Check if a number or string is a palindrome in Java

How to check whether a number or string is a palindrome or not is a popular question. A number is a palindrome if its inverse remains the same as 404. On the other hand, 412 is a non-palindrome because its inverse is 214. To check if a number or a string is palindromed, we must first ask the question how to invert a number or a string in Java? You can solve this problem by using an API or by using only the basics of programming such as loops, conditions, variables, and logical operators. It is recommended to program on your own because this simple problem may well show a real programmer who can code and the other who receives everything on a dish.

recursive and iterative palindrome in java

Check if an integer is a palindrome

This is a Java program that checks whether a number is a palindrome or not. This program does not use any APIs, however, it uses division and modulo to test whether an integer is a palindrome or not. This check is encapsulated in the method reverse() and estPalinsrome(int number).

public class Palindrome {
 
 public static void main(String args[]){
 
 int[] numbers = {0, 2, 113, 11, 1443, 1441, 1654, 19891}; 
 
 for(int number: numbers){
 System.out.println(number +" is a palindrome? " 
 + estPalindrome(number)); 
 } 
 }

 private static boolean isPalindrome(int number) {
 if(number == inverse(number)){
 return true; 
 }
 return false; 
 }
 
 private static int inverse(int number){
 int inverse = 0; 
 while(number != 0){
 inverse = inverse*10 + number%10; 
 number = number/10; 
 }
 inverse return; 
 }
}

Running this program gives this result:

0 is a palindrome? true
2 is a palindrome? true
113 is a palindrome? false
11 is a palindrome? true
1443 is a palindrome? false
1441 is a palindrome? true
1654 is a palindrome? false
19891 is a palindrome? true

This Java program takes an array of integers and checks whether a number is a palindrome or not.

Check if a string is a palindrome

A string is called a palindrome if it is equal to its inverse, For example, "radar" is a palindrome, its inverse remains "radar". Other examples like: "ababa", "she", "anna".

Iterative palindrome

import java.util.Scanner; 

public class Palindrome_string {

 public static void main(String args[])
 {
 String word, mot_inverse = ""; 
 Scanner in = new Scanner(System.in); 
 
 System.out.println("Enter a word "); 
 word = in.nextLine(); 
 
 if (pal(mot))
 System.out.println("'"+mot + "' is a palindrome"); 
 else
 System.out.println("'"+word + "' is not a palindrome"); 
 }
 
 public static boolean pal(String mot){
 int i=0, length=word.length()-1; 
 boolean equal=true; 
 /*test the first character with the last
 *and if they are equal, the program continues
 *to unwind the while loop and test
 *the next character(i+1 with length-(i+1))
 *until i is equal to length/2,
 *, otherwise the equal Boolean receives false
 *so,  The PAL function returns false
 */
 while(i< Length/2 & & equals){
 if(charAt.word(i)==charAt.word(i))
 equals = true; 
 else
 equals = false; 
 i++; 
 }
 equal return; 
 }
}

Result:

Enter a word 
rotor
'rotor' is a palindrome

recursive palindrome

public class Palindrome_recursive {

 public static void main(String[] args) { 
 String[] strings = {"here", "rumor", "Leila",
 "lol", "hi", "no"}; 
 
 for(String string: strings){
 if(estPalindrome(string))
 System.out.println("'"+string + "' is a palindrome"); 
 else
 System.out.println("'"+string + "' is not a palindrome"); 
 }
 }
 
 public static boolean isPalindrome(String s)
 {
 //if the length is 0 or 1
 //then it's a palindrome
 if(s.length() == 0 || s.length() == 1)
 true; 
 if(s.charAt(0) == s.charAt(s.length()-1))
 /*
 * test if the first and the last character
 * are the same then repeat the same processing
 * with the substring with the first and 
 * last character until you reach the 
 * first condition
 */
 return isPalindrome(s.substring(1, s.length()-1)); 

 /*
 * if the if condition is false, then it returns
 * false
 */
 return false; 
 }
}

Result:

'here' is a palindrome
'rumor' is not a palindrome
'Leila' is not a palindrome
'lol' is a palindrome
'hi' is not a palindrome
'no' is a palindrome

Both If codes are case-sensitive and uppercase, you can change them to ignore uppercase by converting both strings to lowercase or uppercase,

As said, a good programmer should always practice on exercises that seem easy and especially Java programming beginners who have just started learning Java programming.

References:
List of French palindromes
Stackoverflow: Check string for palindrome
Java program to check palindrome