Unit 4 Data Collections Practice Exam - 50 Questions | AP CSA 2025-26

Unit 4: Data Collections

50 Questions • AP Exam Difficulty • New 4-Unit Curriculum (2025-26)

1D Arrays • ArrayList • 2D Arrays • Traversals & Algorithms

0 Correct | 0 Incorrect | 0/50 Answered

1D Array Basics & Traversals (Questions 1-14)

Question 1

What is printed?

int[] arr = new int[5];
System.out.println(arr[0]);

A) null
B) 0
C) 5
D) ArrayIndexOutOfBoundsException

Key Concept: Primitive int arrays are initialized to 0 by default. Object arrays (like String[]) are initialized to null.

Question 2

What is printed?

int[] arr = {10, 20, 30, 40, 50};
System.out.println(arr[arr.length - 1]);

A) 40
B) 4
C) 50
D) ArrayIndexOutOfBoundsException

Key Concept: arr.length is 5, so arr[arr.length - 1] = arr[4] = 50. This is the standard way to access the last element.

Question 3

What is printed?

int[] arr = {1, 2, 3, 4, 5};
System.out.println(arr[5]);

A) 5
B) 0
C) null
D) ArrayIndexOutOfBoundsException

Key Concept: Valid indices are 0 to length-1. arr[5] is out of bounds for a 5-element array (indices 0-4).

Question 4

What is printed?

int[] arr = {2, 4, 6, 8, 10};
int sum = 0;
for (int i = 0; i < arr.length; i++) {
    sum += arr[i];
}
System.out.println(sum);

A) 10
B) 30
C) 20
D) 5

Key Concept: Standard traversal pattern. Sum = 2 + 4 + 6 + 8 + 10 = 30.

Question 5

What is printed?

int[] arr = {5, 10, 15, 20};
for (int val : arr) {
    System.out.print(val + " ");
}

A) 5 10 15 20
B) 0 1 2 3
C) 20 15 10 5
D) Compile error

Key Concept: Enhanced for loop (for-each) iterates through values, not indices. val takes each element's value in order.

Question 6

What is printed?

int[] arr = {3, 7, 2, 9, 4};
int max = arr[0];
for (int i = 1; i < arr.length; i++) {
    if (arr[i] > max) {
        max = arr[i];
    }
}
System.out.println(max);

A) 3
B) 7
C) 9
D) 4

Key Concept: Standard find-maximum algorithm. Initialize max to first element, then compare with remaining elements.

Question 7

What is printed?

int[] arr = {1, 2, 3, 4, 5};
for (int i = 0; i < arr.length; i++) {
    arr[i] = arr[i] * 2;
}
System.out.println(arr[2]);

A) 3
B) 6
C) 4
D) 2

Key Concept: The loop doubles each element. arr[2] was 3, becomes 3 * 2 = 6.

Question 8

What is printed?

int[] arr = {1, 2, 3, 4, 5};
for (int val : arr) {
    val = val * 2;
}
System.out.println(arr[2]);

A) 6
B) 2
C) 4
D) 3

Key Concept: Enhanced for loops cannot modify array elements! val is a copy of the element value, not a reference to it. The array remains unchanged.

Question 9

What is printed?

int[] a = {1, 2, 3};
int[] b = a;
b[0] = 99;
System.out.println(a[0]);

A) 1
B) 0
C) 99
D) Compile error

Key Concept: Arrays are objects. int[] b = a creates an alias—both variables reference the same array. Changes through b affect a.

Question 10

What is printed?

int[] arr = {10, 20, 30, 40, 50};
int count = 0;
for (int val : arr) {
    if (val > 25) {
        count++;
    }
}
System.out.println(count);

A) 2
B) 3
C) 4
D) 5

Key Concept: Counting elements matching a condition. Values > 25: 30, 40, 50. Count = 3.

Question 11

What is printed?

