Implementation of Queue using Stack in Java

February 24, 2026

Implementation of Queue Using Stack

Implementation of Queue using Stack in Java is an important problem that helps you understand how different data structures can be combined to simulate each other.

Since a queue follows FIFO (First In, First Out) and a stack follows LIFO (Last In, First Out), implementing a queue using stacks requires logical rearrangement of elements.

This topic is frequently asked in interviews because it tests your understanding of both stacks and queues. It also improves your problem-solving skills in data structure conversions.

Methods to Implement Queue Using Stack in Java

Here we will practice how to Implement Queue Using Stack in Java code:

Using Two Stacks (Efficient and Practical)
Using One Stack with Recursion (Conceptual Method)

Feature	Two Stacks	One Stack + Recursion
Complexity	O(1) amortized	O(n)
Space	O(n)	O(n) + recursion
Efficiency	High	Low

Method to Implement Queue Using Stack in Java

Method 1: Queue Using Two Stacks

Algorithm (Two Stacks Method)

Enqueue:

Push element into stack1

Dequeue:

If stack2 is not empty:
- If stack2 is not empty:
Else:
- While stack1 is not empty:
  - While stack1 is not empty:
- Pop from stack1 and push into stack2
If both stacks are empty:
- Underflow

Java Code:

Run

import java.util.Stack;

class QueueUsingTwoStacks {

    private Stack stack1 = new Stack<>();
    private Stack stack2 = new Stack<>();

    // Enqueue operation
    public void enqueue(int value) {
        stack1.push(value);
        System.out.println(value + " inserted");
    }

    // Dequeue operation
    public int dequeue() {

        if (stack1.isEmpty() && stack2.isEmpty()) {
            System.out.println("Queue is empty (Underflow)");
            return -1;
        }

        // Move elements if needed
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }

        return stack2.pop();
    }

    // Display queue elements
    public void display() {

        if (stack1.isEmpty() && stack2.isEmpty()) {
            System.out.println("Queue is empty");
            return;
        }

        System.out.print("Queue: ");

        // Print stack2 (top to bottom)
        for (int i = stack2.size() - 1; i >= 0; i--) {
            System.out.print(stack2.get(i) + " ");
        }

        // Print stack1 (bottom to top)
        for (int i = 0; i < stack1.size(); i++) {
            System.out.print(stack1.get(i) + " ");
        }

        System.out.println();
    }

    // Main method
    public static void main(String[] args) {

        QueueUsingTwoStacks q = new QueueUsingTwoStacks();

        q.enqueue(10);
        q.enqueue(20);
        q.enqueue(30);

        q.display();

        System.out.println("Removed: " + q.dequeue());
        System.out.println("Removed: " + q.dequeue());

        q.display();

        q.enqueue(40);

        q.display();
    }
}

Input:

enqueue(10)
enqueue(20)
enqueue(30)
dequeue()
dequeue()
enqueue(40)

Output:

10 inserted
20 inserted
30 inserted
Queue: 10 20 30
Removed: 10
Removed: 20
Queue: 30
40 inserted
Queue: 30 40

Time and Space Complexity: For Enqueue and Dequeue:
Space Complexity: O(n)
Time Complexity: O(1)
For Worst Dequeue:
Space Complexity: O(n)
Time Complexity: O(n)

Insertion in Queues Program

Queues using Linked Lists

Implement Queue using two Stacks

Queues using Linked Lists

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Method to Implement Queue Using Stack in Java

Method 2: Queue Using One Stack (Recursion Method)

Algorithm (Recursion Method)

Enqueue:

Push element into stack

Dequeue:

If stack is empty:
- Underflow
Pop top element → temp
If stack is empty now:
- Return temp (bottom element)
Else:
- result = dequeue()
- Push temp back
- Return result

Java Code:

Run

import java.util.Stack;

class QueueUsingOneStack {

    private Stack stack = new Stack<>();

    // Enqueue
    public void enqueue(int value) {
        stack.push(value);
        System.out.println(value + " inserted");
    }

    // Dequeue using recursion
    public int dequeue() {

        if (stack.isEmpty()) {
            System.out.println("Queue is empty (Underflow)");
            return -1;
        }

        int top = stack.pop();

        // If this is last element
        if (stack.isEmpty()) {
            return top;
        }

        int result = dequeue();

        // Push back other elements
        stack.push(top);

        return result;
    }

    // Display
    public void display() {

        if (stack.isEmpty()) {
            System.out.println("Queue is empty");
            return;
        }

        System.out.print("Queue: ");

        for (int i = 0; i < stack.size(); i++) {
            System.out.print(stack.get(i) + " ");
        }

        System.out.println();
    }

    // Main method
    public static void main(String[] args) {

        QueueUsingOneStack q = new QueueUsingOneStack();

        q.enqueue(5);
        q.enqueue(15);
        q.enqueue(25);

        q.display();

