Helper classes (original) (raw)

Brian Goetz brian.goetz at oracle.com
Mon Apr 15 15:12:51 PDT 2013

We probably won't know what the first rule is for quite a while, but I think the line of reasoning you're following here is a good one to explore, and may well lead somewhere good.

Regretfully, I can immediately think of a countervailing design force; that interface instance methods are inherited and interface static methods are not. (This was a deliberate choice; the EG viewed the inheritability of static methods in classes (or the ability to invoke them through instances) to be a bug, and one which would have even more harm under multiple inheritance than under single inheritance.) As such, adding new instance methods after-the-fact has a higher risk of clashing with existing methods in existing implementations, or simply being a mismatch by the time you get far enough down the inheritance tree, because every subtype has to take it. Adding a static to an interface can't interfere with anything down-tree.

But I think you're headed in the right direction.

On 4/15/2013 5:54 PM, Stephen Colebourne wrote:

On 15 April 2013 22:41, Brian Goetz <brian.goetz at oracle.com> wrote:

An example of a method at the other end of the spectrum is Collections.rotate(List, int). Sure, this is useful when you need it, but it just isn't that important, and weighing down an interface with 5000 lines of additional code (OK, Collections is only 4700 LoC) really clogs up the Collection interface. Perhaps there is a semblance of a first rule here. - Collections.rotate(List, int) - static could now be: - List.rotate(List, int) - static but that would just look and behave weirdly, because it should be - List.rotate(int) - instance But, most of us would agree that an instance method for rotating probably isn't justified. Thus it is also not justified as a static method on the same interface. The rule is therefore along the lines of "don't add a static method where one of its arguments is the same type as the interface and the methods purpose is to operate on that argument. Either use a default instance method on the interface, or keep it in a separate utility class". Stephen

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