String[] names = new String[3];
System.out.println(names[0]);

A) null
B) ""
C) 0
D) NullPointerException

Key Concept: Object arrays (including String[]) are initialized to null, not empty strings. Printing null prints "null".

Question 12

What is printed?

int[] arr = {5, 3, 8, 1, 9, 2};
int minIndex = 0;
for (int i = 1; i < arr.length; i++) {
    if (arr[i] < arr[minIndex]) {
        minIndex = i;
    }
}
System.out.println(minIndex);

A) 1
B) 2
C) 3
D) 5

Key Concept: Finding index of minimum. The minimum value is 1 at index 3.

Question 13

What is printed?

int[] arr = {1, 2, 3, 4, 5};
for (int i = arr.length - 1; i >= 0; i--) {
    System.out.print(arr[i] + " ");
}

A) 1 2 3 4 5
B) 5 4 3 2 1
C) 4 3 2 1 0
D) ArrayIndexOutOfBoundsException

Key Concept: Reverse traversal: start at length-1 (last index), decrement to 0. Prints elements in reverse order.

Question 14

What does this method return for arr = {2, 4, 6, 8}?

public static boolean allEven(int[] arr) {
    for (int val : arr) {
        if (val % 2 != 0) {
            return false;
        }
    }
    return true;
}

A) false
B) 0
C) 4
D) true

Key Concept: This checks if ALL elements satisfy a condition. Returns false immediately if any element is odd. All elements are even, so returns true.

ArrayList (Questions 15-28)

Question 15

Which correctly declares an ArrayList of Strings?

A) ArrayList list = new ArrayList();
B) ArrayList list = new ArrayList();
C) ArrayList list = new ArrayList();
D) ArrayList[String] list = new ArrayList[String]();

Key Concept: ArrayList uses angle brackets with the wrapper class name (capital S for String). Java is case-sensitive.

Question 16

What is printed?

ArrayList list = new ArrayList();
list.add(10);
list.add(20);
list.add(30);
System.out.println(list.size());

A) 2
B) 3
C) 30
D) 60

Key Concept: size() returns the number of elements. Three elements were added, so size() returns 3.

Question 17

What is printed?

ArrayList list = new ArrayList();
list.add("A");
list.add("B");
list.add("C");
System.out.println(list.get(1));

A) B
B) A
C) C
D) 1

Key Concept: ArrayList uses get(index) to access elements (not square brackets). Index 1 is "B".

Question 18

What is printed?

ArrayList list = new ArrayList();
list.add(5);
list.add(10);
list.add(15);
list.set(1, 99);
System.out.println(list.get(1));

A) 10
B) 5
C) 99
D) 15

Key Concept: set(index, value) replaces the element at that index. The element at index 1 changes from 10 to 99.

Question 19

What is printed?

ArrayList list = new ArrayList();
list.add("X");
list.add("Y");
list.add("Z");
list.remove(1);
System.out.println(list);

A) [X, Z, null]
B) [X, Z]
C) [Y, Z]
D) [X, Y]

Key Concept: remove(index) removes the element and shifts subsequent elements left. Size decreases by 1. Y at index 1 is removed.

Question 20

What is printed?

ArrayList list = new ArrayList();
list.add(1);
list.add(2);
list.add(0, 99);
System.out.println(list);

A) [1, 2, 99]
B) [99, 2]
C) [1, 99, 2]
D) [99, 1, 2]

Key Concept: add(index, value) inserts at the specified position, shifting existing elements right. 99 is inserted at index 0.

Question 21

What is the problem with this code?

ArrayList list = new ArrayList();
list.add(1);
list.add(2);
list.add(3);
for (int i = 0; i < list.size(); i++) {
    list.remove(i);
}
System.out.println(list);

A) Elements are skipped; prints [2]
B) IndexOutOfBoundsException
C) Prints []
D) Compile error

Key Concept: When removing while traversing forward, elements shift left but i increases. i=0 removes 1, list becomes [2,3]. i=1 removes 3. Result: [2]. Use backward traversal or while loop to fix.

