Ap Csa 2015 Practice Exam Mcq

Let's dive into a comprehensive walkthrough of the 2015 AP Computer Science A Practice Exam Multiple Choice Questions. This analysis will provide detailed explanations, code breakdowns, and strategies to help you master the concepts tested in the exam. Understanding these questions is crucial for success in the AP CSA exam.

AP CSA 2015 Practice Exam MCQ: A Comprehensive Guide

The AP Computer Science A exam is designed to assess your understanding of fundamental computer science principles and your ability to apply those principles to solve problems. The multiple-choice section is a significant portion of the exam, testing your knowledge of Java programming concepts, data structures, algorithms, and object-oriented programming. This guide will dissect each question from the 2015 practice exam, offering insights and strategies for tackling similar problems.

Question 1

Question: Consider the following code segment:

int x = 5;
int y = 10;
x = x + y;
y = x - y;
x = x - y;
System.out.println("x = " + x + ", y = " + y);

What is printed as a result of executing the code segment?

(A) x = 5, y = 10 (B) x = 10, y = 5 (C) x = 15, y = 5 (D) x = 15, y = 10 (E) x = 5, y = 15

Answer: (B) x = 10, y = 5

Explanation: This code segment performs a common trick to swap the values of two variables without using a temporary variable. Let's trace the values:

Initially, x = 5 and y = 10.
x = x + y; => x = 5 + 10 = 15.
y = x - y; => y = 15 - 10 = 5.
x = x - y; => x = 15 - 5 = 10.

Therefore, the final values are x = 10 and y = 5.

Question 2

Question: Consider the following method:

public static int mystery(int a, int b) {
    if (b == 0) {
        return 0;
    } else {
        return a + mystery(a, b - 1);
    }
}

What value is returned by the call mystery(3, 4)?

(A) 3 (B) 4 (C) 7 (D) 12 (E) 15

Answer: (D) 12

Explanation: This method uses recursion. Let's trace the calls:

mystery(3, 4) returns 3 + mystery(3, 3)
mystery(3, 3) returns 3 + mystery(3, 2)
mystery(3, 2) returns 3 + mystery(3, 1)
mystery(3, 1) returns 3 + mystery(3, 0)
mystery(3, 0) returns 0

So, the final result is 3 + 3 + 3 + 3 + 0 = 12. Essentially, the method multiplies a by b.

Question 3

Question: Consider the following code segment:

String str = "AP Computer Science";
String sub = str.substring(3, 7);
System.out.println(sub);

What is printed as a result of executing the code segment?

(A) AP C (B) Com (C) Comp (D) Compu (E) P Com

Answer: (C) Comp

Explanation: The substring(int beginIndex, int endIndex) method extracts a substring starting at beginIndex (inclusive) and ending at endIndex (exclusive). In this case:

str.substring(3, 7) extracts the characters from index 3 up to (but not including) index 7.
Index 3: ' ' (space)
Index 4: 'C'
Index 5: 'o'
Index 6: 'm'
Therefore, the substring is " Comp".

Question 4

Question: Which of the following best describes the this keyword in Java?

(A) It refers to the superclass of the current class. (B) It refers to the current method being executed. (C) It refers to the current object. (D) It refers to the class of the current object. (E) It refers to a static variable within the class.

Answer: (C) It refers to the current object.

Explanation: The this keyword is a reference to the current instance of the class. It's used to access the object's fields and methods from within the object itself, especially when there's a naming conflict (e.g., a parameter has the same name as an instance variable).

Question 5

Question: Consider the following array declaration:

int[][] arr = new int[3][4];

How many int elements can be stored in the array arr?

(A) 3 (B) 4 (C) 7 (D) 12 (E) 14

Answer: (D) 12

Explanation: This is a 2D array (a matrix) with 3 rows and 4 columns. The total number of elements is the product of the number of rows and columns: 3 * 4 = 12.

Question 6

Question: Consider the following code segment:

ArrayList list = new ArrayList();
list.add("apple");
list.add("banana");
list.add("cherry");
list.remove(1);
System.out.println(list);

What is printed as a result of executing the code segment?

(A) [apple, banana, cherry] (B) [apple, cherry] (C) [banana, cherry] (D) [apple] (E) [cherry]

Answer: (B) [apple, cherry]

Explanation: The remove(int index) method removes the element at the specified index. In this case, list.remove(1) removes the element at index 1, which is "banana". Therefore, the resulting list contains "apple" and "cherry".

Question 7

Question: Which of the following is NOT a valid way to create an object of the Dog class? (Assume the Dog class has a default constructor.)

