ArrayList
Traversals

Master the art of iterating through ArrayLists using for loops, enhanced for loops, and while loops: the foundation of data processing in Java.

AP Computer Science A

What Is an ArrayList?

An ArrayList is a resizable, ordered collection of object references in Java. Unlike arrays, ArrayLists can grow and shrink dynamically as elements are added or removed.

Key methods: .get(i) retrieves element at index \(i\), and .size() returns the number of elements.

Why Traversals Matter

A traversal visits every element in a collection, one at a time. Nearly every real-world task: searching, filtering, summing, or transforming data: requires a traversal.

Search

Find a specific value in a list of students

Filter

Remove all items that fail a condition

Transform

Convert every String to uppercase

Three Loop Patterns

Java provides three common ways to traverse an ArrayList. Each has strengths and trade-offs.

for loop

Index-based access. Full control over start, end, and step.

for-each loop

Cleaner syntax. Reads every element automatically.

while loop

Flexible condition. Useful when the stop point is unknown.

The Standard for Loop

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

Initialization: \(i = 0\) starts at the first index.
Condition: \(i < \text{list.size}()\) stops before going out of bounds.
Update: \(i++\) moves to the next element.

The Enhanced for-each Loop

for (String elem : list) {
    System.out.println(elem);
}

No index variable needed. Java automatically assigns each element to elem on every iteration. Cleaner and less error-prone.

Limitation: You cannot add or remove elements during a for-each loop: doing so throws a ConcurrentModificationException.

Traversal with a while Loop

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

Functionally identical to a standard for loop, but the index is managed manually. This is especially useful when removing elements, because you can choose not to increment \(i\) after a removal.

for vs for-each: When to Use Each

	Standard for	Enhanced for-each
Access index?	✓ Yes	✕ No
Modify elements?	✓ via set()	✕ Read-only
Remove elements?	✓ via remove()	✕ Throws exception
Traverse backwards?	✓ Yes	✕ No
Cleaner syntax?	✕ More verbose	✓ Yes

How to Write a Traversal

1

Identify your goal: Are you reading, modifying, removing, or counting elements?

2

Choose the loop type: Read-only → for-each. Need index → standard for. Removing → while or backwards for.

3

Set bounds carefully: Start at \(0\), end at \(\text{size}() - 1\). Use \(<\), never \(\le\).

4

Process each element: Apply your logic inside the loop body.

5

Handle edge cases: What if the list is empty? What if every element matches?

Example 1: Sum All Values

Given ArrayList<Integer> nums containing [3, 7, 2, 9, 1], compute the total sum.

int sum = 0;
for (int n : nums) {
    sum += n;
}
System.out.println(sum);  // 22

Example 2: Removing Elements

Remove all even numbers from ArrayList<Integer> nums. This requires an index because we're modifying the list.

int i = 0;
while (i < nums.size()) {
    if (nums.get(i) % 2 == 0) {
        nums.remove(i);
        // Do NOT increment i
    } else {
        i++;
    }
}

Critical: After \(\text{remove}(i)\), the next element shifts down into index \(i\). If you increment, you skip that element. Only increment when you do not remove.

Example 3: Backwards Removal

An alternative approach: traverse backwards. Removing an element only shifts elements after the removed index, so earlier indices are unaffected.

for (int i = nums.size() - 1; i >= 0; i--) {
    if (nums.get(i) % 2 == 0) {
        nums.remove(i);
    }
}

Example 4: Linear Search

Find the index of the first occurrence of a target value. Return -1 if not found.

public static int search(ArrayList<String> list, String target) {
    for (int i = 0; i < list.size(); i++) {
        if (list.get(i).equals(target)) {
            return i;
        }
    }
    return -1;
}

Always use .equals() to compare Strings and objects: never ==. The == operator checks reference identity, not value equality.

Traversal Decision Flowchart

Common Mistakes

✕ Off-by-one with ≤

Using \(i \le \text{list.size}()\) causes an IndexOutOfBoundsException. Always use \(<\).

✕ Skipping after remove

Incrementing \(i\) after \(\text{remove}(i)\) in a forward loop skips the next element.

✕ Using == for Strings

Using == instead of .equals() compares references, not values. Almost always wrong.

✕ Modifying in for-each

Adding or removing elements during a for-each loop throws a ConcurrentModificationException.

Traversal Summary

Pattern	Best For	Key Syntax
Standard for	Index access, modify, remove	`for (int i = 0; ...)`
Enhanced for-each	Read-only traversal	`for (T x : list)`
While loop	Conditional removal, flexible stop	`while (i < size())`
Backwards for	Clean removal without skip	`for (i = size()-1; ...)`

Question 1 of 5

What is the output of the following code?

ArrayList<Integer> nums = new ArrayList<>();
nums.add(10); nums.add(20); nums.add(30);
for (int i = 0; i < nums.size(); i++) {
    System.out.print(nums.get(i) + " ");
}

▼ Tap to read code

The loop iterates with \(i\) from 0 to 2 (since size() is 3), printing each element: 10 20 30.

Question 2 of 5

Which loop type should you use to remove elements from an ArrayList during traversal?

A for-each loop throws ConcurrentModificationException. A forward for with \(i++\) skips elements. Use a while loop (don't increment on remove) or a backwards for loop.

Question 3 of 5

What happens when this code executes?

ArrayList<String> words = new ArrayList<>();
words.add("Hi"); words.add("Bye");
for (String w : words) {
    if (w.equals("Hi")) {
        words.remove(w);
    }
}

▼ Tap to read code

Removing an element during a for-each loop causes a ConcurrentModificationException. Use a standard for or while loop instead.

Question 4 of 5

Given ArrayList<Integer> a with values [4, 7, 4, 2, 4], how many elements remain after this code runs?

for (int i = a.size() - 1; i >= 0; i--) {
    if (a.get(i) == 4) {
        a.remove(i);
    }
}

▼ Tap to read code

The backwards loop correctly removes all three 4s (at indices 4, 2, and 0), leaving [7, 2]: exactly 2 elements.

Question 5 of 5

What is the value of count after this code runs on the list ["apple", "banana", "avocado", "cherry", "apricot"]?

int count = 0;
for (String s : list) {
    if (s.substring(0, 1).equals("a")) {
        count++;
    }
}

▼ Tap to read code

The strings starting with "a" are: "apple", "avocado", and "apricot". "banana" starts with "b" and "cherry" starts with "c". The count is 3.

ArrayListTraversals

What Is an ArrayList?

Why Traversals Matter

Three Loop Patterns

The Standard for Loop

The Enhanced for-each Loop

Traversal with a while Loop

for vs for-each: When to Use Each

How to Write a Traversal

Example 1: Sum All Values

Example 2: Removing Elements

Example 3: Backwards Removal

Example 4: Linear Search

Traversal Decision Flowchart

Common Mistakes

Traversal Summary

Question 1 of 5

Question 2 of 5

Question 3 of 5

Question 4 of 5

Question 5 of 5

ArrayList
Traversals