Question 22

Which correctly removes all even numbers?

ArrayList list = new ArrayList();
// list contains [2, 5, 8, 3, 6]

A) for (int i = 0; i < list.size(); i++) { if (list.get(i) % 2 == 0) list.remove(i); }
B) for (int val : list) { if (val % 2 == 0) list.remove(val); }
C) for (int i = list.size() - 1; i >= 0; i--) { if (list.get(i) % 2 == 0) list.remove(i); }
D) while (list.size() > 0) { list.remove(0); }

Key Concept: Traverse backwards when removing! This way, index shifts don't affect unprocessed elements. Option B causes ConcurrentModificationException.

Question 23

What is printed?

ArrayList list = new ArrayList();
list.add(10);
list.add(20);
list.add(30);
for (int val : list) {
    System.out.print(val + " ");
}

A) 0 1 2
B) 10 20 30
C) [10, 20, 30]
D) Compile error

Key Concept: Enhanced for loops work with ArrayList. Integer auto-unboxes to int for the loop variable.

Question 24

What is the main difference between arrays and ArrayLists?

A) Arrays can only hold primitives
B) ArrayLists are faster
C) Arrays use methods to access elements
D) ArrayLists can change size; arrays cannot

Key Concept: Arrays have fixed size. ArrayLists grow/shrink automatically. ArrayLists also only hold objects (use wrapper classes for primitives).

Question 25

What is printed?

ArrayList list = new ArrayList();
list.add("cat");
list.add("dog");
list.add("bird");
System.out.println(list.indexOf("dog"));

A) 1
B) 2
C) dog
D) -1

Key Concept: indexOf() returns the first index of the element, or -1 if not found. "dog" is at index 1.

Question 26

What is printed?

ArrayList nums = new ArrayList();
nums.add(5);
nums.add(10);
nums.add(15);
int sum = 0;
for (int i = 0; i < nums.size(); i++) {
    sum += nums.get(i);
}
System.out.println(sum);

A) 15
B) 3
C) 30
D) 10

Key Concept: Standard ArrayList traversal with index. Sum = 5 + 10 + 15 = 30. Use get(i) instead of [i] for ArrayList.

Question 27

What wrapper class is used for int in ArrayList?

A) int
B) Integer
C) Int
D) Number

Key Concept: ArrayList can't hold primitives. Use wrapper classes: Integer for int, Double for double, Boolean for boolean.

Question 28

What is printed?

ArrayList list = new ArrayList();
list.add("A");
list.add("B");
String removed = list.remove(0);
System.out.println(removed + " " + list.size());

A) A 2
B) B 1
C) null 1
D) A 1

Key Concept: remove(index) returns the removed element. "A" is removed and returned. Size goes from 2 to 1.

2D Arrays (Questions 29-42)

Question 29

What is printed?

int[][] arr = new int[3][4];
System.out.println(arr.length + " " + arr[0].length);

A) 3 4
B) 4 3
C) 12 12
D) 3 3

Key Concept: arr.length = number of rows (3). arr[0].length = number of columns in row 0 (4).

Question 30

What is printed?

int[][] arr = {{1, 2, 3}, {4, 5, 6}};
System.out.println(arr[1][2]);

A) 2
B) 5
C) 6
D) 3

Key Concept: arr[row][col]: row 1 is {4, 5, 6}, column 2 is the third element = 6.

Question 31

How many total elements are in this ragged array?

int[][] arr = {{1, 2}, {3, 4, 5}, {6}};

A) 3
B) 6
C) 9
D) Compile error

Key Concept: Java allows ragged (jagged) arrays where rows have different lengths. Total = 2 + 3 + 1 = 6.

