Interoperating between Scala.List and Java.util.List

Question

I am a Scala newbie. I am using Google guava library Collections2.permutations() method which takes as input a java.util.Collection collection

I have the following code, adapted from a corresponding Java program I had written.

import java.util

import com.google.common.collect.Collections2
import collection.JavaConversions._

class OptimalTSP {

  def distance(point1: Point, point2: Point): Double = {
      Math.sqrt(Math.pow(point2.y - point1.y, 2) + Math.pow(point2.x - point1.x, 2))
    }

    def findCheapestPermutation(points: java.util.List[Point]): java.util.List[Point] = {
      var cost: Double = Double.MaxValue
      var minCostPermutation: java.util.List[Point] = null
      val permutations: util.Collection[java.util.List[Point]] = Collections2.permutations(points)
      import scala.collection.JavaConversions._
      for (permutation <- permutations) {
        val permutationCost: Double = findCost(permutation)
        if (permutationCost <= cost) {
          cost = permutationCost
          minCostPermutation = permutation
        }
      }
      println("Cheapest distance: " + cost)
      minCostPermutation
    }
 }

The above works fine, but the full package name java.util.List is explicitly needed. Is there a more idiomatic scala way of doing this, i.e. passing scala List into a method that expects Java Collection?

Are you sure that [this](http://stackoverflow.com/questions/8124440/code-to-enumerate-permutations-in-scala) isn't a better solution? There's already a method for that... — Boris the Spider, Apr 03 '15 at 21:26
@BoristheSpider Thanks for pointing it out, I'm adapting my code to using the List method directly — vsnyc, Apr 03 '15 at 21:36

score 1 · Accepted Answer · answered Apr 04 '15 at 03:55

1

More idiomatically, you can try using permutations and minBy:

 points.permutations.minBy(permutation => findCost(permutation))
 points.permutations.minBy(findCost) //equivalent

answered Apr 04 '15 at 03:55

Ben Reich

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Thanks, that looks much nicer. I'll try it out and drop a comment – vsnyc Apr 04 '15 at 12:55

score 0 · Answer 2 · answered Apr 03 '15 at 21:44

As Boris pointed in the comment permutation method on scala List can be directly used as shown below.

class OptimalTSP {

  def distance(point1: Point, point2: Point): Double = {
      Math.sqrt(Math.pow(point2.y - point1.y, 2) + Math.pow(point2.x - point1.x, 2))
    }

  def findCheapestPermutation(points: List[Point]): List[Point] = {
    var cost: Double = Double.MaxValue
    var minCostPermutation: List[Point] = null
    for (permutation <- points.permutations) {
      val permutationCost: Double = findCost(permutation)
      if (permutationCost <= cost) {
        cost = permutationCost
        minCostPermutation = permutation
      }
    }
    println("Cheapest distance: " + cost)
    minCostPermutation
  }
}

Interoperating between Scala.List and Java.util.List

2 Answers2