Kotlin - Overwrite Obj Props With Modified Obj Props if Not Null

Question

TL;DR:

How do I make this less redundant (any approach that works helps)?

if (personModification.firstName != null) {person.firstName = personModification.firstName}
if (personModification.lastName != null) {person.lastName = personModification.lastName}
if (personModification.job != null) {person.job = personModification.job}

---

The long version: I have a simple problem. I have a class Person:

class Person (val firstName: String?, 
              val lastName: String?, 
              val job: String?)

and I have a class called PersonModification:

class PersonModification(val firstName: String?, 
                         val lastName: String?, 
                         val job: String?)

The task is to overwrite any Person property values with PersonModification values, IF the PersonModification property isn't null. If you care, the business logic behind this is an API endpoint which modifies Person and takes a PersonModification as an argument (but can change all, or any, of the properties, so we don't want to overwrite valid old values with nulls). The solution to this looks like this.

if (personModification.firstName != null) {person.firstName = personModification.firstName}
if (personModification.lastName != null) {person.lastName = personModification.lastName}
if (personModification.job != null) {person.job = personModification.job}

I was told this is redundant (and I agree). The solution pseudocode looks like this:

foreach(propName in personProps){
  if (personModification["propName"] != null) {person["propName"] = personModification["propName"]}
}

Of course, this isn't JavaScript, so it's not that easy. My reflection solution is below, but imo, it's better to have redundancy than do reflection here. What are my other options to remove the redundancy?

---

Refelection:

package kotlin.reflect;

class Person (val firstName: String?, 
              val lastName: String?, 
              val job: String?)

class PersonModification(val firstName: String?, 
                         val lastName: String?, 
                         val job: String?)

// Reflection - a bad solution. Impossible without it.
//https://stackoverflow.com/questions/35525122/kotlin-data-class-how-to-read-the-value-of-property-if-i-dont-know-its-name-at
inline fun <reified T : Any> Any.getThroughReflection(propertyName: String): T? {
    val getterName = "get" + propertyName.capitalize()
    return try {
        javaClass.getMethod(getterName).invoke(this) as? T
    } catch (e: NoSuchMethodException) {
        null
    }
}

fun main(args: Array<String>) {

var person: Person = Person("Bob","Dylan","Artist")
val personModification: PersonModification = PersonModification("Jane","Smith","Placeholder")
val personClassPropertyNames = listOf("firstName", "lastName", "job")

for(properyName in personClassPropertyNames) {
    println(properyName)
    val currentValue = person.getThroughReflection<String>(properyName)
    val modifiedValue = personModification.getThroughReflection<String>(properyName)
    println(currentValue)
    if(modifiedValue != null){
        //Some packages or imports are missing for "output" and "it"
        val property = outputs::class.memberProperties.find { it.name == "firstName" }
        if (property is KMutableProperty<*>) {
            property.setter.call(person, "123")
        }
    }
})
}

You can copy and paste here to run it: https://try.kotlinlang.org/

Answer 1

It should be pretty simple to write a 5 line helper to do this which even supports copying every matching property or just a selection of properties.

Although it's probably not useful if you're writing Kotlin code and heavily utilising data classes and val (immutable properties). Check it out:

fun <T : Any, R : Any> T.copyPropsFrom(fromObject: R, skipNulls: Boolean = true, vararg props: KProperty<*>) {
  // only consider mutable properties
  val mutableProps = this::class.memberProperties.filterIsInstance<KMutableProperty<*>>()
  // if source list is provided use that otherwise use all available properties
  val sourceProps = if (props.isEmpty()) fromObject::class.memberProperties else props.toList()
  // copy all matching
  mutableProps.forEach { targetProp ->
    sourceProps.find {
      // make sure properties have same name and compatible types 
      it.name == targetProp.name && targetProp.returnType.isSupertypeOf(it.returnType) 
    }?.let { matchingProp ->
      val copyValue = matchingProp.getter.call(fromObject);
      if (!skipNulls || (skipNulls && copyValue != null)) {
        targetProp.setter.call(this, copyValue)
      }
    }
  }
}

This approach uses reflection, but it uses Kotlin reflection which is very lightweight. I haven't timed anything, but it should run almost at same speed as copying properties by hand.

Also it uses KProperty instead of strings to define a subset of properties (if you don't want all of them copied) so it has complete refactoring support, so if you rename a property on the class you won't have to hunt for string references to rename.

It will skip nulls by default or you can toggle the skipNulls parameters to false (default is true).

Now given 2 classes:

data class DataOne(val propA: String, val propB: String)
data class DataTwo(var propA: String = "", var propB: String = "")

You can do the following:

  var data2 = DataTwo()
  var data1 = DataOne("a", "b")
  println("Before")
  println(data1)
  println(data2)
  // this copies all matching properties
  data2.copyPropsFrom(data1)
  println("After")
  println(data1)
  println(data2)
  data2 = DataTwo()
  data1 = DataOne("a", "b")
  println("Before")
  println(data1)
  println(data2)
  // this copies only matching properties from the provided list 
  // with complete refactoring and completion support
  data2.copyPropsFrom(data1, DataOne::propA)
  println("After")
  println(data1)
  println(data2)

Output will be:

Before
DataOne(propA=a, propB=b)
DataTwo(propA=, propB=)
After
DataOne(propA=a, propB=b)
DataTwo(propA=a, propB=b)
Before
DataOne(propA=a, propB=b)
DataTwo(propA=, propB=)
After
DataOne(propA=a, propB=b)
DataTwo(propA=a, propB=)