ArrayList

Why Are ArrayLists Important?

In AP Computer Science A, ArrayLists are a core part of the exam. Unlike arrays, ArrayLists are dynamic in size, making them more flexible for storing and managing data.

📌 Key Advantages of ArrayLists:
✅ Resizable (Unlike arrays, they grow and shrink automatically)
✅ Built-in Methods for adding, removing, and modifying elements
✅ Easier to use for real-world applications

💡 Important ArrayList Algorithms for APCSA

Below are must-know algorithms and operations tested in multiple-choice questions (MCQs) and Free Response Questions (FRQs).

1️⃣ Creating and Initializing an ArrayList

📌 Syntax:

import java.util.ArrayList; 

ArrayList<Integer> numbers = new ArrayList<>();  // Creating an Integer ArrayList
ArrayList<String> words = new ArrayList<>();     // Creating a String ArrayList

🔹 Important Methods:

  • add(value) → Adds an element at the end
  • add(index, value) → Inserts at a specific position
  • size() → Returns the number of elements

✅ Example:

ArrayList<Integer> list = new ArrayList<>();
list.add(10);      // Adds 10
list.add(0, 5);    // Inserts 5 at index 0
System.out.println(list); // Output: [5, 10]

2️⃣ Accessing Elements in an ArrayList

Method: get(index)
📌 Example:

ArrayList<String> colors = new ArrayList<>();
colors.add("Red");
colors.add("Blue");
System.out.println(colors.get(1));  // Output: Blue

🔹 Common Mistakes:
⚠ IndexOutOfBoundsException occurs if accessing an index out of range.

3️⃣ Updating and Modifying an ArrayList

Method: set(index, value)
📌 Example:

ArrayList<Integer> nums = new ArrayList<>();
nums.add(3);
nums.add(7);
nums.set(1, 10);  // Changes element at index 1 to 10
System.out.println(nums); // Output: [3, 10]

4️⃣ Removing Elements from an ArrayList

Methods:

  • remove(index) → Removes element at the given index
  • remove(Object) → Removes the first occurrence of the given object

📌 Example:

ArrayList<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Orange");
fruits.remove(1);  // Removes "Banana"
System.out.println(fruits); // Output: [Apple, Orange]

5️⃣ Finding an Element in an ArrayList

Method: contains(value)
📌 Example:

ArrayList<Integer> list = new ArrayList<>();
list.add(100);
list.add(200);
System.out.println(list.contains(200)); // Output: true

🔹 Alternative: Use indexOf(value), which returns -1 if not found.

6️⃣ Iterating Over an ArrayList

Using a for loop:

ArrayList<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");

for (int i = 0; i < names.size(); i++) {
    System.out.println(names.get(i));
}

Using an enhanced for-each loop:

for (String name : names) {
    System.out.println(name);
}

7️⃣ Swapping Two Elements in an ArrayList

📌 Example:

ArrayList<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);

// Swap elements at index 0 and index 2
int temp = list.get(0);
list.set(0, list.get(2));
list.set(2, temp);

System.out.println(list); // Output: [3, 2, 1]

8️⃣ Reversing an ArrayList

📌 Example:

ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(1);
numbers.add(2);
numbers.add(3);

for (int i = 0; i < numbers.size() / 2; i++) {
    int temp = numbers.get(i);
    numbers.set(i, numbers.get(numbers.size() - 1 - i));
    numbers.set(numbers.size() - 1 - i, temp);
}

System.out.println(numbers); // Output: [3, 2, 1]

9️⃣ Sorting an ArrayList

📌 Using Collections.sort():

import java.util.Collections;

ArrayList<Integer> nums = new ArrayList<>();
nums.add(50);
nums.add(10);
nums.add(40);

Collections.sort(nums);  // Sorts in ascending order
System.out.println(nums); // Output: [10, 40, 50]

🔹 For descending order:

Collections.sort(nums, Collections.reverseOrder());

🔟 Removing Duplicates from an ArrayList

📌 Example:

ArrayList<Integer> list = new ArrayList<>();
list.add(10);
list.add(20);
list.add(10);

ArrayList<Integer> uniqueList = new ArrayList<>();
for (Integer num : list) {
    if (!uniqueList.contains(num)) {
        uniqueList.add(num);
    }
}

System.out.println(uniqueList); // Output: [10, 20]

🔴 Common APCSA FRQ Questions Involving ArrayLists

✔ Processing an ArrayList of Objects (e.g., filtering data)
✔ Shifting elements in an ArrayList
✔ Simulating a Queue using an ArrayList
✔ Finding max/min values in an ArrayList
✔ Reordering elements based on conditions

Final Tips for Mastering ArrayLists

✅ Practice Coding – Don’t just read, code these examples in Java!
✅ Understand Edge Cases – Watch out for IndexOutOfBoundsException.
✅ Use size() Instead of Fixed Lengths – Remember, ArrayLists resize dynamically.
✅ Check FRQ Practice Problems – Many APCSA FRQs involve ArrayLists.

Master these fundamental algorithms with Code Scholars, and you’ll be well-prepared for the APCSA Exam!

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