Comparable or Comparator in Java?

I was writing some code recently where users can place bids on items.

I wanted to store the bids in a PriorityQueue to enable easy retrieval of the max bid on an item. 

Bid.java

I wanted to implement either a Comparator or make Bid Comparable to compare two bids and order them by highest prices.

Attempt 1: Make Bid implement Comparable<Bid>:

However:

- this doesn't really make sense because it only makes sense to compare two bids if they are associated with the same Item.
- given the current class structure, there is no way to handle this in the compareTo(Bid bid) method.

Therefore we should write a Comparator specifically for our use case to be passed into the Priority Queue. :-)

So in general it depends on whether the implementation of compare two is specific to the object or the usage.