Question 32

What is printed?

int[][] grid = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
int sum = 0;
for (int row = 0; row < grid.length; row++) {
    for (int col = 0; col < grid[row].length; col++) {
        sum += grid[row][col];
    }
}
System.out.println(sum);

A) 9
B) 45
C) 15
D) 36

Key Concept: Standard row-major traversal visits every element. Sum of 1-9 = 45.

Question 33

What is printed?

int[][] arr = {{1, 2, 3}, {4, 5, 6}};
for (int[] row : arr) {
    for (int val : row) {
        System.out.print(val + " ");
    }
}

A) 1 4 2 5 3 6
B) 1 2 3 4 5 6
C) 1 2 4 5
D) Compile error

Key Concept: Enhanced for loop on 2D array: outer loop gets each row (int[]), inner loop gets each element. Row-major order.

Question 34

What traversal order does this code use?

for (int col = 0; col < arr[0].length; col++) {
    for (int row = 0; row < arr.length; row++) {
        System.out.print(arr[row][col] + " ");
    }
}

A) Row-major order
B) Column-major order
C) Diagonal order
D) Random order

Key Concept: Outer loop on columns, inner loop on rows = column-major order. Processes all rows of column 0, then column 1, etc.

Question 35

What is printed? (Main diagonal sum)

int[][] arr = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
int sum = 0;
for (int i = 0; i < arr.length; i++) {
    sum += arr[i][i];
}
System.out.println(sum);

A) 6
B) 12
C) 15
D) 45

Key Concept: Main diagonal: row index equals column index. arr[0][0]=1, arr[1][1]=5, arr[2][2]=9. Sum = 15.

Question 36

What is printed? (Aliasing in 2D arrays)

int[][] arr = {{1, 2, 3}, {4, 5, 6}};
arr[0] = arr[1];
arr[1][0] = 99;
System.out.println(arr[0][0]);

A) 1
B) 4
C) 99
D) ArrayIndexOutOfBoundsException

Key Concept: arr[0] = arr[1] makes both rows reference the same array (aliasing). Changing arr[1][0] also changes arr[0][0].

Question 37

What is printed?

int[][] grid = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
int count = 0;
for (int[] row : grid) {
    for (int val : row) {
        if (val % 2 == 0) {
            count++;
        }
    }
}
System.out.println(count);

A) 4
B) 5
C) 3
D) 9

Key Concept: Counting even numbers: 2, 4, 6, 8 are even. Count = 4.

Question 38

What is printed?

String[][] words = {{"A", "B"}, {"C", "D", "E"}};
System.out.println(words[1].length);

A) 2
B) 3
C) 5
D) 1

Key Concept: Each row can have different length. Row 1 has 3 elements: "C", "D", "E".

Question 39

What is printed?

int[][] matrix = {{1, 2}, {3, 4}};
for (int r = 0; r < matrix.length; r++) {
    for (int c = 0; c < matrix[r].length; c++) {
        if (r == c) {
            matrix[r][c] *= 2;
        }
    }
}
System.out.println(matrix[0][0] + matrix[1][1]);

A) 5
B) 6
C) 8
D) 10

Key Concept: Diagonal elements (r == c) are doubled. matrix[0][0] = 1*2 = 2, matrix[1][1] = 4*2 = 8. Sum = 10.

Question 40

What is printed?

int[][] arr = new int[2][3];
System.out.println(arr[1][2]);

A) null
B) 0
C) 2
D) ArrayIndexOutOfBoundsException

Key Concept: int arrays default to 0. This is a valid index (row 1, col 2 of a 2x3 array).

Question 41

What does this code compute?

int[][] grid = {{1, 2, 3}, {4, 5, 6}};
int result = 0;
for (int c = 0; c < grid[0].length; c++) {
    result += grid[0][c];
}
System.out.println(result);

A) Sum of column 0
B) Sum of all elements
C) Sum of row 0
D) Number of columns

Key Concept: grid[0][c] accesses only row 0, iterating through all columns. Sum = 1 + 2 + 3 = 6.

