Java Program for Palindrome String

Problem: Write a java program to check whether the given string is palindrome or not.

There are multiple ways to check for a palindrome string in Java. Let’s discuss some of the methods with examples.

Check Palindrome String in Java using for Loop

In this method, we reverse the string using a loop and compare the reverse with the original input string.

To reverse the string we need to loop through each character of the original string from the back and concatenate it into the reverse string.

import java.util.Scanner;
 
public class PalindromeString {
 
    public static void main(String args[]){
 
        Scanner in = new Scanner(System.in);
        
        String str, rev="";
 
        System.out.println("Enter a String");
        str= in.nextLine();
 
        for(int i=str.length()-1 ; i>=0; i--){
            rev = rev + str.charAt(i);
        }
 
        if(rev.equalsIgnoreCase(str))
            System.out.println("Palindrome String");
        else
            System.out.println("Not a Palindrome String");
    }
}

Output

Enter a String
mom
Palindrome String

Enter a String
wow wow
Palindrome String

Enter a String
pencil programmer
Not a Palindrome String

Check Palindrome String in Java Without Finding its Reverse

To check whether a string or word is palindrome or not without computing reverse, we need to compare 1st half of the string with another half.

We can achieve this by using for loop from 0 to half the length of the input string (i.e. str.length()/2).

import java.util.Scanner;
 
public class PalindromeStringWR {
    public static void main(String args[]){
 
        Scanner in = new Scanner(System.in);
        int l;
        boolean flag = true;             //Assuming string is palindrome
        String str;
 
        System.out.println("Enter a String");
        str= in.nextLine();
 
        l = str.length();               //length of string
 
        for(int i=0; i<=l/2; i++){
            //If at a particular position from front if the character doesn't match
            //with the character at same position from last, then string is not palindrome
            if(str.charAt(i) != str.charAt(l-1-i)){
                flag = false;
                break;
            }
        }
 
        if(flag == true)
            System.out.println("Palindrome String");
        else
            System.out.println("Not a Palindrome String");
    }
}

Output

Enter a String
madam
Palindrome String

Enter a String
hello!
Not a Palindrome String

Check Palindrome String in Java using StringBuilder

The StringBuilder class in Java has an inbuilt reverse() method, that can be used to reverse a string.

All we need to do is to compare the input string with the return result of the reverse() method.

Here is the Java code for the same.

import java.util.Scanner;
 
public class PalindromeStringSB {
 
    public static void main(String args[]){
 
        Scanner in = new Scanner(System.in);
        String str, rev="";
 
        System.out.println("Enter a String");
        str= in.nextLine();
 
        StringBuilder stringBuilder = new StringBuilder(str);
        rev = stringBuilder.reverse().toString();
 
        if(str.equalsIgnoreCase(rev))
            System.out.println("Palindrome String");
        else
            System.out.println("Not a Palindrome String");
    }
}

Output

Enter a String
lol
Palindrome String

Enter a String
please comment
Not a Palindrome String

In this tutorial, we learned different ways to check palindrome string in Java programming language.