Re: "Program to an interface" - When to break a design pattern

Tom Anderson <>
Thu, 5 May 2011 23:29:40 +0100
On Thu, 5 May 2011, Steven Simpson wrote:

On 05/05/11 20:21, Zapanaz wrote:

So in this case, it seems to me, that using LinkedHashMap in the method
signature makes sense. The fact that the return retains the insertion
order is an integral part of what the method does.

Hypothetically, what you might want is an intermediate interface between
Map and LinkedHashMap, e.g. InsertionOrderedMap, which extends Map but
adds no methods. This might provide some sort of compile-time
protection, that mere documentation cannot provide.

However, IMHO, its value is dubious, for a few reasons:

   * An implementation of InsertionOrderedMap can be compiled without
     meeting the insertion-order requirement.
   * Another implementation of Map can be compiled without extending
     InsertionOrderedMap while meeting the insertion-order requirement.

These points are true of any interface! Look:

public class DubiousComparator implements Comparator<String> {
  public int compare(String a, String b) {
  return 1;

public class NotAComparator /* does not implement Comparator<String> */ {
  public int compare(String a, String b) {
  return a.compareTo(b);

Interfaces are useful because they let you make a promise; alas, making
programmers keep their promises is beyond any compiler.

   * Specifically for LinkedHashMap, it can actually provide access
     order instead, so it couldn't implement both InsertionOrderedMap
     and a similar AccessOrderedMap without breaking at least one
     requirement, whichever way a particular instance was configured.

I think it would be sufficient to have an interface OrderedMap, which is,
very roughly, to a Map what a List is to a Set. Except really, we would
also want an OrderedSet. SortedMap/Set could perhaps be a subtype of
OrderedMap/Set; or, borrowing from Smalltalk, we could have a
SequenceableMap/Set, from which we derive different kinds of sequencing. I
think these things would be useful, but not enormously so. I feel
Zapanaz's pain, but i think it's just One Of Those Things.


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