Palindrome Number Program in Java

What is a palindrome number

A palindrome number reads the same forwards and backwards — like 121, 1331 or 7. To check, we reverse the number using maths and compare it with the original. If they match, it is a palindrome. It is a classic loop-and-digit exercise.

Reversing a number

We reverse a number with three steps in a loop:

• digit = num % 10 — get the last digit.
• rev = rev * 10 + digit — build the reversed number.
• num = num / 10 — remove the last digit.

Repeat until num becomes 0, and rev holds the reversed number.

The Java program

public class Palindrome {
    public static void main(String[] args) {
        int num = 121;
        int original = num;    // keep a copy
        int rev = 0;

        while (num > 0) {
            int digit = num % 10;
            rev = rev * 10 + digit;
            num = num / 10;
        }
        if (rev == original)
            System.out.println(original + " is a palindrome");
        else
            System.out.println(original + " is not a palindrome");
    }
}

We save original, reverse the number into rev, then compare. Since 121 reversed is 121, they match and it is a palindrome.

Dry run

A string alternative

int num = 121;
String s = String.valueOf(num);
String r = new StringBuilder(s).reverse().toString();
System.out.println(s.equals(r) ? "Palindrome" : "Not");

You can also convert the number to a string and compare it with its reverse. The maths method avoids strings, but this is shorter and just as valid.

Common mistakes

• Not saving the original. The loop reduces the number to 0 — keep a copy to compare.
• Wrong reverse formula. Use rev * 10 + digit, not rev + digit.
• Swapping % and /. % 10 gets the digit; / 10 removes it.
• Negative numbers. Negatives are usually treated as not palindromes because of the sign.

Summary

• A palindrome number reads the same both ways (121, 1331).
• Reverse the number with % 10, rev*10 + digit, and / 10.
• Save the original to compare at the end.
• If the reverse equals the original, it is a palindrome.
• A string reverse-and-compare is an easy alternative.

Next, reverse text in reverse a string, or practise digits in the Armstrong number program.