Non-value field is accessed in 'hashCode()' - scala

Question

It happen with id. (Non-value field is accessed in 'hashCode()')
How can I fix it?
Here is my code:
import java.util.Date
case class Category() {
var id: Integer = _
var parentId: Integer = _
var name: String = _
var status: Boolean = _
var sortOrder: Integer = _
var createTime: Date = _
var updateTime: Date = _
def this(id: Integer, parentId: Integer, name: String, status: Boolean, sortOrder: Integer, createTime: Date, updateTime: Date) {
this()
this.id = id
this.parentId = parentId
this.name = name
this.status = status
this.sortOrder = sortOrder
this.createTime = createTime
this.updateTime = updateTime
}
override def hashCode(): Int = {
if (id != null) id.hashCode() else 0
}
}
If I change var id to val id, it will be ok. But I need setter, I cant't set it as a val.

Answer 1

Non value field in hashCode means you're relaying on a mutable field to generate the classes hashcode, which may be dangerous, depending how you use that class. For example, assume you have a HashMap[Category, String], you place an entry in the hash map and them mutate the id field, now that hashmap can no longer find the class instance by it's hashcode.
You're treating a case class like it's a regular, mutable Java class. Case classes provides the mechanism of encapsulating immutable data in a syntax convenient way (where you also get a few nice things for free, such as hashCode, equals, etc). What you can do is make your case class accept parameters via it's primary constructor:
case class Category(id: Int,
parentId: Int,
name: String,
status: Boolean,
sortOrder: Int,
createTime: Date,
updateTime: Date) {
override def hashCode(): Int = id.hashCode()
}
If you want to update Category, you can use the utility copy method defined on a case class:
val cat = Category(id = 1,
parentId = 1,
name = "Hello",
status = true,
sortOrder = 1,
createTime = new Date(123132123L),
updateTime = new Date(52234234L))
val newCat: Category = cat.copy(id = 2)
Another way of approaching this (which I like less TBH) would be to make all your fields an Option[T] with a default value set to None:
case class Category(id: Option[Int] = None,
parentId: Option[Int] = None,
name: Option[String] = None,
status: Option[Boolean] = None,
sortOrder: Option[Int] = None,
createTime: Option[Date] = None,
updateTime: Option[Date] = None) {
override def hashCode(): Int = id.map(_.hashCode()).getOrElse(0)
}
And then updating them once you can populate the class with data:
val cat = Category()
val newCat = cat.copy(id = Some(1))

Answer 2

After I try many times, I think this should be a good choice.
Any ideas?
override def hashCode(): Int = {
if (getId != null) getId.hashCode() else 0
}