Union types / extension interfaces - oop

Question

I have several data class with fields, which are used in forms and need them to have a method return true if any of the fields has been filled.
I don't want to rewrite this for all the classes, so I'm doing it like this at the moment:
data class Order(var consumer: String, var pdfs: List<URI>): Form {
override val isEmpty(): Boolean
get() = checkEmpty(consumer, pdfs)
}
data class SomethingElse(var str: String, var set: Set<String>): Form {
override val isEmpty(): Boolean
get() = checkEmpty(str, set)
}
interface Form {
val isEmpty: Boolean
fun <T> checkEmpty(vararg fields: T): Boolean {
for (f in fields) {
when (f) {
is Collection<*> -> if (!f.isEmpty()) return false
is CharSequence -> if (!f.isBlank()) return false
}
}
return true;
}
}
This is obviously not very pretty nor type-safe.
What's a more idiomatic way of doing this, without abstracting every property into some kind of Field-type?
Clarification: What I'm looking for is a way to get exhaustive when, for example by providing all the allowed types (String, Int, List, Set) and a function for each to tell if they're empty. Like an "extension-interface" with a method isEmptyFormField.

Answer 1

It's kinda hacky but should work.
Every data class creates set of method per each constructor parameters. They're called componentN() (where N is number starting from 1 indicating constructor parameter).
You can put such methods in your interface and make data class implicitly implement them. See example below:
data class Order(var consumer: String, var pdfs: List) : Form
data class SomethingElse(var str: String, var set: Set) : Form
interface Form {
val isEmpty: Boolean
get() = checkEmpty(component1(), component2())
fun checkEmpty(vararg fields: T): Boolean {
for (f in fields) {
when (f) {
is Collection -> if (!f.isEmpty()) return false
is CharSequence -> if (!f.isBlank()) return false
}
}
return true;
}
fun component1(): Any? = null
fun component2(): Any? = null
}
You can also add fun component3(): Any? = null etc... to handle cases with more that 2 fields in data class (e.g. NullObject pattern or handling nulls directly in your checkEmpty() method.
As I said, it's kinda hacky but maybe will work for you.

Answer 2

If all you are doing is checking for isEmpty/isBlank/isZero/etc. then you probably don't need a generic checkEmpty function, etc.:
data class Order(var consumer: String, var pdfs: List<URI>) : Form {
override val isEmpty: Boolean
get() = consumer.isEmpty() && pdfs.isEmpty()
}
data class SomethingElse(var str: String, var set: Set<String>) : Form {
override val isEmpty: Boolean
get() = str.isEmpty() && set.isEmpty()
}
interface Form {
val isEmpty: Boolean
}
However, if you are actually do something a bit more complex then based on your added clarification I believe that "abstracting every property into some kind of Field-type" is exactly what you want just don't make the Field instances part of each data class but instead create a list of them when needed:
data class Order(var consumer: String, var pdfs: List<URI>) : Form {
override val fields: List<Field<*>>
get() = listOf(consumer.toField(), pdfs.toField())
}
data class SomethingElse(var str: String, var set: Set<String>) : Form {
override val fields: List<Field<*>>
get() = listOf(str.toField(), set.toField())
}
interface Form {
val isEmpty: Boolean
get() = fields.all(Field<*>::isEmpty)
val fields: List<Field<*>>
}
fun String.toField(): Field<String> = StringField(this)
fun <C : Collection<*>> C.toField(): Field<C> = CollectionField(this)
interface Field<out T> {
val value: T
val isEmpty: Boolean
}
data class StringField(override val value: String) : Field<String> {
override val isEmpty: Boolean
get() = value.isEmpty()
}
data class CollectionField<out C : Collection<*>>(override val value: C) : Field<C> {
override val isEmpty: Boolean
get() = value.isEmpty()
}
This gives you type-safety without changing your data class components, etc. and allows you to "get exhaustive when".

Answer 3

You can use null to mean "unspecified":
data class Order(var consumer: String?, var pdfs: List<URI>?) : Form {
override val isEmpty: Boolean
get() = checkEmpty(consumer, pdfs)
}
data class SomethingElse(var str: String?, var set: Set<String>?) : Form {
override val isEmpty: Boolean
get() = checkEmpty(str, set)
}
interface Form {
val isEmpty: Boolean
fun <T> checkEmpty(vararg fields: T): Boolean = fields.all { field -> field == null }
}
The idea here is the same as that of an Optional<T> in Java but without the extra object, etc.
You now have to worry about null safety but if your fields are meant to have a concept of absent/empty then this seems appropriate (UsingAndAvoidingNullExplained · google/guava Wiki).