Java - interface design

Question

I have a simple (and possibly quite common) design issue. Let's say I have an interface that looks like this:

public interface AnalysisResult {

    BigDecimal getMeasure();

}

This, However, is later implemented by several different concrete implementations - some including average field as well, some containing counts of various properties etc. Now, I am given a set of AnalysisResult and I would like to be able to output the results according to the type, i.e. Mean result would say what the mean is on top of the Measure etc. One way to do this would be to expose a method on the interface say outputResult(), but the problem is I might want to output the result in the HTML format, CSV and many others, so I would require a method for each. Also, there might be the case I want to output something based on the type, such as : 'This is the Mean result'... Would you say that in this case it would be easier to simply use instanceof and do the work on the outputter side?

score 1 · Accepted Answer · edited May 23 '17 at 10:24

1

I'd go for a further extension.. something like

public interface Measure {
   public String getOutputAsTxt();
   public String getOutputAsCsv();
   public String getOutputAsFunnyFormat();
};

public interface AnalysisResult {

  public Measure getMeasure();

}

Thus moving the output specialization to the particular Measure. Yes this add a bit of code but keeps everything neat and separated.

Or I'd try to explore a bit the Visitor pattern (this is a sort of simple implementation).

For further information, please refer to the chosen answer in this SO question (every time I read it I feel the urge to +1 for its awesomess)

edited May 23 '17 at 10:24

Community

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answered Dec 18 '12 at 08:50

BigMike

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Visitor pattern was also I had in mind. I do not want the Measure to mix the output logic – Bober02 Dec 18 '12 at 08:56
@Bober02 funny in linked the question the best answer is not the chosen one, but is the one with the most upvotes, it's c++ but can easily be converted in java. I'd go for that way it you don't want to specialize returned objects. As usual it depends on how many format you'll have and how often you'll add/change them. – BigMike Dec 18 '12 at 08:58
I had another question in mind: I would like the Outputter class to take the list of analyssi results, and possibly output some meta information based on type, such as column names. Well, the main requirement is that the collection must only contain one specific type of objects, which I think cannot be achieved if you simply a visitor pattern, as this accepts objects only based on the parent interface – Bober02 Dec 18 '12 at 09:43

score 0 · Answer 2 · answered Dec 18 '12 at 08:53

results according to the type :

This is a separate behavior so would suggest you to implement something like Strategy Pattern. Have a base interface having method print(). Then you can implement this method in multiple Implementing classes like CSVStylePrint, HTMLStylePrint etc.

This way your printing functionality is separate and you follow OCP (Open Closed principle : i.e. Open for extension but closed for changes) principle. OCP is the underlying design principle used in Strategy Pattern.

score 0 · Answer 3 · answered Dec 18 '12 at 08:55

0

I suggest to use a method public String getOutput(String format) in the interface.

Let the implementations decide what to return based on the 'format' argument.

This will also help to call method specific to a particular implementation.

answered Dec 18 '12 at 08:55

Renjith

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score 0 · Answer 4 · answered Dec 18 '12 at 08:59

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Save result data in a common xml format and use XSLT to transform raw data to your desired format. JUnit does this with test results. No code change needed when you later on decide to add a different format.

answered Dec 18 '12 at 08:59

Frank

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Java - interface design

4 Answers4