Tommy Yamakaitis Tommy Yamakaitis - 3 months ago 22
Java Question

isPalindrome - Collection and List Reversal

This is a homework lab for school. I am trying to reverse a LinkedList, and check if it is a palindrome (the same backwards and forwards). I saw similar questions online, but not many that help me with this. I have made programs that check for palindromes before, but none that check an array or list. So, first, here is my isPalindrome method:

public static <E> boolean isPalindrome(Collection<E> c) {
Collection<E> tmp = c;
Collections.reverse((List<E>) c);
if(tmp == c) { return true; } else { return false; }

My professor wants us to set the method up to accept all collections which is why I used Collection and cast it as a list for the reverse method, but I'm not sure if that is done correctly. I know that it does reverse the list. Here is my main method:

public static void main(String...strings) {
Integer[] arr2 = {1,3,1,1,2};
LinkedList<Integer> ll2 = new LinkedList<Integer>(Arrays.asList(arr2));
if(isPalindrome(ll2)) { System.out.println("Successful!"); }

The problem is, I am testing this with an array that is not a palindrome, meaning it is not the same backwards as it is forwards. I already tested it using the array {1,3,1} and it works fine because that is a palindrome. Using {1,3,1,1,2} still returns true for palindrome, though it is clearly not. Here is my output using the {1,3,1,1,2} array:

[1, 3, 1, 1, 2]
[2, 1, 1, 3, 1]

So, it seems to be properly reversing the List, but when it compares them, it assumes they are equal? I believe there is an issue with the tmp == c and how it checks whether they are equal. I assume it just checks if it contains the same elements, but I'm not sure. I also tried tmp.equals(c), but it returned the same results. I'm just curious is there is another method that I can use or do I have to write a method to compare tmp and c?

Thank you in advance!


In your code c and tmp are links to same collection and tmp == c will be always true. Your must clone your collection to new instance, for example: List<E> tmp = new ArrayList(c);.