A good candidate for that would be the data object itself. Just to take another example than simple validation: Let's say we have the common case that we need to store a password, which should be salted and hashed for security reasons. Why not put the method to do that into the data object? This way it can be ensured that the salting and hashing is done every time the password field of the data object is set. And this method can also be called to hash the string entered during login and then compare it to the value stored in the data object. This may be contrary to the dogma that data objects are supposed to be simple containers for the data only, but in object oriented design it makes sense when an object has methods or properties to validate its state or make changes to its state.
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