Java Palindrome Method

This Java article tests for palindromes. It introduces an isPalindrome method and skips non-letter chars while scanning.
Palindromes. Often as a programming exercise, we write code to detect palindromes. A palindrome spells the same word (or sentence) forwards and backwards.
An algorithm. By scanning forward from the start, and backwards from the end of a string, we can compare the equality of each character. We can ignore spaces and punctuation as we go along.
This code sample introduces 2 methods. In isPalindrome we tell whether the string argument is a palindrome. It uses a while-true loop (with nested loops) to check characters.While

Non-letter chars: The two nested while loops, that test for Character.isLetter, advance past non-letter chars—this skips spaces.

ToLowerCase: We invoke Character.toLowerCase to convert uppercase letters to lowercase ones. So "X" and "x" are equal.

Main: Here we test an array of Strings for palindromes. We write our results to console.

Java program that checks for palindromes public class Program { public static boolean isPalindrome(String value) { // Scan towards the middle, adjusting the start and end indexes. int min = 0; int max = value.length() - 1; while (true) { if (min > max) { return true; } // Move forward past non-letter chars at the start. while (!Character.isLetter(value.charAt(min))) { min++; } // Move backward past non-letter chars at the end. while (!Character.isLetter(value.charAt(max))) { max--; } char a = Character.toLowerCase(value.charAt(min)); char b = Character.toLowerCase(value.charAt(max)); // If letters are not equal, this is not a palindrome. if (a != b) { return false; } // Advance towards center. min++; max--; } } public static void main(String[] args) { // Some example lines. String[] lines = { "civic", "A man, a plan, a canal: Panama.", "A Toyota. Race fast, safe car. A Toyota.", "Cigar? Toss it in a can. It is so tragic.", "Dammit, I'm mad!", "Delia saw I was ailed.", "Desserts, I stressed!", "Draw, O coward!", "Lepers repel.", "Live not on evil.", "Lonely Tylenol.", "Murder for a jar of red rum.", "Never odd or even.", "No lemon, no melon.", "Senile felines.", "So many dynamos!", "Step on no pets.", "Was it a car or a cat I saw?", "Dot Net Perls is not a palindrome.", "Why are you reading this?", "This article is not useful." }; // Test each line to see if it is a palindrome. for (String line : lines) { if (isPalindrome(line)) { System.out.println("Palindrome: " + line); } else { System.out.println("Not palindrome: " + line); } } } } Output Palindrome: civic Palindrome: A man, a plan, a canal: Panama. Palindrome: A Toyota. Race fast, safe car. A Toyota. Palindrome: Cigar? Toss it in a can. It is so tragic. Palindrome: Dammit, I'm mad! Palindrome: Delia saw I was ailed. Palindrome: Desserts, I stressed! Palindrome: Draw, O coward! Palindrome: Lepers repel. Palindrome: Live not on evil. Palindrome: Lonely Tylenol. Palindrome: Murder for a jar of red rum. Palindrome: Never odd or even. Palindrome: No lemon, no melon. Palindrome: Senile felines. Palindrome: So many dynamos! Palindrome: Step on no pets. Palindrome: Was it a car or a cat I saw? Not palindrome: Dot Net Perls is not a palindrome. Not palindrome: Why are you reading this? Not palindrome: This article is not useful.
Considerations. Often when implementing a palindrome method, non-letter characters are not considered. But when analyzing a sentence, punctuation and spaces usually appear.

So: It is possible to preprocess a string, with Replace or a regular expression, to eliminate these characters.


However: For performance, and clarity, it is better to skip past these characters within the isPalindrome method.

When learning a language, I often implement many small methods based on known algorithms. These include a ROT13 cipher and a palindrome-testing method. We can fill in gaps in our knowledge.ROT13
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