ArrayList

📋 ArrayList Algorithms

Part 1 of 7 — Removing, Searching, and Common Patterns

The Remove-While-Traversing Trap

// BUGGY: Skips elements after removal!
ArrayList<String> words = new ArrayList<>();
// words = ["cat", "dog", "cat", "bird"]
for (int i = 0; i < words.size(); i++) {
    if (words.get(i).equals("cat")) {
        words.remove(i);
        // After removing index 0: ["dog", "cat", "bird"]
        // i becomes 1, skipping "dog" -> misses second "cat"!
    }
}

Three Correct Solutions

// Solution 1: Decrement i after removal
for (int i = 0; i < words.size(); i++) {
    if (words.get(i).equals("cat")) {
        words.remove(i);
        i--;  // Recheck this index
    }
}

// Solution 2: Traverse backward
for (int i = words.size() - 1; i >= 0; i--) {
    if (words.get(i).equals("cat")) {
        words.remove(i);  // Removal does not affect earlier indices
    }
}

// Solution 3: Use while loop
int i = 0;
while (i < words.size()) {
    if (words.get(i).equals("cat")) {
        words.remove(i);  // Do NOT increment i
    } else {
        i++;
    }
}

🔑 Backward traversal is the safest and simplest approach for removing elements, since removing a later element does not affect the indices of earlier ones.

Concept Check 🎯

Common ArrayList Algorithms

Finding the Maximum

public int findMax(ArrayList<Integer> list) {
    int max = list.get(0);
    for (int i = 1; i < list.size(); i++) {
        if (list.get(i) > max) {
            max = list.get(i);
        }
    }
    return max;
}

Removing All Occurrences

public void removeAll(ArrayList<String> list, String target) {
    for (int i = list.size() - 1; i >= 0; i--) {
        if (list.get(i).equals(target)) {
            list.remove(i);
        }
    }
}

Building a Filtered List

public ArrayList<Integer> getEvens(ArrayList<Integer> list) {
    ArrayList<Integer> evens = new ArrayList<>();
    for (int val : list) {
        if (val % 2 == 0) {
            evens.add(val);
        }
    }
    return evens;
}

Removing Duplicates

public ArrayList<String> removeDups(ArrayList<String> list) {
    ArrayList<String> result = new ArrayList<>();
    for (String s : list) {
        if (!result.contains(s)) {
            result.add(s);
        }
    }
    return result;
}

Applied Recall ✍️

1. When removing elements from an ArrayList in a forward loop, you must _______ the index after each removal to avoid skipping.

2. The safest way to remove elements while traversing is to iterate _______.

3. To check if an ArrayList contains a specific value, use the _______ method.

Choose the Pattern 🔍

AP Exam Strategy: ArrayList Algorithms

• The remove-while-traversing bug is one of the most commonly tested topics
• Always compare Strings with .equals(), never ==
• Know three solutions: backward loop, i-- after remove, while loop with conditional increment
• FRQ tip: if the question says "remove all X," traverse backward
• The enhanced for loop is read-only for modification purposes — do not add or remove
• list.remove(i) returns the removed element — useful for moving elements between lists

AP-Style Application 🎯

💻 ArrayList Algorithms

Part 2 of 7 — Key Processes

Understanding the processes related to ArrayList Algorithms helps explain how and why patterns develop. This part explores the mechanisms driving key phenomena.

Key Concepts

Concept Check 🎯

Key Processes — Deeper Dive

Process 1

The primary mechanism that drives patterns in ArrayList Algorithms. Understanding this concept is essential for mastering ArrayList Algorithms in AP Computer Science A.

Process 2

A secondary process that shapes outcomes in ArrayList Algorithms. This builds on the previous concept and connects to broader themes in the course.

Cause and effect

The relationship between actions and outcomes in ArrayList Algorithms. This is frequently tested on the AP exam and connects to multiple units in the curriculum.

Applied Recall (exact term answers) ✍️

1. What term refers to the primary mechanism that drives patterns in ArrayList Algorithms?

💻 ArrayList Algorithms

Part 3 of 7 — Patterns & Examples

This part examines specific patterns and real-world examples related to ArrayList Algorithms. Case studies help illustrate abstract concepts.

