Subtype dependency on corresponding subtypes

 Robert Mark Bram <>
Sun, 12 Aug 2007 08:25:06 -0000
Hi All,

I have a design question.

I am writing an application that will search through the contents of
various files against a given search criteria. I envisage that there
will be different search strategies, each of which have different
search criteria. A couple of examples would be as per below.

Plain Text Strategy. Criteria: a single string (search term), case
sensitive flag.
Regular Expression Strategy: a single regular expression, number of
lines before and after each result to include, case sensitive flag.
XML Strategy: a single string (search term), name of the xml element
to be searched within.

I envisage having an abstract search type and concrete subclasses
based on each strategy. For example:

public interface Search {
   SearchResult search(SearchCriteria criteria);

public class PlainTextSearchStrategy implements Search {
   public SearchResult search(SearchCriteria criteria) {
      // do the search..

public class RegularExpressionSearchStrategy implements Search
{ ... }

public class XmlSearchStrategy implements Search { ... }

My difficulty is the SearchCriteria. Each search strategy type would
have its own search criteria type and I do not think there is any
common behaviour (or data) that can be defined in the criteria super
type. As such, SearchCriteria is just a marker interface, and this
bothers me. The search strategy sub-types would need to cast
SearchCriteria to the specific type they need, so there is no type
safety beyond ensuring the marker interface is implemented. For

public class PlainTextSearchStrategy implements Search {
   public SearchResult search(SearchCriteria criteria) {
      PlainTextSearchCriteria ptCriteria = (PlainTextSearchCriteria)
      // do the search..

Is there a design better suited to this problem of ensuring a sub-type
can rely on corresponding subtypes being present? As I understand it,
generics won't help me here either. Do I need to live with the

Any advice would be much appreciated!


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