Enqueue Operation in DSA Java - Build from Scratch

Enqueue Operation in a Queue

📖 Scenario: Imagine a line of people waiting to buy tickets at a movie theater. The queue grows as new people arrive at the end of the line.

🎯 Goal: You will create a simple queue using an array and write code to add (enqueue) new people to the end of the queue.

📋 What You'll Learn

Create an integer array called queue with size 5

Create an integer variable called rear and set it to -1

Write a method called enqueue that adds a number to the queue if there is space

Print the queue contents after adding new elements

💡 Why This Matters

🌍 Real World

Queues are used in real life to manage lines, like waiting for tickets, customer service, or print jobs.

💼 Career

Understanding queue operations is important for software development roles that involve data processing, task scheduling, and resource management.

1

Create the queue array and rear pointer

Create an integer array called queue with size 5 and an integer variable called rear set to -1.

DSA Java

// Create the queue array and rear pointer
// Your code here

Hint

Use int[] queue = new int[5]; to create the array and int rear = -1; to track the last element.

2

Create a method to add elements to the queue

Create a static method called enqueue that takes an integer value and adds it to the queue if rear is less than 4. Increase rear by 1 and store value at queue[rear].

DSA Java

public class Main {
    static int[] queue = new int[5];
    static int rear = -1;

    public static void enqueue(int value) {
        // Your code here
    }

    public static void main(String[] args) {
    }
}

Hint

Check if rear is less than 4 before adding. Increase rear and assign value to queue[rear].

3

Add elements to the queue using enqueue

In the main method, call enqueue three times with values 10, 20, and 30.

DSA Java

public class Main {
    static int[] queue = new int[5];
    static int rear = -1;

    public static void enqueue(int value) {
        if (rear < 4) {
            rear++;
            queue[rear] = value;
        }
    }

    public static void main(String[] args) {
        // Your code here
    }
}

Hint

Call enqueue three times with the values 10, 20, and 30 inside main.

4

Print the queue contents after enqueue operations

In the main method, after the enqueue calls, use a for loop with variable i from 0 to rear to print each element of queue separated by spaces.

DSA Java

public class Main {
    static int[] queue = new int[5];
    static int rear = -1;

    public static void enqueue(int value) {
        if (rear < 4) {
            rear++;
            queue[rear] = value;
        }
    }

    public static void main(String[] args) {
        enqueue(10);
        enqueue(20);
        enqueue(30);
        // Your code here
    }
}

Hint

Use a for loop from 0 to rear and print each queue[i] followed by a space.

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