Order class by field - scala

Question

I have created a class Path which should be uniquely identifiable by occupation and last.
case class Path(occupation: BitSet, last: Int) {
var cost = 0
def setCost(cost: Int) {
this.cost = cost
}
def getCost(): Int = {
return cost
}
}
}
Also, I would like it to be sortable by count, which I made a field.
implicit val pathOrd = Ordering.by((p: Path) => p.getCost)
The problem is that when I am sorting it(as you can see in the line above), I get java.lang.NullPointerException on that line.
Why is this happening?
Can I store my data better?

Answer 1

Your code gives no exceptions to me using this code:
# class Path(occupation: BitSet, last: Int) {
var cost = 0
def setCost(cost: Int) {
this.cost = cost
}
def getCost(): Int = {
return cost
}
}
defined class Path
# List(new Path(BitSet(), 3))
res6: List[Path] = List(cmd5$Path#3ef8de39)
# implicit val pathOrd = Ordering.by((p: Path) => p.getCost)
pathOrd: Ordering[Path] = scala.math.Ordering$$anon$9#5f478e42
# res6.sorted
res9: List[Path] = List(cmd5$Path#3ef8de39)
You are missing val for occupation and last
class Path(val occupation: BitSet, val last: Int)
I would suggest you to create a case class
case class Path(occupation: BitSet, last: Int)
It will have equals, and hashCode based on it's fields, toString, apply, unapply and copy methods.
I am not sure if you really need to modify the cost, if it is calculated based on other values you could make it a method
case class Path(occupation: BitSet, last: Int) {
def cost: Int = 42
}
if it's not then it should be a field. I want to encourage you to use immutable structures, which would mean doing:
case class Path(occupation: BitSet, last: Int, cost: Int)
Adding a field with setter and getter in scala is as simple as this:
class Path(val occupation: BitSet, val last: Int) {
var cost = 0
}
You can use it like this:
val path = new Path(BitSet(), 3)
path.cost = 12
println(path.cost)

Answer 2

Here is a simpler version of the code that uses Scala's conversions for using var to indicate mutable state in a class:
case class Path
(occupation: BitSet, // Immutable (will never change)
var cost: Int = 0, // Mutable
var last: Int = 0 // Mutable
)
// Implementing the Ordering TypeClass
implicit val pathOrd = Ordering.by((p: Path) => p.cost)
val t = new Path(BitSet(1,2), 0, 0)
t.cost = 2 // changing cost
val data = Seq(new Path(BitSet(1,2), 0, 0), new Path(BitSet(1), 3, 0), new Path(BitSet(1), 2, 0))
println(data.sorted)