ArrayList FRQ Practice – AP Computer Science A

The ArrayList is one of the most frequently tested topics on the AP Computer Science A exam.
Students must understand how to add, remove, modify, filter, and search elements inside an ArrayList — often with loops and conditions combined.

This page gives you, exam-style FRQs to strengthen your ArrayList logic and exam performance.

Why Practice ArrayList FRQs?

ArrayList FRQs commonly test:

• Adding items
• Removing items
• Filtering elements
• Counting and searching
• Processing lists of objects
• In-place mutation of a list
• Iteration using for-loop or enhanced for-loop
• Avoiding ConcurrentModification issues

Mastering these patterns will significantly improve your FRQ score.

How to Use This Page

• Solve each problem first without looking at the answer
• Compare your approach with the step-by-step solution
• Review the “Common Mistakes” section
• Retry the problem and apply the corrections

FRQ 1 — Count Strings Containing a Letter (Easy)

Problem

Write countContaining(ArrayList<String> list, String letter) that returns how many elements in the list contain the given letter.

Example:
[“cat”, “dog”, “car”, “tree”], letter = “a” → returns 2

Approach

• Loop through list
• Use contains()
• Increase count when true

Java Solution

public int countContaining(ArrayList<String> list, String letter) {
    int count = 0;
    for (String s : list) {
        if (s.contains(letter)) {
            count++;
        }
    }
    return count;
}

Common Mistakes

• Using == for Strings
• Not understanding case-sensitivity
• Using indexOf incorrectly

FRQ 2 — Remove All Short Words (Medium)

Problem

Write a method removeShortWords(ArrayList<String> words, int minLength) that removes all words shorter than minLength.

Modify the list in-place.

Example:
Input: [“hi”, “apple”, “go”, “phone”], minLength = 3
Output: [“apple”, “phone”]

Approach

• Iterate from the end to the beginning
• Remove elements that are too short
• Backwards loop avoids skipping elements

Java Solution

public void removeShortWords(ArrayList<String> words, int minLength) {
    for (int i = words.size() - 1; i >= 0; i--) {
        if (words.get(i).length() < minLength) {
            words.remove(i);
        }
    }
}

Common Mistakes

• Using enhanced for-loop (causes ConcurrentModificationException)
• Looping forward and skipping elements
• Forgetting that remove(i) shifts the list

FRQ 3 — Double All Even Numbers (Medium)

Problem

Write a method doubleEvens(ArrayList<Integer> nums) that doubles every even number in the list.

Approach

• Loop with index
• Use get(i)
• Replace with set(i, newValue)

Java Solution

public void doubleEvens(ArrayList<Integer> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums.get(i) % 2 == 0) {
            nums.set(i, nums.get(i) * 2);
        }
    }
}

Common Mistakes

• Trying to modify values inside enhanced for-loop
• Using remove inside this logic
• Forgetting to use set()

FRQ 4 — Extract All Positive Numbers (Medium)

Problem

Write getPositives(ArrayList<Integer> nums) that returns a new ArrayList containing only the positive values.

Original list must remain unchanged.

Approach

• Create new list
• Add only positive numbers
• Return new list

Java Solution

public ArrayList<Integer> getPositives(ArrayList<Integer> nums) {
    ArrayList<Integer> result = new ArrayList<>();
    for (int n : nums) {
        if (n > 0) {
            result.add(n);
        }
    }
    return result;
}

Common Mistakes

• Modifying the original list
• Returning null for empty lists

FRQ 5 — Find the Longest String (Hard)

Problem

Write longestString(ArrayList<String> words) that returns the longest string.
If the list is empty, return "".

Approach

• Handle empty list case
• Start with first element as longest
• Compare lengths

Java Solution

public String longestString(ArrayList<String> words) {
    if (words.size() == 0) return "";

    String longest = words.get(0);

    for (String s : words) {
        if (s.length() > longest.length()) {
            longest = s;
        }
    }
    return longest;
}

Common Mistakes

• Forgetting to check if list is empty
• Using >= which changes which string is returned

More FRQs Coming Soon

• Remove duplicates
• Collapse consecutive duplicates
• Filter words by prefix
• List of objects (Student, Item, Score objects)
• Merge two ArrayLists
• Manual sorting logic

Related FRQ Pages

• Arrays FRQs
• Strings FRQs
• 2D Arrays FRQs
• Mixed FRQs
• Full Mock FRQ Set

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