Queue Concept and FIFO Principle in DSA Java - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of enqueue and dequeue operations on a queue

What is the printed state of the queue after the following operations?

DSA Java

import java.util.LinkedList;
import java.util.Queue;

public class Main {
    public static void main(String[] args) {
        Queue<Integer> queue = new LinkedList<>();
        queue.add(10);
        queue.add(20);
        queue.add(30);
        queue.remove();
        queue.add(40);
        System.out.println(queue);
    }
}

A [10, 20, 40]

B [10, 20, 30, 40]

C [30, 40]

D [20, 30, 40]

💡 Hint

Remember that queue removes elements in FIFO order.

The queue starts with 10, 20, 30. Removing the first element (10) leaves 20, 30. Adding 40 puts it at the end, so the queue is [20, 30, 40].

🧠 Conceptual

intermediate

Understanding FIFO principle in queue operations

Which of the following best describes the FIFO principle in queues?

A Elements are removed in the same order they were added.

B Elements are removed randomly from the queue.

C The last element added is the first to be removed.

D The queue removes the largest element first.

💡 Hint

Think about how people line up in a queue at a store.

FIFO means First In, First Out. The first element added to the queue is the first one to be removed.

🔧 Debug

advanced

Identify the error in queue usage

What error will this Java code produce when run?

DSA Java

import java.util.LinkedList;
import java.util.Queue;

public class Main {
    public static void main(String[] args) {
        Queue<Integer> queue = new LinkedList<>();
        queue.add(1);
        queue.add(2);
        queue.remove();
        queue.remove();
        queue.remove();
    }
}

A Throws NoSuchElementException at runtime.

B No error, code runs fine.

C Compilation error: remove() method not found.

D Throws NullPointerException at runtime.

💡 Hint

Check what happens when removing from an empty queue.

Removing more elements than present causes NoSuchElementException because remove() expects an element to remove.

❓ Predict Output

advanced

Queue state after mixed operations

What is the output of the queue after these operations?

DSA Java

import java.util.LinkedList;
import java.util.Queue;

public class Main {
    public static void main(String[] args) {
        Queue<String> queue = new LinkedList<>();
        queue.offer("A");
        queue.offer("B");
        queue.poll();
        queue.offer("C");
        queue.offer("D");
        queue.poll();
        System.out.println(queue);
    }
}

A [D]

B [C, D]

C [A, C, D]

D [B, C, D]

💡 Hint

poll() removes the head element or returns null if empty.

First poll removes "A", second poll removes "B", leaving "C" and "D" in the queue.

🚀 Application

expert

Queue usage in real-world scenario

A printer queue processes print jobs in the order they arrive. If jobs arrive in this order: Job1, Job2, Job3, and Job2 is canceled before printing, what is the order of jobs printed?

A Job1, Job2, Job3

B Job2, Job1, Job3

C Job1, Job3

D Job3, Job1

💡 Hint

Canceled jobs are removed before printing.

Job2 is canceled before printing, so only Job1 and Job3 remain in order.

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