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Brian Goetz <brian.goetz@oracle.com> 2024-04-01 16:34Z:</div>
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> [...]</div>
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> - Concision. If a fully-general constructor takes many parameters, but not all are essential to the use case, then the construction site becomes a site of accidental complexity. Being able to handle common grouping of parameters simplifies use sites.</div>
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> - Flexibility. Related to the above, not only might the user not need to specify a given constructor parameter, but they want the flexibility of saying "let the implementation pick the best value". Constructors with fewer parameters reserve more flexibility
for the implementation.</div>
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> - Alternative representations. Some objects may take multiple representations as input, such as accepting a Date, a LocalDate, or a LocalDateTime.</div>
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> The first two cases are generally handled with "telescoping constructor nests", where we have:</div>
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> Foo(A a)</div>
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> Foo(A a, B b)</div>
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> Foo(A a, B b, C d, D d)</div>
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> [...]</div>
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> The "Flexibility" requirement does not really apply to deconstructors; having a deconstructor that accepts additional bindings does not constrain anything, not in the same way as a constructor taking needlessly specific arguments. Imagine if ArrayList had
only constructors that take int (for array capacity); this is terrible for the constructor, because it forces a resource management decision onto users who will not likely make a very good decision, and one that is hard to change later, but pretty much harmless
for deconstructors.</div>
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> The "Concision" requirement does not really apply as much to deconstructors as constructors; matching with `Foo(var a, _, _)` is not nearly as painful as invoking with lots of parameters, each of which require an explicit choice by the user.</div>
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> So the main reason for overloading deconstructors is to match representations with the constructor overloads -- but with a given "representation set", there probably does not need to be as many deconstructors as constructors. What we really need is to match
the "maximal" constructor in a telescoping nest with a corresponding deconstructor, or for a tree-shaped set, one for each "maximal" representation.</div>
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> So for a class with constructors</div>
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> Foo()</div>
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> Foo(A a)</div>
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> Foo(A a, B B)</div>
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> Foo(X x)</div>
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> Foo(X x, Y y)</div>
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> we would want dtors for (A,B) and (X,Y), but don't really need the others.</div>
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Well that does depend on whether you are going to allow deconstructors to be not total. If they don't have to be total, the deconstructor `Foo(X x)` could be short for `Foo(X x, null)` if constructor `Foo(X x)` calls `Foo(x, null)`.</div>
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However, it would become more complex when `Foo(X x)` calls `Foo(x, Foo.DEFAULT_Y)`. Now the deconstructor `Foo(X x)` doesn't really have a longer form equivalent.</div>
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Kind regards</div>
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Robbe Pincket</div>
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