OOP - session 2.1 - JVM, Array, Stack

Which of the following is the correct way to declare and allocate an array of 10 integers in Java?

• int arr = new int[10];

• int arr[] = new int[10];

• int arr = [10];

• int[] arr = int[10];

Answer explanation

In Java, arrays are declared by specifying the data type followed by square brackets (which can appear after the type or the variable name) and are allocated using the new keyword with the size inside square brackets.

Which statement best describes arrays in Java?

• Arrays are primitive types and their size can be changed at runtime.

• Arrays are objects stored on the heap and have a fixed size once created.

• Arrays are automatically resized like Python lists when elements are added.

• Arrays are stored in the stack and allow dynamic memory allocation.

Answer explanation

Unlike Python lists, Java arrays are objects that reside on the heap, and their size is fixed upon creation. They are not dynamically resizable.

int[] numbers = new int[5];

What are the default values of the elements in the numbers array?

• They are undefined and accessing them will cause a runtime error.

• All elements are set to 0.

• All elements are set to null.

• They contain random garbage values.

Answer explanation

In Java, when an array of primitive type (such as int) is allocated, each element is automatically initialized to a default value (0 for numeric types).

Given an array declared as
String[] names = new String[3];

what happens if you try to access names[3]?

• The compiler automatically resizes the array to include the index.

• It returns null because the element has not been assigned a value.

• It throws an ArrayIndexOutOfBoundsException at runtime.

• It returns an empty string.

Answer explanation

Java enforces strict bounds checking on arrays. Since valid indices for an array of length 3 are 0, 1, and 2, attempting to access names[3] will result in an ArrayIndexOutOfBoundsException.

Which of the following statements about multi-dimensional arrays in Java is true?

• Multi-dimensional arrays in Java are arrays of arrays, meaning the subarrays (rows) can be of different lengths (jagged arrays).

• Java does not support multi-dimensional arrays.

• Multi-dimensional arrays in Java are implemented as linked lists under the hood.

• Multi-dimensional arrays in Java are truly multi-dimensional like in C, and their rows must all be of equal length.

Answer explanation

In Java, multi-dimensional arrays are implemented as arrays of arrays. This means each subarray (or “row”) can be independently allocated, allowing for jagged arrays where rows can have different lengths.

Which of the following best describes a stack data structure in Java?

• A data structure that follows the Last In, First Out (LIFO) principle.

• A structure that supports random access to its elements.

• A non-linear data structure optimized for search operations.

• A data structure that follows the First In, First Out (FIFO) principle.

Answer explanation

A stack is a linear data structure where the most recently added element is the first to be removed, following the LIFO principle.

Which Java class provides a direct implementation of a stack?

• java.util.ArrayList

• java.util.Stack

• java.util.LinkedList

• java.util.Queue

Answer explanation

The java.util.Stack class is the built-in implementation in Java that provides methods like push(), pop(), and peek() to perform standard stack operations.

Which method is used in the java.util.Stack class to add an element to the top of the stack?

• add()

• insert()

• push()

• append()

Answer explanation

In Java's Stack class, the push() method is specifically used to add (or "push") an element onto the top of the stack.

What exception is thrown when attempting to remove (pop) an element from an empty Java stack?

• ArrayIndexOutOfBoundsException

• EmptyStackException

• NullPointerException

• NoSuchElementException

Answer explanation

If you call pop() (or peek()) on an empty Java stack, it throws an EmptyStackException, indicating that the operation cannot be completed because the stack is empty.

Python lists can be used to implement stack behavior. Which of the following correctly compares the two approaches?

• Java’s Stack does not allow duplicate elements, unlike Python lists.

• Python lists require manual resizing, whereas Java’s Stack automatically resizes.

• Both Python lists (using append() and pop()) and Java’s Stack (using push() and pop()) support dynamic resizing and similar LIFO operations.

• Python lists and Java stacks both use the same method names for stack operations.

Answer explanation

Python lists support dynamic resizing and can perform stack operations using append() and pop(), which are conceptually similar to Java’s push() and pop() methods. Note that while the method names differ, the underlying LIFO behavior is analogous.