Queue Concept and FIFO Principle in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Java

queue.[1](10);

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

Aadd

Benqueue

Cpush

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using push() which is for stacks, not queues.

Using enqueue() which is not a Java Queue method.

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

2fill in blank

medium

Complete the code to remove the front element from the queue.

DSA Java

int front = queue.[1]();

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

Aremove

Bpop

Cdequeue

Dpoll

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove() which throws exception if queue is empty.

Using pop() which is for stacks.

The poll() method removes and returns the front element 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

int front = queue.[1]();

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

Apeek

Bget

Ctop

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using top() which is for stacks.

Using get() which is not a Queue method.

The peek() method returns the front element without removing it, or null if empty.

4fill in blank

hard

Fill both blanks to create a queue and add an element 5 to it.

DSA Java

Queue<Integer> queue = new [1]<>();
queue.[2](5);

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

ALinkedList

BArrayList

Cadd

Doffer

Attempts:

3 left

💡 Hint

Common Mistakes

Using ArrayList which does not implement Queue.

Using add() which can throw exceptions if capacity is limited.

Use LinkedList to implement Queue and offer() to add elements safely.

5fill in blank

hard

Fill all three blanks to remove the front element and print the queue.

DSA Java

int removed = queue.[1]();
System.out.println("Removed: " + [2]);
System.out.println("Queue: " + [3]);

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

Aremove

Bremoved

Cqueue

Dpoll

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove() which throws exception if queue is empty.

Printing wrong variables or methods.

poll() removes the front element safely, then we print the removed element and the current queue.

Practice

1. What does the FIFO principle mean in the context of a queue?

easy

A. Last In, First Out - the last element added is the first removed

B. First In, Last Out - the first element added is the last removed

C. First In, First Out - the first element added is the first removed

D. Last In, Last Out - the last element added is the last removed

Solution

1. Step 1: Understand FIFO meaning

   FIFO means the first element that enters the queue is the first to leave.

2. Step 2: Match FIFO to queue behavior

   Queues follow FIFO, so the first added element is removed first.

3. Final Answer:

   First In, First Out - the first element added is the first removed -> Option C

4. Quick Check:

   FIFO = First In, First Out [OK]

Hint: Remember queue = line; first come, first served [OK]

Common Mistakes:

• Confusing FIFO with LIFO
• Thinking queue removes last added first
• Mixing stack behavior with queue

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

easy

A. q.add(10);

B. q.enqueue(10);

C. q.push(10);

D. q.insert(10);

Solution

1. Step 1: Recall Java Queue methods

   Java Queue interface uses add() or offer() to add elements.

2. Step 2: Identify correct method

   push() and enqueue() are not Queue methods; insert() is invalid.

3. Final Answer:

   q.add(10); -> Option A

4. Quick Check:

   Use add() to insert in Java Queue [OK]

Hint: Use add() or offer() to add 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

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.print(q.poll() + " -> ");
    System.out.print(q.poll() + " -> ");
    System.out.print(q.poll() + " -> ");
  }
}

medium

A. 15 -> 10 -> 5 ->

B. 5 -> 10 -> 15 ->

C. 10 -> 15 -> 5 ->

D. 5 -> 15 -> 10 ->

Solution

1. Step 1: Add elements to queue

   Elements 5, 10, 15 are added in order to the queue.

2. Step 2: Remove elements using poll()

   poll() removes elements in FIFO order: 5, then 10, then 15.

3. Final Answer:

   5 -> 10 -> 15 -> -> Option B

4. Quick Check:

   Queue poll() removes in FIFO order [OK]

Hint: poll() removes first added element in queue [OK]

Common Mistakes:

• Thinking poll() removes last element
• Confusing queue with stack order
• Assuming poll() does not remove element

4. Identify the error in the following Java code snippet using a Queue:

Queue<String> q = new LinkedList<>();
q.add("apple");
q.add("banana");
String first = q.remove();
String second = q.remove();
String third = q.remove();

medium

A. No error, code runs fine

B. remove() method does not exist in Queue

C. Queue cannot store Strings

D. Third remove() causes NoSuchElementException

Solution

1. Step 1: Check number of elements added

   Only two elements "apple" and "banana" are added to the queue.

2. Step 2: Analyze remove() calls

   First two remove() calls remove "apple" and "banana". Third remove() tries to remove from empty queue causing exception.

3. Final Answer:

   Third remove() causes NoSuchElementException -> Option D

4. Quick Check:

   Removing from empty queue throws exception [OK]

Hint: Removing more than added elements causes exception [OK]

Common Mistakes:

• Assuming remove() returns null if empty
• Thinking queue can't store Strings
• Believing remove() method is missing

5. You have a queue that stores customer IDs in order of arrival. You want to process customers but skip any ID that is negative (invalid). Which approach correctly processes the queue while maintaining FIFO order and skipping negatives?

Queue<Integer> q = new LinkedList<>();
// Assume q is filled with IDs including negatives
while (!q.isEmpty()) {
  int id = q.poll();
  // What to do here?
}

hard

A. If id is negative, ignore and continue; else process it

B. If id is negative, remove all remaining elements

C. If id is negative, stop processing the queue

D. If id is negative, add it back to the end of the queue

Solution

1. Step 1: Understand queue processing with condition

   We want to skip negative IDs but keep processing others in order.

2. Step 2: Choose correct action for negative IDs

   Ignoring negative IDs and continuing preserves FIFO order and processes valid IDs only.

3. Final Answer:

   If id is negative, ignore and continue; else process it -> Option A

4. Quick Check:

   Skip negatives, process others in FIFO [OK]

Hint: Skip invalid IDs, don't re-add or stop queue [OK]

Common Mistakes:

• Re-adding negative IDs causing infinite loop
• Stopping queue on first negative ID
• Removing all elements on negative ID