Question

Write a java program that uses a stack to reverse an integer of three digits. The program must prompt the user to input an integer of 3 digits and display it in reverse.
- Your program must include the Stack class, the Reverse class, and the Test class.
- In the Test class, the program must prompt the user to input a 3-digit number and display it in reverse.
- Class Reverse must use the Stack class to reverse the number.
Note: the input must be of type integer.
Here is a sample run:
Enter a 3-digit integer: 123
The reversed number: 321

Answer 1

Here is the complete java code with comments:

NOTE: Not only it works for 3 digits, but for even more number of digits :)

//To get input
import java.util.*;
//To use power function
import java.lang.Math;

//This is the stack class
class StackClass {
    // This arraylist will work as a stack
    public ArrayList<Integer> mystack = new ArrayList<Integer>();

    // This will add to the stack till the number becomes 0
    public StackClass(int n) {
        while (n != 0) {
            mystack.add(n % 10);
            n = n / 10;
        }
    }

}

// Reverse Class
class Reverse {
    int reversedNum;

    Reverse(StackClass st) {
        reversedNum = 0;
        for (int i = 0; i < st.mystack.size(); i++) {
            /*
             * The numeber gets converted like this: If the number is 3 2 1 in the stack
             * then it will be converted like this revNum = 3*(100) + 2*(10) + 1(1) = 321
             * which is reverse of 123
             */
            reversedNum = (int) (reversedNum + st.mystack.get(i) * (Math.pow(10, st.mystack.size() - i - 1)));
        }
    }
}

public class Test {
    // This is the main function
    public static void main(String[] args) {
        // The scanner is used to take the input
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a 3 digit number: ");
        int n = sc.nextInt();
        StackClass stobj = new StackClass(n);
        Reverse obj = new Reverse(stobj);
        System.out.println("The reversed number: " + obj.reversedNum);
        sc.close();
    }
}

Here is the output for the above code: