out.println(EmailFilter.sanitizeSubject("Check out this FREE offer!", "FREE"));
        System.out.println();

// Test 2: Multiple occurrences
        System.out.println("Test 2: Multiple Flagged Words");
        System.out.println(EmailFilter.sanitizeSubject("URGENT: Please read URGENT", "URGENT"));
        System.out.println();

// Test 3: No occurrences
        System.out.println("Test 3: No Flagged Words Found");
        System.out.println(EmailFilter.sanitizeSubject("Hello World", "Secret"));
    }
}
====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper {
    private String[] input1 = {
        "spam spam spam", "Danger Zone", "The end is near", "HiddenSecret", "banana"
    };
    private String[] input2 = {"spam", "Danger", "near", "Secret", "ana"};
    private String[] expected = {
        "[REDACTED] [REDACTED] [REDACTED]",
        "[REDACTED] Zone",
        "The end is [REDACTED]",
        "Hidden[REDACTED]",
        "b[REDACTED]na"
    };

@Test
    public void test0() {
        boolean passed = test(0);
        assertTrue(passed);
    }

@Test
    public void test1() {
        boolean passed = test(1);
        assertTrue(passed);
    }

@Test
    public void test2() {
        boolean passed = test(2);
        assertTrue(passed);
    }

@Test
    public void test3() {
        boolean passed = test(3);
        assertTrue(passed);
    }

@Test
    public void test4() {
        boolean passed = test(4);
        assertTrue(passed);
    }

private boolean test(int i) {
        String desc = "sanitizeSubject(\"" + input1[i] + "\", \"" + input2[i] + "\")";
        String actual = "";

try {
            actual = "" + EmailFilter.sanitizeSubject(input1[i], input2[i]);
        } catch (Exception e) {
            actual = "The code caused an exception";
        }

boolean passed = getResults(expected[i], actual, desc);
        return passed;
    }

private boolean stringMatch(String str1, String str2) {
        str1 = str1.replaceAll(" ", "");
        str2 = str2.replaceAll(" ", "");

return str1.contains(str2);
    }
}

🔗

Subsection 15.6.3 Practice FRQ - The SequenceAnalyzer Class

In bioinformatics, scientists analyze DNA strings composed of the characters A, C, G, and T. A "doublet" is defined as any two-character sequence where a character is immediately followed by the same character (e.g., "AA", "GG").

🔗

Write the method countValidDoublets. This method analyzes a given DNA string and counts how many doublets exist, but only if the doublet appears after the first occurrence of a "start marker" character.

🔗

public class SequenceAnalyzer {
    /**
     * Counts doublets (two identical consecutive characters) that appear AFTER the first occurrence
     * of startMarker. Precondition: dna is not null, dna.length() >= 2.
     */
    public static int countValidDoublets(String dna, String startMarker) {

        /* TO BE IMPLEMENTED */

    }
}

Activity 15.6.2.

🔗

public class SequenceAnalyzer {
    /**
     * Counts doublets (two identical consecutive characters) that appear AFTER the first occurrence
     * of startMarker. Precondition: dna is not null, dna.length() >= 2.
     */
    public static int countValidDoublets(String dna, String startMarker) {}

public static void main(String[] args) {
        // Test 1: Standard DNA sequence
        // Marker 'T' is at index 1. Doublets after 'T': 'GG'
        String dna1 = "ATGGC";
        System.out.println("DNA: " + dna1 + " | Marker: T");
        System.out.println(
                "Doublets found: " + SequenceAnalyzer.countValidDoublets(dna1, "T")); // Expected: 1
        System.out.println();

// Test 2: Marker at the very beginning
        // Marker 'A' is at index 0. Doublets: 'CC' and 'CC' (for 'CCC')
        String dna2 = "ACCCG";
        System.out.println("DNA: " + dna2 + " | Marker: A");
        System.out.println(
                "Doublets found: " + SequenceAnalyzer.countValidDoublets(dna2, "A")); // Expected: 2
        System.out.println();

// Test 3: Marker not found
        String dna3 = "GATTACA";
        System.out.println("DNA: " + dna3 + " | Marker: X");
        System.out.println(
                "Doublets found: " + SequenceAnalyzer.countValidDoublets(dna3, "X")); // Expected: 0
    }
}
====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper {
    private String[] input1 = {"AATTAA", "GGGGG", "ATGCGG", "ATGC", "AABBAA"};
    private String[] input2 = {"T", "G", "GG", "C", "B"};
    private String[] expected = {"2", "4", "1", "0", "2"};

