Question

I’m using compareTo() in a doubly-linked-list to compare a code and a read string, and can’t figure out why it isn’t working. It’s a lot of code involved, but I don’t want to make you guys read through it all, so I’ll try to post just what is relevant and hope it’s enough.

This is the line of code. I know it’s incomplete, but everything else is working properly.

if(p.getCode().compareTo(str)==0){

And they ARE equal. I printed them out the right before this comparison happens and they are the same.

I know not much has been given, and I’m kind of asking you to trust me that they really are equal, but it’s to save you time. I’m wondering whether there is some sort of special rules I should know about regarding compareTo() I am missing…

Thanks.

Answer 1

compareTo is provided by the Comparable interface. There is no language enforced requirement that Comparable.compareTo return 0 when Object.equals returns true. That is, two instances of an object may be completely identical and an implementation of Comparator.compareTo does not have to return 0.

Assuming that p.getCode returns a String and the argument str is a String, then the implementation of String does ensure that equals and compareTo are consistent.

So there are other possibilities:

1. You have leading or trailing spaces in one or both arguments. If you are going down the route of adding debug logging, I often find it helpful to use pipe (‘|’) characters around each string do that spaces will be visible.
2. There is a difference in uppercase vs. lowercase characters. String comparing is case sensitive and a special case-insensitive Comparator must be used.