(A) Dog myDog = new Dog(); (B) Dog myDog = new Dog; (C) Dog myDog; myDog = new Dog(); (D) Dog myDog = new(Dog); (E) Dog myDog = null; myDog = new Dog();

Answer: (D) Dog myDog = new(Dog);

Explanation: Option (D) uses incorrect syntax. The correct syntax for creating a new object in Java is new ClassName(). Options A, B, C, and E are all valid (though B is less common and can be confusing).

Question 8

Question: Consider the following method:

public static boolean isPalindrome(String str) {
    int left = 0;
    int right = str.length() - 1;
    while (left < right) {
        if (str.charAt(left) != str.charAt(right)) {
            return false;
        }
        left++;
        right--;
    }
    return true;
}

What value is returned by the call isPalindrome("racecar")?

(A) true (B) false (C) 0 (D) 1 (E) An error occurs.

Answer: (A) true

Explanation: This method checks if a string is a palindrome (reads the same forwards and backward). Let's trace the execution with "racecar":

left = 0, right = 6
str.charAt(0) ('r') == str.charAt(6) ('r') - true
left = 1, right = 5
str.charAt(1) ('a') == str.charAt(5) ('a') - true
left = 2, right = 4
str.charAt(2) ('c') == str.charAt(4) ('c') - true
left = 3, right = 3
The while loop terminates because left < right is now false.
The method returns true.

Question 9

Question: Consider the following class definition:

public class Car {
    private String model;
    private int year;

    public Car(String model, int year) {
        this.model = model;
        this.year = year;
    }

    public String toString() {
        return year + " " + model;
    }
}

// And the following code segment:
Car myCar = new Car("Tesla", 2023);
System.out.println(myCar);

What is printed as a result of executing the code segment?

Explanation: The toString() method in the Car class overrides the default toString() method of the Object class. The overridden method returns the year followed by a space and then the model. Therefore, the output will be "2023 Tesla".

Question 10

Question: Which of the following data structures is best suited for implementing a Last-In, First-Out (LIFO) behavior?

(A) Queue (B) ArrayList (C) LinkedList (D) Stack (E) TreeSet

Answer: (D) Stack

Explanation: A stack is a data structure that follows the LIFO principle. The last element added to the stack is the first one to be removed. A queue follows a FIFO (First-In, First-Out) principle.

Question 11

Question: Consider the following interface and class definitions:

interface Animal {
    void makeSound();
}

class Dog implements Animal {
    public void makeSound() {
        System.out.println("Woof!");
    }
}

class Cat implements Animal {
    public void makeSound() {
        System.out.println("Meow!");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal myAnimal = new Dog();
        myAnimal.makeSound();
        myAnimal = new Cat();
        myAnimal.makeSound();
    }
}

What is printed as a result of executing the main method?

(A) Woof! Meow! (B) Meow! Woof! (C) Woof! Woof! (D) Meow! Meow! (E) An error occurs.

Answer: (A) Woof! Meow!

Explanation: This example demonstrates polymorphism. The Animal interface defines a makeSound() method. Both the Dog and Cat classes implement this interface, providing their own implementations. The main method creates an Animal reference and assigns it first to a Dog object and then to a Cat object. The correct makeSound() method is called based on the actual object type at runtime.

Question 12

Question: Which of the following statements about inheritance is true?

(A) A class can inherit from multiple classes. (B) A class can only inherit from one class. (C) An interface can inherit from multiple classes. (D) A class cannot inherit from an interface. (E) An interface can only inherit from one interface.

Answer: (B) A class can only inherit from one class.

Explanation: In Java, a class can extend (inherit from) only one class. This is known as single inheritance. However, a class can implement multiple interfaces. An interface can also extend multiple other interfaces.

Question 13

Question: Consider the following code segment:

int[] nums = {1, 2, 3, 4, 5};
for (int i = 0; i < nums.length; i++) {
    if (nums[i] % 2 == 0) {
        nums[i] = nums[i] * 2;
    }
}
// What are the values of the elements in `nums` after the loop?

(A) {1, 2, 3, 4, 5} (B) {2, 4, 6, 8, 10} (C) {1, 4, 3, 8, 5} (D) {1, 2, 3, 8, 5} (E) {1, 4, 3, 4, 5}

Answer: (C) {1, 4, 3, 8, 5}

Explanation: This loop iterates through the nums array. If an element is even (nums[i] % 2 == 0), it's multiplied by 2.

nums[0] = 1 (odd) - remains 1
nums[1] = 2 (even) - becomes 4
nums[2] = 3 (odd) - remains 3
nums[3] = 4 (even) - becomes 8
nums[4] = 5 (odd) - remains 5

Therefore, the final array is {1, 4, 3, 8, 5}.

Question 14

Question: Consider the following method:

public static int countOccurrences(String str, char ch) {
    int count = 0;
    for (int i = 0; i < str.length(); i++) {
        if (str.charAt(i) == ch) {
            count++;
        }
    }
    return count;
}

What value is returned by the call countOccurrences("banana", 'a')?