@Test
    public void test0() {
        boolean passed = test(0);
        assertTrue(passed);
    }

@Test
    public void test1() {
        boolean passed = test(1);
        assertTrue(passed);
    }

@Test
    public void test2() {
        boolean passed = test(2);
        assertTrue(passed);
    }

@Test
    public void test3() {
        boolean passed = test(3);
        assertTrue(passed);
    }

@Test
    public void test4() {
        boolean passed = test(4);
        assertTrue(passed);
    }

private boolean test(int i) {
        String desc = "countValidDoublets(\"" + input1[i] + "\", \"" + input2[i] + "\")";
        String actual = "";

try {
            actual = "" + SequenceAnalyzer.countValidDoublets(input1[i], input2[i]);
        } catch (Exception e) {
            actual = "The code caused an exception";
        }

boolean passed = getResults(expected[i], actual, desc);
        return passed;
    }

private boolean stringMatch(String str1, String str2) {
        str1 = str1.replaceAll(" ", "");
        str2 = str2.replaceAll(" ", "");

return str1.contains(str2);
    }
}

🔗

Subsection 15.6.4 Practice FRQ - TextExpander

A software company is creating a "Short-Hand Expander" that converts common abbreviations into their full-word equivalents. You will write a method for the TextExpander class that finds and replaces specific abbreviations.

🔗

Write the method expandText, which takes three String parameters: message, abbr, and fullWord. The method should return a new String where every occurrence of the abbreviation abbr in message is replaced by fullWord.

🔗

/**
 * Returns a version of message where every instance of abbr
 * is replaced by fullWord.
 * Precondition: message, abbr, and fullWord are not null.
 */
public String expandText(String message, String abbr, String fullWord) {

    /* TO BE IMPLEMENTED BELOW */
}

Constraints:

🔗

If the abbreviation is not found, the original message should be returned.

🔗
The search should be case-sensitive.

🔗

Activity 15.6.3.

🔗

public class TextExpander {
    /**
     * Returns a version of message where every instance of abbr is replaced by fullWord.
     * Precondition: message, abbr, and fullWord are not null.
     */
    public static String expandText(String message, String abbr, String fullWord) {}

public static void main(String[] args) {
        // Test 1: Standard replacement
        String m1 = "See you l8r!";
        System.out.println("Input: " + m1);
        System.out.println("Result: " + expandText(m1, "l8r", "later"));
        System.out.println();

// Test 2: Multiple occurrences
        String m2 = "u should call u-r mom";
        System.out.println("Input: " + m2);
        System.out.println("Result: " + expandText(m2, "u", "you"));
        System.out.println();

// Test 3: Abbreviation not found
        String m3 = "Hello world";
        System.out.println("Input: " + m3);
        System.out.println("Result: " + expandText(m3, "bye", "goodbye"));
    }
}
====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper {
    private String[] input1 = {"asap please", "Call me asap", "aaaaa", "JK...JK", "100%"};
    private String[] input2 = {"asap", "asap", "aa", "JK", "100"};
    private String[] input3 = {"as soon as possible", "soon", "b", "Just Kidding", "One Hundred"};
    private String[] expected = {
        "as soon as possible please",
        "Call me soon",
        "bba",
        "Just Kidding...Just Kidding",
        "One Hundred%"
    };

@Test
    public void test0() {
        boolean passed = test(0);
        assertTrue(passed);
    }

@Test
    public void test1() {
        boolean passed = test(1);
        assertTrue(passed);
    }

@Test
    public void test2() {
        boolean passed = test(2);
        assertTrue(passed);
    }

@Test
    public void test3() {
        boolean passed = test(3);
        assertTrue(passed);
    }

@Test
    public void test4() {
        boolean passed = test(4);
        assertTrue(passed);
    }

private boolean test(int i) {
        String desc = "expandText(\"" + input1[i] + "\", \"" + input2[i] + "\"" + input3[i] + "\")";
        String actual = "";

try {
            actual = "" + TextExpander.expandText(input1[i], input2[i], input3[i]);
        } catch (Exception e) {
            actual = "The code caused an exception";
        }

boolean passed = getResults(expected[i], actual, desc);
        return passed;
    }

private boolean stringMatch(String str1, String str2) {
        str1 = str1.replaceAll(" ", "");
        str2 = str2.replaceAll(" ", "");

return str1.contains(str2);
    }
}

🔗

You have attempted of activities on this page.

🔗

Prev Top Next