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
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