java配置/参数传递设计

Question

Often I find the need to engineer objects with configurable functionality.

To exemplify, assume I'm creating a DateIterator . The configurable option(s) might be whether to iterate the closed interval [start, end] or the open-end interval [start, end) .

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• (1) The, in my opinion, ungraceful solution - limited to only one true/false configuration option

new DateIterator(boolean openInterval);

• (2) The typesafe enum way - typically a bit bulky

new DateIterator(Interval.OPEN_END);

• (3) The unconventional attempt - nice but not too straight forward

new DateIterator().openEnd();

• (4) The inheritance approach - often over-engineering

new OpenEndedDateIterator();

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To this comes a few alternatives which I consider inferior, like integer-based configuration new DateIterator(Interval.OPEN_END); or property based configuration.

Are there any other approaches? Which approach you do you prefer?

Answer 1

I'd say the Builder pattern makes sense here:

DateIterator di = 
  DateIterator.builder()
              .withStartDate(new Date())
              .withOpenEnd()
              .build();

That way your actual DateIterator can be immutable, while the builder returned by DateIterator.builder() does the configuration work.

Answer 2

While there is no good answer, and it's largely a matter of taste, I do follow the following rule of thumb, with large space for exceptions to avoid over engineering :

1. If the only configuration you have is one "parameter", taking a fixed set, and alters behavior (like in your example), go with subclasses. While it may be over-engineered, if many methods in your class start with an "if (this.parameter == x) ... else if (this.parameter == y) .. " it will make code unreadable.
2. If your parameter is not a fixed set, but a string or number, and you NEED it for the class to work properly put it in the constructor, if it's not mandatory i like solution number (3), the unconventional attempt :)
3. If you have more than one parameter in a fixed set (like START_OPEN_ENDED and STOP_OPEN_ENDED), creating subclasses may mean to create a subclass for each permutation. In that case consider encapsulation. For example (I would not do it in this specific case probably, but it's a good example), create a single class DateComparator, with a subclass for open ended, and encapsulate a DateComparator for the start and one for the end, having only one DateIterator.

Again, these are the rule of thumb I use, in no way mandatory, and often I find myself not respecting them verbatim.