Unit 4 — Data Collections

Unit 4 is about storing and processing many values: arrays, ArrayLists, and 2D arrays. You’ll practice traversal patterns, common algorithms (searching, counting, min/max, swapping), and reasoning about indexes.

Topic pages

Go in order (4.1 → 4.15).

Array Creation and Access4.1 Traversing Arrays4.2 Enhanced for Loop for Arrays4.3 Developing Algorithms Using Arrays4.4 ArrayList Creation and Access4.5 Traversing ArrayLists4.6 Enhanced for Loop for ArrayLists4.7 Developing Algorithms Using ArrayLists4.8 ArrayList Methods4.9 ArrayList and Arrays4.10 Exploring 2D Arrays4.11 2D Array Creation and Access4.12 Traversing 2D Arrays4.13 Enhanced for Loop for 2D Arrays4.14 Developing Algorithms Using 2D Arrays4.15

How to use the quiz

Do MCQ first, then check answers at the end.
Pay attention to index bounds and loop conditions.
FRQ is focused on a classic traversal + algorithm pattern.

Unit 4 Quiz — Multiple Choice (15)

Choose the best answer. (One correct option each.)

1) Which statement correctly creates an int array of length 5?

Topic 4.1

A. int arr = new int[5]; B. int[] arr = new int[5]; C. int[] arr = new int(5); D. int arr[] = new int;

Answer: B. Arrays use [] and new int[length].

2) What is the valid index range for an array with length `n`?

Topic 4.1

A. 1 to n B. 0 to n C. 1 to n-1 D. 0 to n-1

Answer: D. Java arrays are zero-indexed: last index is length - 1.

3) What is printed?

Topic 4.2

int[] a = {2, 4, 6, 8};
int sum = 0;
for (int i = 0; i < a.length; i++) {
  sum += a[i];
}
System.out.println(sum);

A. 20 B. 8 C. 6 D. 24

4) Which loop visits every element of an array exactly once?

Topic 4.2

A. for (int i = 1; i <= a.length; i++) B. for (int i = 0; i <= a.length; i++) C. for (int i = 0; i < a.length; i++) D. for (int i = a.length; i >= 0; i--)

5) What is true about the enhanced for loop with arrays?

Topic 4.3

A. It allows you to change the array’s length B. It visits elements in order, but you don’t directly use an index variable C. It can skip elements automatically D. It always runs backwards through the array

6) Which task usually requires an indexed loop rather than enhanced for?

Topic 4.4

A. Summing the values B. Finding the max value C. Counting values that match a condition D. Swapping adjacent elements

7) Which statement correctly creates an `ArrayList<Integer>`?

Topic 4.5

A. ArrayList<Integer> list = new ArrayList<Integer>(); B. ArrayList<int> list = new ArrayList<int>(); C. ArrayList<Integer> list = new ArrayList<int>(); D. ArrayList list = new ArrayList<Integer>();

Answer: A. Use wrapper type Integer (not int).

8) For an `ArrayList` named `list`, which method returns the number of elements?

Topic 4.5

A. list.length B. list.length() C. list.size() D. list.count()

Answer: C. ArrayLists use size().

9) What is the value of `list` after this code executes?

Topic 4.9

ArrayList<Integer> list = new ArrayList<>();
list.add(10);
list.add(20);
list.add(1, 15);

A. [10, 20, 15] B. [10, 15, 20] C. [15, 10, 20] D. [10, 15]

10) Which call removes the element at index `i` from an `ArrayList` named `list`?

Topic 4.9

A. list.delete(i) B. list.remove() C. list.remove(i + 1) D. list.remove(i)

Answer: D. remove(index) removes the element at that index.

11) Which best describes the relationship between arrays and ArrayLists?

Topic 4.10

A. Arrays have fixed length; ArrayLists can change size B. Arrays can store objects but ArrayLists cannot C. ArrayLists use length; arrays use size() D. Arrays cannot be traversed with loops

12) In a 2D array `int[][] m`, what does `m.length` represent?

Topic 4.11

A. Number of columns B. Total number of elements C. Number of rows D. Number of rows times columns

Answer: C. m.length is the number of row arrays.

13) Which expression gets the number of columns in row `r` of `m`?

Topic 4.12

A. m.length B. m[r].length C. m.length[r] D. m[r].size()

14) Which nested loop correctly traverses every element of a rectangular 2D array `m`?

Topic 4.13

A. for (int r = 0; r <= m.length; r++) for (int c = 0; c <= m[r].length; c++) B. for (int r = 1; r < m.length; r++) for (int c = 1; c < m[r].length; c++) C. for (int r = 0; r < m[0].length; r++) for (int c = 0; c < m.length; c++) D. for (int r = 0; r < m.length; r++) for (int c = 0; c < m[r].length; c++)

15) Which is true about enhanced for loops with 2D arrays?

Topic 4.14

A. The outer enhanced loop gives rows (1D arrays), then inner gives elements B. You must use indexes with enhanced loops C. Enhanced loops can resize the 2D array D. Enhanced loops visit columns first by default

Unit 4 Quiz — FRQ (1)

This FRQ uses a common Unit 4 pattern: traversal + conditional update.

FRQ 1) Replace negatives with 0 (Array)

Write a method public static void clampNegatives(int[] a) that updates the array so that any negative value is replaced with 0. The method does not return anything.

Write your solution here:

Sample solution

public static void clampNegatives(int[] a) {
  for (int i = 0; i < a.length; i++) {
    if (a[i] < 0) {
      a[i] = 0;
    }
  }
}

Uses an indexed loop because we need to write back into the array.

Unit 4 — Data Collections

Topic pages

How to use the quiz

Unit 4 Quiz — Multiple Choice (15)

1) Which statement correctly creates an int array of length 5?

2) What is the valid index range for an array with length n?

3) What is printed?

4) Which loop visits every element of an array exactly once?

5) What is true about the enhanced for loop with arrays?

6) Which task usually requires an indexed loop rather than enhanced for?

7) Which statement correctly creates an ArrayList<Integer>?

8) For an ArrayList named list, which method returns the number of elements?

9) What is the value of list after this code executes?

10) Which call removes the element at index i from an ArrayList named list?

11) Which best describes the relationship between arrays and ArrayLists?

12) In a 2D array int[][] m, what does m.length represent?

13) Which expression gets the number of columns in row r of m?

14) Which nested loop correctly traverses every element of a rectangular 2D array m?

15) Which is true about enhanced for loops with 2D arrays?

Unit 4 Quiz — FRQ (1)

FRQ 1) Replace negatives with 0 (Array)

Sample solution

Done?

2) What is the valid index range for an array with length `n`?

7) Which statement correctly creates an `ArrayList<Integer>`?

8) For an `ArrayList` named `list`, which method returns the number of elements?

9) What is the value of `list` after this code executes?

10) Which call removes the element at index `i` from an `ArrayList` named `list`?

12) In a 2D array `int[][] m`, what does `m.length` represent?

13) Which expression gets the number of columns in row `r` of `m`?

14) Which nested loop correctly traverses every element of a rectangular 2D array `m`?