Enqueue Operation in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the queue.

DSA Java

queue.[1](element);

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

Ainsert

Badd

Cpush

Denqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using push which is for stacks

Using enqueue which is not a Java Queue method

The add method adds an element to the end of the queue in Java's Queue interface.

2fill in blank

medium

Complete the code to check if the queue is full before enqueueing.

DSA Java

if (queue.size() [1] queueCapacity) {
    System.out.println("Queue is full");
}

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

A==

B<=

C!=

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' which checks if size is less than or equal to capacity

Using '!=' which does not check fullness

We check if the current size is greater than or equal to the capacity to know if the queue is full.

3fill in blank

hard

Fix the error in the enqueue method to correctly add an element.

DSA Java

public void enqueue(int element) {
    if (queue.size() < capacity) {
        queue.[1](element);
    } else {
        System.out.println("Queue is full");
    }
}

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

Ainsert

Badd

Coffer

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using push which is for stacks

Using add which throws exception if full

The offer method is preferred for queues as it returns false if the queue is full instead of throwing an exception.

4fill in blank

hard

Fill both blanks to create a method that enqueues an element and prints the queue size.

DSA Java

public void enqueue(int element) {
    if (queue.[1](element)) {
        System.out.println("Queue size: " + queue.[2]());
    } else {
        System.out.println("Queue full");
    }
}

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

Aoffer

Badd

Csize

Dpeek

Attempts:

3 left

💡 Hint

Common Mistakes

Using add which throws exception if full

Using peek instead of size

offer adds the element safely, and size returns the current number of elements.

5fill in blank

hard

Fill all three blanks to implement enqueue with capacity check and print the queue after enqueue.

DSA Java

public void enqueue(int element) {
    if (queue.[1]() < capacity) {
        queue.[2](element);
        System.out.println("Queue: " + queue.[3]());
    } else {
        System.out.println("Queue is full");
    }
}

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

Asize

Badd

CtoString

Doffer

Attempts:

3 left

💡 Hint

Common Mistakes

Using offer instead of add here

Using peek instead of toString

Check size before adding with add, then print queue using toString.

Practice

1. What does the enqueue operation do in a queue data structure?

easy

A. Adds an element to the end of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Returns the size of the queue

Solution

1. Step 1: Understand the queue structure

   A queue works like a line where elements are added at the back and removed from the front.

2. Step 2: Define enqueue operation

   Enqueue means adding a new element at the end of the queue to wait its turn.

3. Final Answer:

   Adds an element to the end of the queue -> Option A

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue always adds at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct Java syntax to enqueue an element 5 into a queue named q?

easy

A. q.enqueue(5);

B. q.add(5);

C. q.push(5);

D. q.insert(5);

Solution

1. Step 1: Recall Java Queue methods

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

2. Step 2: Identify correct method

   enqueue() and insert() are not standard Queue methods; push() is for stacks.

3. Final Answer:

   q.add(5); -> Option B

4. Quick Check:

   Java Queue enqueue = add() [OK]

Hint: Use add() to enqueue in Java Queue [OK]

Common Mistakes:

• Using push() which is for stacks
• Using enqueue() which is not a Java Queue method
• Using insert() which doesn't exist in Queue

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

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

public class TestQueue {
  public static void main(String[] args) {
    Queue<Integer> q = new LinkedList<>();
    q.add(10);
    q.add(20);
    q.add(30);
    q.add(40);
    q.add(50);
    q.add(60);
    System.out.println(q);
  }
}

medium

A. [10 -> 20 -> 30 -> 40 -> 50 -> 60 -> null]

B. [60, 50, 40, 30, 20, 10]

C. SyntaxError

D. [10, 20, 30, 40, 50, 60]

Solution

1. Step 1: Understand Queue implementation

   LinkedList implements Queue and stores elements in insertion order.

2. Step 2: Trace enqueue operations

   Elements 10, 20, 30, 40, 50, 60 are added in that order, so printing the queue shows them in the same order.

3. Final Answer:

   [10, 20, 30, 40, 50, 60] -> Option D

4. Quick Check:

   Queue prints elements in insertion order [OK]

Hint: Queue prints elements in order added [OK]

Common Mistakes:

• Assuming reverse order print
• Expecting arrow notation in output
• Thinking code has syntax errors

4. Identify the error in the following Java code that tries to enqueue an element into a queue:

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

public class QueueTest {
  public static void main(String[] args) {
    Queue<Integer> q = new LinkedList<>();
    q.enqueue(100);
    System.out.println(q);
  }
}

medium

A. Cannot print queue directly

B. LinkedList cannot be used as a Queue

C. Queue interface has no method named enqueue()

D. Missing import for enqueue method

Solution

1. Step 1: Check Queue interface methods

   Java's Queue interface does not have an enqueue() method; it uses add() or offer().

2. Step 2: Verify LinkedList usage

   LinkedList implements Queue correctly, so no issue there; printing queue directly works fine.

3. Final Answer:

   Queue interface has no method named enqueue() -> Option C

4. Quick Check:

   enqueue() method missing in Queue [OK]

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

Common Mistakes:

• Trying to call enqueue() on Java Queue
• Thinking LinkedList can't be a Queue
• Assuming enqueue is a standard method

5. You have a queue implemented using a linked list with a fixed maximum size of 3. What will happen if you try to enqueue a fourth element without dequeuing any element first?

hard

A. The enqueue operation will fail or throw an error due to overflow

B. The new element will be added, increasing the queue size beyond 3

C. The oldest element will be removed automatically to add the new element

D. The queue will reset and start empty before adding the new element

Solution

1. Step 1: Understand fixed size queue behavior

   A fixed size queue cannot hold more elements than its maximum capacity.

2. Step 2: Effect of enqueue beyond capacity

   Trying to add a fourth element when size is 3 causes overflow; enqueue fails or throws an error.

3. Final Answer:

   The enqueue operation will fail or throw an error due to overflow -> Option A

4. Quick Check:

   Fixed size queue overflow blocks enqueue [OK]

Hint: Fixed size queue blocks enqueue when full [OK]

Common Mistakes:

• Assuming queue auto-removes oldest element
• Thinking queue size can grow unlimited
• Believing queue resets automatically