Collections.reverse method not reversing entry

Question

My code below is supposed to accept an integer from the user, and then print whatever integer they enter in reverse order. I am getting no errors, but when I run this program the integer is not being printed in reverse. Can someone please help me figure out why?

import java.util.*;

public class ReverseDigits {

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

        System.out.println("Please enter a whole number.");
        response = sc.next();

        reverseDigit(response);
    }

    public static void reverseDigit(String digit) {
        ArrayList al = new ArrayList();
        al.add(digit);
        Collections.reverse(al);
        System.out.println("Your number in reverse order is: " + al);
    }

}

score 6 · Accepted Answer · edited May 23 '17 at 12:23

6

You misinterpreted what Collections.reverse does. This method reverses the list that you gave, not its content. Since you called this method with a list having a single element, the result will be the same list.

If you want to reverse a String, please refer to this question.

As a side-note: do not use raw types like ArrayList, this will get into trouble. Prefer the type-safe way ArrayList<String>.

edited May 23 '17 at 12:23

Community

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answered Oct 31 '15 at 18:01

Tunaki

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Thank you so much! I will read up more on ArrayLists, and your link solved my problem. – carchelhf Oct 31 '15 at 18:05

score 0 · Answer 2 · answered Oct 31 '15 at 18:04

0

Try this code.

ArrayList al = new ArrayList();
al.add(new StringBuffer(digit).reverse().toString());

answered Oct 31 '15 at 18:04

Ravindra babu

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Please don't use StringBuffer, it was replaced more than ten years ago. – Peter Lawrey Oct 31 '15 at 18:05

score 0 · Answer 3 · answered Oct 31 '15 at 18:05

If you have a collection of one, it is the same in reverse order as forward.

If you want to reverse the string representation of an integer, you can use StringBuilder to reverse the digits.

StringBuilder sb = new StringBuilder();
sb.append(digits);
sb.reverse();
System.out.println("Your number in reverse order is: "+ sb);

score 0 · Answer 4 · answered Oct 31 '15 at 18:06

0

If you want to reverse a String why are you adding it to a list ?

Use this :

         String s = sc.next();
         StringBuilder sb = new StringBuilder(s);
         System.out.println(sb.reverse());

answered Oct 31 '15 at 18:06

Rahman

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score 0 · Answer 5 · answered Oct 31 '15 at 19:17

The reverse(List<?>) method is used to reverse the order of the elements in the specified list. Since there is only one item in your collection, reverse order is same as the initial list. You can use the below code as you are trying to reverse an integer.

package com.stackoverflow.answer;

import java.util.*;

public class ReverseDigits {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Please enter a whole number: ");
        int number = scanner.nextInt();
        System.out.println(String.format("Your number in reverse order is: %d", reverse(number)));
        scanner.close();
    }

    public static int reverse(int x) {
        return reverse(x, 0);
    }

    private static int reverse(int x, int y) {
        return x == 0 ? y : reverse(x / 10, y * 10 + x % 10);
    }

}

Collections.reverse method not reversing entry

5 Answers5