Wrapping methods without Decorator Pattern or AOP

Question

I have a Dao interface which doesn't have any methods defined.

Then I have multiple classes which implement this interface, like PetDao and HouseDao.

For some of the implementation classes, and some of their methods, I need to do an operation.

Let's say this operation is defined in the Operation class, and the method is doOperation().

PetDao has several methods: Pet selectPet(int petId), void insertPet(PetDto pet), and I only want to call doOperation() for insertPet.

HouseDao has several methods: House selectHouse(int houseId), void deleteHouse(int houseId), and I only want to call doOperation() for deleteHouse.

I cannot modify the Dao interface. Neither of these classes are instantiated manually, they are all injected.

I could change PetDao and HouseDao code to statically call doOperation(). But this would mean that the Dao classes would have another responsibility than the originally intended, and that I would have a big impact on the source code (there are multiple Daos with multiple methods that I need to track).

I cannot use the Decorator Pattern because Dao interface does not have any methods.

Are there other options, besides AOP, to achieve this?

Why don't you just create a new interface with your doOperation method in it, and have each DTO implement that interface? — Robert Harvey
– Robert Harvey, Commented Jul 9, 2021 at 18:57
But this would mean that the Dao classes would have another responsibility than the originally intended -- I'm not particularly concerned about this arbitrary distinction, if your proposed solution actually solves the problem you are having in a reasonably good way. — Robert Harvey
– Robert Harvey, Commented Jul 9, 2021 at 18:59
Your question is missing some crucial information. E.g.: "Dao interface does not have any methods" - why? "I have multiple classes which implement this interface" - why do they implement it? "I cannot modify the Dao interface." - why? — Filip Milovanović
– Filip Milovanović, Commented Jul 10, 2021 at 9:58

Ian Jacobs · Accepted Answer · 2021-07-09 22:10:03Z

Injecting a class isn't really a violation of the Single Responsibility Principle.

The new logic is encapsulated in your Operation class, that's it's responsibility. Calling a dependency doesn't really change what this class responsibile for.

Something like the below isn't really changing the responsibilities of your DAO class.

(forgive the C# syntax)

public class HouseDao:IDao { public HouseDao(Operation myOperation) { <snip> operation = myOperation; } <snip> public void deleteHouse(int houseId) { //dostuff operation.doOperation() } }

All that said, if you really need an alternative, implementing this as a Visitor (https://www.baeldung.com/java-visitor-pattern) may be more along the lines of what you want.

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