Why Queue Exists and What Problems It Solves in DSA Java - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the end of the queue.

DSA Java

queue.[1](element);

Drag options to blanks, or click blank then click option'

Aenqueue

Binsert

Cadd

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using push which is for stacks.

Using enqueue which is not a Java Queue method.

In Java's Queue interface, add() adds an element to the end of the queue.

2fill in blank

medium

Complete the code to remove the element at the front of the queue.

DSA Java

queue.[1]();

Drag options to blanks, or click blank then click option'

Apoll

Bremove

Cpop

Ddequeue

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which is for stacks.

Using remove which throws exception if queue is empty.

The poll() method removes and returns the head of the queue or returns null if empty.

3fill in blank

hard

Fix the error in the code to check the front element without removing it.

DSA Java

Element front = queue.[1]();

Drag options to blanks, or click blank then click option'

Apeek

Bfront

Cget

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using get which does not exist for Queue.

Using top which is for stacks.

The peek() method returns the head of the queue without removing it, or null if empty.

4fill in blank

hard

Fill both blanks to create a queue and add an element to it.

DSA Java

Queue<String> queue = new [1]<>();
queue.[2]("apple");

Drag options to blanks, or click blank then click option'

ALinkedList

Badd

CArrayList

Doffer

Attempts:

3 left

💡 Hint

Common Mistakes

Using ArrayList which does not implement Queue.

Using offer which is an alternative to add but not the answer here.

Use LinkedList to create a queue and add() to add elements.

5fill in blank

hard

Fill all three blanks to remove and print all elements from the queue until it is empty.

DSA Java

while (!queue.[1]()) {
    String item = queue.[2]();
    System.out.[3](item);
}

Drag options to blanks, or click blank then click option'

AisEmpty

Bpoll

Cprintln

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove() which throws exception if empty.

Using print instead of println.

Use isEmpty() to check if queue is empty, poll() to remove elements safely, and println() to print.

Practice

1. Why do we use a Queue data structure instead of a simple Array for certain problems?

easy

A. Because a queue can only store one element at a time.

B. Because a queue stores elements randomly without any order.

C. Because a queue allows direct access to any element by index.

D. Because a queue processes elements in the order they arrive, following FIFO.

Solution

1. Step 1: Understand the behavior of a queue

   A queue processes elements in First-In-First-Out (FIFO) order, meaning the first element added is the first to be removed.

2. Step 2: Compare with array behavior

   An array allows random access but does not enforce order of processing; a queue ensures order is maintained.

3. Final Answer:

   Because a queue processes elements in the order they arrive, following FIFO. -> Option D

4. Quick Check:

   Queue = FIFO order [OK]

Hint: Queues always process in arrival order (FIFO) [OK]

Common Mistakes:

• Thinking queues allow random access like arrays
• Confusing FIFO with LIFO (stack behavior)
• Assuming queues store only one element

2. Which of the following is the correct way to add an element to a Java Queue named q?

easy

A. q.push(10);

B. q.add(10);

C. q.enqueue(10);

D. q.insert(10);

Solution

1. Step 1: Recall Java Queue methods

   Java's Queue interface uses add() or offer() to insert elements, not push() or enqueue().

2. Step 2: Identify correct method

   Among the options, only add(10) is a valid method to add an element to a Queue.

3. Final Answer:

   q.add(10); -> Option B

4. Quick Check:

   Java Queue insertion = add() [OK]

Hint: Use add() or offer() to insert in Java Queue [OK]

Common Mistakes:

• Using push() which is for stacks
• Using enqueue() which is not a Java Queue method
• Using insert() which does not exist in Queue interface

3. What will be the output of the following Java code?

import java.util.*;
public class Main {
  public static void main(String[] args) {
    Queue<Integer> q = new LinkedList<>();
    q.add(5);
    q.add(10);
    q.add(15);
    System.out.println(q.poll());
    System.out.println(q.peek());
  }
}

medium

A. 5
10

B. 10
15

C. 5
5

D. 15
10

Solution

1. Step 1: Understand queue operations poll() and peek()

   poll() removes and returns the head (first element), peek() returns the head without removing it.

2. Step 2: Trace the queue state

   Queue after adds: [5, 10, 15]. poll() returns 5 and removes it. Queue now: [10, 15]. peek() returns 10 without removal.

3. Final Answer:

   5 10 -> Option A

4. Quick Check:

   poll()=5, peek()=10 [OK]

Hint: poll() removes head, peek() shows head [OK]

Common Mistakes:

• Confusing poll() with peek()
• Assuming poll() does not remove element
• Mixing order of elements after poll

4. Identify the error in the following Java code that uses a queue:

import java.util.*;
public class Main {
  public static void main(String[] args) {
    Queue<Integer> q = new LinkedList<>();
    q.add(1);
    q.add(2);
    int x = q.get();
    System.out.println(x);
  }
}

medium

A. Method get() does not exist for Queue interface.

B. Queue cannot store integers.

C. LinkedList cannot be used as a Queue.

D. Missing import statement for Queue.

Solution

1. Step 1: Check Queue methods

   The Queue interface does not have a method named get(). To retrieve and remove the head, use poll() or remove().

2. Step 2: Verify other code parts

   Queue can store integers, LinkedList implements Queue, and import is correct.

3. Final Answer:

   Method get() does not exist for Queue interface. -> Option A

4. Quick Check:

   Queue method get() = error [OK]

Hint: Use poll() or remove(), not get() for Queue [OK]

Common Mistakes:

• Using get() method on Queue
• Thinking LinkedList can't be a Queue
• Ignoring correct imports

5. You need to process customer service requests in the order they arrive, but sometimes you must prioritize urgent requests immediately. Which data structure best solves this problem and why?

hard

A. Use a simple Queue because it processes requests in arrival order.

B. Use a Stack because it processes the most recent request first.

C. Use a Priority Queue because it allows urgent requests to be processed first while maintaining order.

D. Use an Array because it allows random access to requests.

Solution

1. Step 1: Understand the problem requirements

   Requests must be processed in arrival order, but urgent requests need immediate attention, breaking simple FIFO order.

2. Step 2: Choose suitable data structure

   A Priority Queue allows assigning priority to requests so urgent ones come first, while still managing order among same priority requests.

3. Final Answer:

   Use a Priority Queue because it allows urgent requests to be processed first while maintaining order. -> Option C

4. Quick Check:

   Urgent first + order = Priority Queue [OK]

Hint: Priority Queue handles urgent and normal order [OK]

Common Mistakes:

• Using simple Queue ignores urgency
• Using Stack reverses order incorrectly
• Using Array lacks order and priority