This fast algorithm goes through string from both ends and expects them to be the same. As soon as they start to differ we know that this is not a palindrome.

package com.farenda.tutorials.algorithms.strings;

public class PalindromeDetector {

    public boolean isPalindrome(String s) {
        for (int i = 0, j = s.length()-1; i <= j; ++i, --j) {
            // front char != back char
            if (s.charAt(i) != s.charAt(j)) {
                return false;
            }
        }
        return true;
    }
}

Notice that we compare the chars only to the middle of the string, because after that they start to repeat – remember, palindrome from both sides is the same. ;-)

This implementation is presumably slower than the previous one, but is much more readable and requires no explanation:

public boolean isSlowPalindrome(String s) {
    return new StringBuilder(s).reverse().toString().equals(s);
}

To check that it really works, let’s write some JUnit tests:

package com.farenda.tutorials.algorithms.strings;

import org.junit.Test;

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

public class PalindromeDetectorTest {

    private PalindromeDetector detector = new PalindromeDetector();

    @Test
    public void shouldFindPalindromes() {
        assertTrue(detector.isPalindrome(""));
        assertTrue(detector.isPalindrome("a"));
        assertTrue(detector.isPalindrome("aba"));
        assertTrue(detector.isPalindrome("abba"));
        assertTrue(detector.isPalindrome("madamimadam"));

        assertTrue(detector.isSlowPalindrome("madamimadam"));
    }

    @Test
    public void shouldDetectNotPalindromes() {
        assertFalse(detector.isPalindrome("ab"));
        assertFalse(detector.isPalindrome("abab"));
        assertFalse(detector.isPalindrome("random string"));

        assertFalse(detector.isSlowPalindrome("random string"));
    }
}

• How to reverse String in Java
• JUnit Tutorial