Array Deque/dequeue Example in Java

Introduction

Array Deque/Dequeue is a classic Java console program that demonstrates the concept with complete source code and sample output. Classic data structures such as stack, queue and linked list implemented in Java.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Definition

A double-ended queue is an abstract data type that generalizes a queue, for which elements can be added to or removed from either the front (head) or back (tail). It is also often called a head-tail linked list.

Array Deque/Dequeue Example Program

import java.util.Scanner;

class DequeueClass {

    private int[] array;
    private int val1, val2;

    public DequeueClass() {
        val1 = 0;
        val2 = 0;
        resize();
    }

    public boolean isEmpty() {
        return val2 == 0;
    }

    public void clear() {
        val1 = 0;
        val2 = 0;
        resize();
    }

    public int getSize() {
        return val2;
    }

    private void resize() {
        int[] num = new int[Math.max(2 * val2, 1)];
        for (int i = 0; i < val2; i++) {
            num[i] = array[(val1 + i) % array.length];
        }
        array = num;
        val1 = 0;
    }

    public int get(int index) {
        return array[(val1 + index) % array.length];
    }

    public int set(int a, int b) {
        int y = array[(val1 + a) % array.length];
        array[(val1 + a) % array.length] = b;
        return y;
    }

    public void add(int num1, int num2) {
        if (val2 + 1 > array.length) {
            resize();
        }
        if (num1 < val2 / 2) {

            val1 = (val1 == 0) ? array.length - 1 : val1 - 1;

            for (int i = 0; i <= num1 - 1; i++) {
                array[(val1 + i) % array.length] = array[(val1 + i + 1) % array.length];
            }
        } else {

            for (int i = val2; i > num1; i--) {
                array[(val1 + i) % array.length] = array[(val1 + i - 1) % array.length];
            }
        }
        array[(val1 + num1) % array.length] = num2;
        val2++;
    }

    public int remove(int position) {
        int x = array[(val1 + position) % array.length];
        if (position < val2 / 2) {

            for (int i = position; i > 0; i--) {
                array[(val1 + i) % array.length] = array[(val1 + i - 1) % array.length];
            }

            val1 = (val1 + 1) % array.length;
        } else {

            for (int i = position; i < val2 - 1; i++) {
                array[(val1 + i) % array.length] = array[(val1 + i + 1) % array.length];
            }
        }
        val2--;
        if (3 * val2 < array.length) {
            resize();
        }
        return x;
    }

    public void display() {
        int num = val1;
        for (int i = 0; i < val2; i++) {
            System.out.print(array[num % array.length] + " ");
            num++;
        }
        System.out.println();
    }
}

public class ArrayDequeueExample {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        DequeueClass dequeueClass = new DequeueClass();

        char character;

        do {
            System.out.println("\nMethods in Array Dequeue : \n");
            System.out.println("1. Size");
            System.out.println("2. Check if array is empty");
            System.out.println("3. Add Element");
            System.out.println("4. Get Element");
            System.out.println("5. Set Element");
            System.out.println("6. Remove Element");
            System.out.println("7. Clear");

            int choice = scan.nextInt();
            switch (choice) {
                case 1:
                    System.out.println("\nSize is " + dequeueClass.getSize());
                    break;
                case 2:
                    System.out.println("\nArray is Empty or Not : " + dequeueClass.isEmpty());
                    break;
                case 3:
                    System.out.println("Enter the index and element");
                    dequeueClass.add(scan.nextInt(), scan.nextInt());
                    break;
                case 4:
                    System.out.println("Enter index");
                    break;
                case 5:
                    System.out.println("Enter the index and element");
                    break;
                case 6:
                    System.out.println("\nEnter index");
                    dequeueClass.remove(scan.nextInt());
                    break;
                case 7:
                    System.out.println("\nArray cleared successfully");
                    dequeueClass.clear();
                    break;
                default:
                    System.out.println("Entry is Wrong \n ");
                    break;
            }

            dequeueClass.display();

            System.out.println("\nDo you wish to continue (Type Y or N) \n");
            character = scan.next().charAt(0);
        } while (character == 'Y' || character == 'y');
    }
}

Sample Output

Methods in Array Dequeue :

1. Size
2. Check if array is empty
3. Add Element
4. Get Element
5. Set Element
6. Remove Element
7. Clear
3
Enter the index and element
0
100
100

Do you wish to continue (Type Y or N)

Y

Methods in Array Dequeue :

1. Size
2. Check if array is empty
3. Add Element
4. Get Element
5. Set Element
6. Remove Element
7. Clear
3
Enter the index and element
1
1000
100 1000

Do you wish to continue (Type Y or N)

Y

Methods in Array Dequeue :

1. Size
2. Check if array is empty
3. Add Element
4. Get Element
5. Set Element
6. Remove Element
7. Clear
1

Size is 2
100 1000

Do you wish to continue (Type Y or N)

N

When to use

Use this array deque/dequeue example when learning or revising core Java syntax.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. import java.util.Scanner; imports a class used later in the program.

3. val1 = 0; updates a variable used in the calculation or output.

4. val2 = 0; updates a variable used in the calculation or output.

5. val1 = 0; updates a variable used in the calculation or output.

6. val2 = 0; updates a variable used in the calculation or output.

7. int[] num = new int[Math.max(2 * val2, 1)]; updates a variable used in the calculation or output.

8. The if statement runs the nested code only when the condition is true.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.