Java Basics Practical - Palindromes

exercises
lab
java
strings
recursion

Practical exercises for week 2. The practical also introduces IntelliJ IDEA (the course’s supported IDE — see the install guide on Blackboard) and the Java 21 API docs (in particular the String class in java.lang, needed below) — neither has gradable content, so isn’t repeated here.

On the HTML site, fill in each blank with your answer (as a quoted string, e.g. "true"), then click Run Code to check it. In the PDF, the Working callout is shown as a static answer key instead (interactive checking needs a browser).

Question 1 — Palindromes

A palindrome reads the same forwards and backwards, e.g. "AaaA", "madamimadam", "racecar".

Implement four methods, all with the signature public static boolean isPalindromeN(String word) (for \(N=1..4\)), each using a different technique:

  1. isPalindrome1 — a for loop.
  2. isPalindrome2 — a while loop.
  3. isPalindrome3 — recursive, with no helper methods.
  4. isPalindrome4 — recursive, using a private helper method.

All four should agree on at least these cases. What does each return?

isPalindromeN("AaA") →

isPalindromeN("A") →

isPalindromeN("") →

isPalindromeN("Abbb") →

Task 0 — isPalindrome1 (for loop):

public static boolean isPalindrome1(String word) {
    int len = word.length();
    for (int i = 0; i < len / 2; i++) {
        if (word.charAt(i) != word.charAt(len - i - 1)) {
            return false;
        }
    }
    return true;
}

Task 1 — isPalindrome2 (while loop):

public static boolean isPalindrome2(String word) {
    int len = word.length();
    int i = 0;
    while (i < len / 2) {
        if (word.charAt(i) != word.charAt(len - i - 1)) {
            return false;
        }
        i++;
    }
    return true;
}

Task 2 — isPalindrome3 (recursive, no helper methods):

public static boolean isPalindrome3(String word) {
    if (word.length() < 2) {
        return true; // base case
    }
    if (word.charAt(0) != word.charAt(word.length() - 1)) {
        return false; // base case
    }
    return isPalindrome3(word.substring(1, word.length() - 1)); // recursive step
}

Task 3 — isPalindrome4 (recursive, with a private helper):

public static boolean isPalindrome4(String word) {
    return helper(word, 0);
}

private static boolean helper(String word, int i) {
    if (i >= word.length() / 2) {
        return true; // base case
    }
    if (word.charAt(i) != word.charAt(word.length() - i - 1)) {
        return false; // base case
    }
    return helper(word, i + 1); // recursive step
}

All four agree on the required test cases: "AaA" → true, "A" → true, "" → true, "Abbb" → false.

Code review

The practical’s second half is a paired activity, not a gradable exercise: swap solutions with a partner, and for each other’s code —

  • Understand it, and ask questions about anything unclear.
  • Check correctness against the test cases above (and a few of your own).
  • Check it follows the course style guide.
  • Discuss/suggest improvements if you find issues.

Then discuss which of the four isPalindromeN techniques you each preferred, and why.