Question 42

What does this code compute?

int[][] grid = {{1, 2, 3}, {4, 5, 6}};
int result = 0;
for (int r = 0; r < grid.length; r++) {
    result += grid[r][0];
}
System.out.println(result);

A) Sum of column 0
B) Sum of all elements
C) Sum of row 0
D) Number of rows

Key Concept: grid[r][0] accesses column 0, iterating through all rows. Sum = 1 + 4 = 5.

Mixed Applications & Algorithms (Questions 43-50)

Question 43

What is printed? (Linear search)

int[] arr = {5, 3, 8, 1, 9};
int target = 8;
int index = -1;
for (int i = 0; i < arr.length; i++) {
    if (arr[i] == target) {
        index = i;
        break;
    }
}
System.out.println(index);

A) -1
B) 8
C) 2
D) 3

Key Concept: Linear search checks each element until found. 8 is at index 2. break exits the loop early.

Question 44

What is printed?

ArrayList list = new ArrayList();
list.add(10);
list.add(20);
list.add(30);
list.add(1, 15);
System.out.println(list.get(2));

A) 15
B) 20
C) 30
D) 10

Key Concept: After add(1, 15), list is [10, 15, 20, 30]. Element at index 2 is 20.

Question 45

What is printed?

int[] arr = {1, 2, 3, 4, 5};
int[] copy = arr;
copy[0] = 100;
System.out.println(arr[0] + " " + copy[0]);

A) 1 100
B) 100 1
C) 1 1
D) 100 100

Key Concept: int[] copy = arr creates an alias, not a true copy. Both variables point to the same array object.

Question 46

What is printed?

int[][] arr = {{1, 2}, {3, 4}, {5, 6}};
int sum = 0;
for (int r = 0; r < arr.length; r++) {
    sum += arr[r][arr[r].length - 1];
}
System.out.println(sum);

A) 12
B) 9
C) 6
D) 21

Key Concept: arr[r][arr[r].length - 1] gets the last element of each row. 2 + 4 + 6 = 12.

Question 47

What is printed?

ArrayList list = new ArrayList();
list.add("A");
list.add("B");
list.add("C");
list.add("B");
list.remove("B");
System.out.println(list);

A) [A, C]
B) [A, B, C]
C) [A, C, B]
D) [B, C, B]

Key Concept: remove(Object) removes the FIRST occurrence only. First "B" at index 1 is removed. Second "B" remains.

Question 48

What is printed?

int[] arr = {4, 2, 7, 1, 5};
double avg = 0;
for (int val : arr) {
    avg += val;
}
avg = avg / arr.length;
System.out.println(avg);

A) 3
B) 3.8
C) 4
D) 19

Key Concept: Sum = 4+2+7+1+5 = 19. Average = 19/5 = 3.8. Using double preserves decimal.

Question 49

What is printed?

int[][] grid = {{1, 2, 3}, {4, 5, 6}};
int max = grid[0][0];
for (int[] row : grid) {
    for (int val : row) {
        if (val > max) {
            max = val;
        }
    }
}
System.out.println(max);

A) 1
B) 3
C) 5
D) 6

Key Concept: Standard find-max in 2D array using enhanced for loops. Maximum value is 6.

Question 50

Which is true about arrays and ArrayLists?

A) Both use square brackets to access elements
B) Both can hold primitive types directly
C) Both can be traversed with enhanced for loops
D) Both automatically resize when full

Key Concept: Enhanced for loops work with both arrays and ArrayLists. Arrays use [], ArrayList uses get(). ArrayList auto-resizes, arrays don't. ArrayList needs wrapper classes for primitives.

🎉 Unit 4: Data Collections Complete!

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New 4-Unit AP CSA Curriculum (2025-26) • APCSExamPrep.com

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Unit 4 Data Collections Practice Exam - 50 Questions | AP CSA 2025-26

Unit 4: Data Collections

🎉 Unit 4: Data Collections Complete!

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