Key Concepts

Concept Check 🎯

Patterns & Examples — Deeper Dive

Spatial pattern

The geographic distribution related to ArrayList Algorithms. Understanding this concept is essential for mastering ArrayList Algorithms in AP Computer Science A.

Case study

A specific real-world example that illustrates ArrayList Algorithms. This builds on the previous concept and connects to broader themes in the course.

Comparison

Analyzing similarities and differences across examples of ArrayList Algorithms. This is frequently tested on the AP exam and connects to multiple units in the curriculum.

Applied Recall (exact term answers) ✍️

1. What term refers to the geographic distribution related to ArrayList Algorithms?

💻 ArrayList Algorithms

Part 4 of 7 — Connections & Interactions

ArrayList Algorithms connects to other topics in AP Computer Science A. Understanding these connections reveals how different processes interact.

Key Concepts

Concept Check 🎯

Connections & Interactions — Deeper Dive

Interconnection

How ArrayList Algorithms links to other course topics. Understanding this concept is essential for mastering ArrayList Algorithms in AP Computer Science A.

Scale interaction

How ArrayList Algorithms operates differently at local, national, and global scales. This builds on the previous concept and connects to broader themes in the course.

Feedback loop

How outcomes of ArrayList Algorithms can reinforce or modify the original process. This is frequently tested on the AP exam and connects to multiple units in the curriculum.

Applied Recall (exact term answers) ✍️

1. What term refers to how ArrayList Algorithms links to other course topics?

💻 ArrayList Algorithms

Part 5 of 7 — Change Over Time

ArrayList Algorithms has evolved over time. Understanding historical and contemporary changes helps explain current patterns and predict future trends.

Key Concepts

Concept Check 🎯

Change Over Time — Deeper Dive

Continuity

Aspects of ArrayList Algorithms that have remained stable over time. Understanding this concept is essential for mastering ArrayList Algorithms in AP Computer Science A.

Change

How ArrayList Algorithms has transformed due to new forces and conditions. This builds on the previous concept and connects to broader themes in the course.

Trend

The direction of change in ArrayList Algorithms over time. This is frequently tested on the AP exam and connects to multiple units in the curriculum.

Applied Recall (exact term answers) ✍️

1. What term refers to aspects of ArrayList Algorithms that have remained stable over time?

💻 ArrayList Algorithms

Part 6 of 7 — Problem-Solving Workshop

Apply ArrayList Algorithms concepts to data interpretation and analytical scenarios. Practice the types of questions seen on the AP exam.

Key Concepts

Concept Check 🎯

Problem-Solving Workshop — Deeper Dive

Data interpretation

Analyzing maps, graphs, and tables related to ArrayList Algorithms. Understanding this concept is essential for mastering ArrayList Algorithms in AP Computer Science A.

Argumentation

Making evidence-based claims about ArrayList Algorithms. This builds on the previous concept and connects to broader themes in the course.

Spatial reasoning

Using geographic thinking to analyze ArrayList Algorithms. This is frequently tested on the AP exam and connects to multiple units in the curriculum.

Applied Recall (exact term answers) ✍️

1. What term refers to analyzing maps, graphs, and tables related to ArrayList Algorithms?

💻 ArrayList Algorithms

Part 7 of 7 — AP Review

Comprehensive review of ArrayList Algorithms for the AP exam. Focus on key concepts, common question types, and exam strategies.

Key Concepts

Concept Check 🎯

AP Review — Deeper Dive

Key vocabulary

Essential terms and definitions for ArrayList Algorithms. Understanding this concept is essential for mastering ArrayList Algorithms in AP Computer Science A.

Common question types

The most frequent ways ArrayList Algorithms is tested on the AP exam. This builds on the previous concept and connects to broader themes in the course.

Exam strategy

Approaches for answering ArrayList Algorithms questions effectively. This is frequently tested on the AP exam and connects to multiple units in the curriculum.

Applied Recall (exact term answers) ✍️

1. What term refers to essential terms and definitions for ArrayList Algorithms?

2. What concept describes the most frequent ways ArrayList Algorithms is tested on the AP exam?