Property include/exclude on Kotlin data classes

Question

Suppose I only want one or two fields to be included in the generated equals and hashCode implementations (or perhaps exclude one or more fields). For a simple class, e.g.:

data class Person(val id: String, val name: String) 

Groovy has this:

@EqualsAndHashCode(includes = 'id') 

Lombok has this:

@EqualsAndHashCode(of = "id") 

What is the idiomatic way of doing this in Kotlin?

My approach so far

data class Person(val id: String) {    // at least we can guarantee it is present at access time    var name: String by Delegates.notNull()     constructor(id: String, name: String): this(id) {       this.name = name    } } 

Just feels wrong though... I don't really want name to be mutable, and the extra constructor definition is ugly.

Answer 1

I've used this approach.

data class Person(val id: String, val name: String) {    override fun equals(other: Person) = EssentialData(this) == EssentialData(other)    override fun hashCode() = EssentialData(this).hashCode()    override fun toString() = EssentialData(this).toString().replaceFirst("EssentialData", "Person") }  private data class EssentialData(val id: String) {    constructor(person: Person) : this(id = person.id)  } 

Answer 2

This approach may be suitable for property exclusion:

class SkipProperty<T>(val property: T) {   override fun equals(other: Any?) = true   override fun hashCode() = 0 } 

SkipProperty.equals simply returns true, which causes the embeded property to be skipped in equals of parent object.

data class Person(     val id: String,      val name: SkipProperty<String> )