        System.out.println("Removed: " + q.dequeue());

        q.display();

        q.enqueue(35);

        q.display();
    }
}

Input:

enqueue(5)
enqueue(15)
enqueue(25)
dequeue()
enqueue(35)

Output:

5 inserted
15 inserted
25 inserted
Queue: 5 15 25
Removed: 5
Queue: 15 25
35 inserted
Queue: 15 25 35

Time and Space Complexity: For Enqueue:
Space Complexity: O(n)
Time Complexity: O(1)
For Dequeue:
Space Complexity: O(n)
Time Complexity: O(n)

Points to remember:

Before operations, check:

Stack is not empty (for dequeue)
Avoid deep recursion
Handle underflow
Prevent null references

Example:

if (stack.isEmpty()) {
    System.out.println("Queue is empty");
}

Frequently Asked Questions

1. What is Implementation of Queue using Stack in Java?

Answer:

It is a method of creating a queue by using stack operations to maintain FIFO order.

2. Which is the best way to implement queue using stack?

Answer:

Using two stacks is the most efficient and commonly used method.

3. What is the time complexity of queue using stack?

Answer:

Enqueue is O(1), dequeue is O(1) amortized using two stacks.

4. Can we implement queue using one stack?

Answer:

Yes, using recursion, but it is less efficient and not recommended for large data.

5. Is queue using stack used in real applications?

Answer:

Mostly for learning and interviews. Real systems prefer built in queue structures.

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Stacks

Introduction to Stack in Data Structure
Click Here
Operations on a Stack
Click Here
Stack: Infix, Prefix and Postfix conversions
Click Here
Stack Representation in –
C | C++ | Java
Representation of a Stack as an Array. –
C | C++ | Java
Representation of a Stack as a Linked List. –
C | C++ | Java
Infix to Postfix Conversion –
C | C++ | Java
Infix to prefix conversion in –
C | C++ | Java
Postfix to Prefix Conversion in –
C | C++ | Java

Queues

Queues in Data Structures (Introduction)
Click Here
Queues Program in C and implementation
Click Here
Implementation of Queues using Arrays | C Program
Click Here
Types of Queues in Data Structure
Click Here
Application of Queue Data Structure
Click Here
Insertion in Queues Program (Enqueuing) –
C | C++ | Java
Deletion (Removal) in Queues Program(Dequeuing) –
C | C++ | Java
Reverse a Queue –
C | C++ | Java
Queues using Linked Lists –
C | C++ | Java
Implement Queue using Stack –
C | C++ | Java
Implement Queue using two Stacks –
C | C++ | Java

Circular Queues

Circular queue in Data Structure
Click Here
Applications of Circular Queues
Click Here
Circular queue in –
C | C++ | Java
Circular queue using Array –
C | C++ | Java
Circular Queue using Linked Lists –
C | C++ | Java

Priority Queue

Application of Priority Queue
Priority Queue Example
Priority Queue Introduction –
C | C++ | Java
Priority Queue Implementation using Array –
C | C++ | Java
Priority Queue using Linked List –
C | C++ | Java
Priority Queue Insertion and Deletion-
C | C++ | Java

Stacks

Introduction to Stack in Data Structure
Operations on a Stack
Stack: Infix, Prefix and Postfix conversions
Stack Representation in – C | C++ | Java
Representation of a Stack as an Array. – C | C++ | Java
Representation of a Stack as a Linked List. – C | C++ | Java
Infix to Postfix Conversion – C | C++ | Java
Infix to prefix conversion in – C | C++ | Java
Postfix to Prefix Conversion in – C | C++ | Java

Queues

Queues in Data Structures (Introduction)
Queues Program in C and implementation
Implementation of Queues using Arrays | C Program
Types of Queues in Data Structure
Application of Queue Data Structure
Insertion in Queues Program (Enqueuing) – C | C++ | Java
Deletion (Removal) in Queues Program(Dequeuing) – C | C++ | Java
Reverse a Queue – C | C++ | Java
Queues using Linked Lists – C | C++ | Java
Implement Queue using Stack – C | C++ | Java
Implement Queue using two Stacks – C | C++ | Java

Circular Queues

Circular queue in Data Structure
Applications of Circular Queues
Circular queue in – C | C++ | Java
Circular queue using Array – C | C++ | Java
Circular Queue using Linked Lists – C | C++ | Java

Priority Queue

Application of Priority Queue
Priority Queue Example
Priority Queue Introduction – C | C++ | Java
Priority Queue Implementation using Array – C | C++ | Java
Priority Queue using Linked List – C | C++ | Java
Priority Queue Insertion and Deletion- C | C++ | Java

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Implementation of Queue using Stack in Java