Re: Abstract Class versus an Interface, when no Members in Common

Wanja Gayk <>
Tue, 8 Nov 2011 23:13:41 +0100
In article <22857359-211e-443e-9c5d->, says...

I have a method that needs to be able to return either of two very
different types of data, in one case a class consisting of a two-
dimensional array of <int>s accompanied by a single <byte> value, and
in the other case just a one-dimensional array of <int>s and that's
all. So I created an abstract class called <SearchResult> that has no
members, not even any constructors, and made two classes extend
<SearchResult>, the one class having the two-dimensional array and the
<byte> value, and the other having the one-dimensional array. Then I
have my method return a value of <SearchResult>.

I guess generics could be the remedy to your Problem.

Let me sketch something like this.

Say we have too classes "Foo" and "Bar", both have nothing in commmon.
However, we'd like to have a common interface to obtain both the very
special result of our query and some common information, say about our
readLimit and if it was exceeded.
That could look like this:

interface SearchResult{
  <T> Iterable<T> items();
  ReadLimit readLimit();

interface ReadLimit{
  int maxItems();
  boolean isExceeded();

We then have some consumers, that call some finder to obtain a
searchResult and get the totally different types from it, whilst also
using the common information:

class FooConsumer{

 void doSomething(){
  SearchResult<Foo> result
   = finder.find(Foo.class, new BlueFooPredicate(),new ReadLimit(1000));
  for(Foo foo : result.items()){
   assert Color.Blue.equals(foo.getColor());
   alert("More than "+result.readLimit().maxItems()+" entries found!");

class BarConsumer{

 //implementation perhaps injected by a DI-Framework
 private Finder finder;

 void doSomething(){
  SearchResult<Bar> result
    = finder.find(Bar.class,
                  new SteelBarPredicate(), new ReadLimit(500));

  for(Bar bar : result.items()){
   assert Material.Steel.equals(bar.getMaterial());
   alert("More than "+result.readLimit().maxItems()+" entries found!");

Kind regards,

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