Custom FilterFieldTypes

[pocketone]

Currently a FilterField is only allowed to return a BigDecimal, Date or List of Strings.

Actually the only requirement for a return type should be to implement Comparable.

An open question is: What if we want to support other FilterOperands than greater than, lower than or equals. E.g. it could be desirable to implement "startsWith". Should compareTo return 0 (equals) in the case of a match?