I have a @ManyToMany entity where I want to apply a numerical order to a String column. The most basic idea would be:



But this doesn't work because codes 1,2,10 are ordered like 1,10,2.

I then thought I'd use the @Sort annotation. First attempt was:


But this doesn't seem to trigger the comparator and the items in the list are in insertion order 1,10,2.

I read in the documentation one needed to use a SortedSet, so I changed the code to:



Unfortunately, changing to this will give me new error elsewhere. In the code:

I'll get the Exception:


This is only solved if CentreImpl implements Comparable, but that makes the use of SortType.COMPARATOR useless.

Does anyone know how can we use the Comparator without having to implement Comparable on the underlying class?

You should specify the comparator to use as a parameter in when creating the TreeSet:



This is according to the javadoc for the TreeSet class which mentions that if you use the no-argument constructor, the elements must implement Comparable. If you supply your own comparator implementation it should work for any objects that the comparator supports.

You are right, nordborg.

This leads me to another question, however. Implementing the SortedSet with the Comparator means all operations (like add, remove) rely on the comparator instead of equals. This leads to some problems: I only want the Comparator to sort the collection (by code), not to assess equality.

For example, if CentreImpl's equals is based on name and code, the following test will break (saying size is 1):



So, how can one sort using a comparator but keep the default equals as the base for other operations?

This can be done if it is possible to implement the comparator in a way that always distinguishes non-equal elements. In the example you give, if the codes are equal the comparator should also compare the names.