Enqueue Operation in DSA Java - Execution Trace

Concept Flow - Enqueue Operation

Create new node with value

↓

Is queue empty?

Yes→Set head and tail to new node

No↓

Set tail.next to new node

↓

Update tail to new node

↓

Done

This flow shows how a new element is added to the end of the queue by creating a node and updating pointers.

Execution Sample

DSA Java

class Queue {
  Node head, tail;
  void enqueue(int val) {
    Node newNode = new Node(val);
    if (tail == null) {
      head = tail = newNode;
    } else {
      tail.next = newNode;
      tail = newNode;
    }
  }
}

This code adds a new value to the end of a linked-list based queue.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with value 10	None	None	null
2	Check if queue is empty (tail == null)	None	None	null
3	Queue empty: set head and tail to new node	10	head -> 10, tail -> 10	10 -> null
4	Create new node with value 20	10	None	10 -> null
5	Check if queue is empty (tail == null)	10	None	10 -> null
6	Queue not empty: set tail.next to new node	10, 20	tail.next -> 20	10 -> 20 -> null
7	Update tail to new node	10, 20	tail -> 20	10 -> 20 -> null
8	Create new node with value 30	10, 20	None	10 -> 20 -> null
9	Check if queue is empty (tail == null)	10, 20	None	10 -> 20 -> null
10	Queue not empty: set tail.next to new node	10, 20, 30	tail.next -> 30	10 -> 20 -> 30 -> null
11	Update tail to new node	10, 20, 30	tail -> 30	10 -> 20 -> 30 -> null
12	End of enqueue operations	10, 20, 30	head -> 10, tail -> 30	10 -> 20 -> 30 -> null

💡 All enqueue operations complete; tail updated to last node; queue correctly linked.

Variable Tracker

Variable	Start	After Step 3	After Step 7	After Step 11	Final
head	null	Node(10)	Node(10)	Node(10)	Node(10)
tail	null	Node(10)	Node(20)	Node(30)	Node(30)
newNode	null	Node(10)	Node(20)	Node(30)	null

Key Moments - 3 Insights

Why do we check if tail is null before adding a new node?

Because if tail is null, the queue is empty, so both head and tail must point to the new node. This is shown in execution_table rows 2 and 3.

Why do we update tail.next before moving tail to the new node?

Updating tail.next links the old last node to the new node, maintaining the chain. Then tail moves to the new node. See rows 6 and 7.

What happens if we forget to update tail after adding a node?

The tail pointer would still point to the old last node, breaking the queue's end. New nodes won't be added correctly. This is avoided by step 7 and 11.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what does the queue look like?

A 10 -> null

B null

C 20 -> null

D 10 -> 20 -> null

💡 Hint

Check the Visual State column at step 3.

At which step does the tail pointer first move from Node(10) to Node(20)?

A Step 5

B Step 7

C Step 6

D Step 8

💡 Hint

Look at Pointer Changes column for tail updates.

If the queue was not empty, which operation links the new node to the queue?

A Creating new node

B Setting head to new node

C Setting tail.next to new node

D Setting tail to null

💡 Hint

Refer to execution_table rows 6 and 10 for linking nodes.

Concept Snapshot

Enqueue Operation in Queue:
- Create new node with value
- If queue empty (tail == null), set head and tail to new node
- Else, set tail.next to new node
- Update tail to new node
- Maintains FIFO order by adding at end

Full Transcript

The enqueue operation adds a new element to the end of a queue implemented as a linked list. First, a new node is created with the given value. If the queue is empty (tail pointer is null), both head and tail pointers are set to this new node. Otherwise, the current tail's next pointer is set to the new node, and then the tail pointer is updated to point to the new node. This process ensures the queue maintains the correct order of elements, adding new ones at the end. The execution table shows each step, including pointer updates and the visual state of the queue after each operation.

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