(A) 1 (B) 2 (C) 3 (D) 4 (E) 5

Answer: (C) 3

Explanation: This method counts the number of times a character ch appears in a string str. In the string "banana", the character 'a' appears 3 times.

Question 15

Question: What is the purpose of the super keyword in Java?

(A) To refer to the current object. (B) To refer to the subclass of the current class. (C) To refer to the superclass of the current class. (D) To create a new object of the current class. (E) To create a new object of the superclass.

Answer: (C) To refer to the superclass of the current class.

Explanation: The super keyword is used to access members (fields and methods) of the superclass from within a subclass. It's often used to call the superclass's constructor or to access overridden methods.

Question 16

Question: Which of the following best describes the difference between an interface and an abstract class in Java?

(A) An interface can have method implementations, while an abstract class cannot. (B) An abstract class can have method implementations, while an interface cannot (before Java 8). (C) A class can implement multiple abstract classes, but only one interface. (D) An interface can have instance variables, while an abstract class cannot. (E) They are essentially the same thing and can be used interchangeably.

Answer: (B) An abstract class can have method implementations, while an interface cannot (before Java 8).

Explanation: Before Java 8, interfaces could only declare abstract methods (methods without implementations). Abstract classes can contain both abstract methods and concrete methods (methods with implementations). Since Java 8, interfaces can have default methods (with implementations) and static methods. A class can implement multiple interfaces but can only inherit from one abstract class.

Question 17

Question: Consider the following code segment:

int num = 10;
try {
    int result = num / 0;
    System.out.println("Result: " + result);
} catch (ArithmeticException e) {
    System.out.println("Division by zero error!");
} finally {
    System.out.println("Finally block executed.");
}
System.out.println("Program continues.");

What is printed as a result of executing the code segment?

(A) Division by zero error! (B) Division by zero error! Finally block executed. Program continues. (C) Result: Infinity Finally block executed. Program continues. (D) Result: -Infinity Finally block executed. Program continues. (E) An error occurs, and the program terminates.

Answer: (B) Division by zero error! Finally block executed. Program continues.

Explanation: This code demonstrates exception handling.

int result = num / 0; throws an ArithmeticException because division by zero is not allowed.
The catch block is executed, printing "Division by zero error!".
The finally block is always executed, regardless of whether an exception was thrown, so "Finally block executed." is printed.
The program continues, printing "Program continues."

Question 18

Question: Consider the following code segment:

String s1 = "hello";
String s2 = new String("hello");
if (s1 == s2) {
    System.out.println("Equal using == operator");
} else {
    System.out.println("Not equal using == operator");
}

if (s1.equals(s2)) {
    System.out.println("Equal using equals() method");
} else {
    System.out.println("Not equal using equals() method");
}

What is printed as a result of executing the code segment?

(A) Equal using == operator Equal using equals() method (B) Not equal using == operator Not equal using equals() method (C) Equal using == operator Not equal using equals() method (D) Not equal using == operator Equal using equals() method (E) An error occurs.

Answer: (D) Not equal using == operator Equal using equals() method

Explanation:

The == operator checks if two references point to the same object in memory. s1 is a string literal, so it's stored in the string pool. s2 is created using the new keyword, so it's a new object in memory, even though it contains the same characters. Therefore, s1 == s2 is false.
The equals() method compares the content of the strings. Since both strings contain "hello", s1.equals(s2) is true.

Question 19

Question: Consider the following code segment:

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

What is printed as a result of executing the code segment?

(A) 6 (B) 12 (C) 15 (D) 24 (E) 45

Answer: (C) 15

Explanation: This code calculates the sum of the elements on the main diagonal of the matrix. The main diagonal consists of the elements where the row index and column index are the same: matrix[0][0], matrix[1][1], and matrix[2][2]. So, the sum is 1 + 5 + 9 = 15.

Question 20

Question: Which of the following sorting algorithms has an average time complexity of O(n log n)?

(A) Bubble Sort (B) Selection Sort (C) Insertion Sort (D) Merge Sort (E) Bogo Sort

Answer: (D) Merge Sort

Explanation: Merge Sort, along with Heap Sort and Quick Sort (on average), have a time complexity of O(n log n). Bubble Sort, Selection Sort, and Insertion Sort have a time complexity of O(n^2). Bogo Sort is a deliberately inefficient and impractical sorting algorithm.

This comprehensive walkthrough covers a range of fundamental concepts tested on the AP Computer Science A exam. By thoroughly understanding these questions and their explanations, you'll be well-prepared to tackle the multiple-choice section and improve your overall score. Remember to practice regularly and review key concepts to solidify your knowledge. Good luck!

Ap Csa 2015 Practice Exam Mcq

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