Java Program to Check Palindrome

A string is called a palindrome string if the reverse of that string is the same as the original string. For example, radar, level, etc.

Similarly, a number that is equal to the reverse of that same number is called a palindrome number. For example, 3553, 12321, etc.

To check a Palindrome in Java, we first reverse the string or number and compare the reversed string or number with the original value.

Example 1: Java Program to Check Palindrome String

class Main {
  public static void main(String[] args) {

    String str = "Radar", reverseStr = "";
    
    int strLength = str.length();

    for (int i = (strLength - 1); i >=0; --i) {
      reverseStr = reverseStr + str.charAt(i);
    }

    if (str.toLowerCase().equals(reverseStr.toLowerCase())) {
      System.out.println(str + " is a Palindrome String.");
    }
    else {
      System.out.println(str + " is not a Palindrome String.");
    }
  }
}

Output

Radar is a Palindrome String.

In the above example, we have a string "Radar" stored in str. Here, we have used the

1. for loop to reverse the string

• The loop runs from the end to the beginning of the string.
• The charAt() method accesses each character of the string.
• Each character of the string is accessed in reverse order and stored in reverseStr.

2. if statement to compare str and reverseStr

• The toLowerCase() method converts both str and reverseStr to lowercase. This is because Java is case sensitive and 'r' and 'R' are two different values.
• The equals() method checks if two strings are equal.

Example 2: Java Program to Check Palindrome Number

class Main {
  public static void main(String[] args) {
    
    int num = 3553, reversedNum = 0, remainder;
    
    // store the number to originalNum
    int originalNum = num;
    
    // get the reverse of originalNum
    // store it in variable
    while (num != 0) {
      remainder = num % 10;
      reversedNum = reversedNum * 10 + remainder;
      num /= 10;
    }
    
    // check if reversedNum and originalNum are equal
    if (originalNum == reversedNum) {
      System.out.println(originalNum + " is Palindrome.");
    }
    else {
      System.out.println(originalNum + " is not Palindrome.");
    }
  }
}

Output

3553 is Palindrome.

In the above example, we have a number 3553 stored in num and originalNum variables. Here, we have used the

• while loop to reverse num and store the reversed number in reversedNum
• if...else to check if reversedNum is same as the originalNum