My Adventures in Coding

December 30, 2011

Scala – How to combine two lists of different types so they can be sorted by a common field

Filed under: Scala — Brian @ 1:48 pm
Tags: ,

Recently I was pairing with a co-worker on a new view for our application. The purpose of this view was just to show a timeline of all activities (regardless of type) occurring in the system. We were confident there would be a way to accomplish this task easily in Scala!

For this example lets say we are writing a program for a car dealership. The manager of the dealership would like a page showing test drives and sales throughout the day. The point of this page is not to connect test drives to sales, but instead a view used to give a timeline of the activity taking place on the car lot.

In the example below, even though we have two lists of different types testDrives=List[TestDrives] and purchases=List[Purchases], they both have a common field which is “date“. To do the sort we first combine the two lists with (testDrives ::: purchases), creating a new list of type List[{def date: Date}]. The objects in the new list all have “date“. So now sorting the list is easy!

import scala.collection.JavaConversions._
import java.util.Date

case class TestDrive(car: String, priceQuoted: Int, date: Date)
case class Purchase(car: String, priceSold: Int, date: Date)

val date = new Date
val dateFormat = new java.text.SimpleDateFormat("yyyy/MM/dd HH:mm:ss")

val testDrives = List(TestDrive("Charger", 22000, new Date(date.getTime - 1000000)), TestDrive("Camero", 250000, new Date(date.getTime - 4000000)), TestDrive("Pony", 2500, new Date(date.getTime - 10000000)))

val purchases = List(Purchase("Infiniti G35x", 19500, new Date(date.getTime - 500000)), Purchase("Infiniti G37x", 28500, new Date(date.getTime - 6000000)), Purchase("Altima", 35000, new Date(date.getTime - 9000000)))

// Combine the TestDrives and Purchases lists and sort descending by date
for (something <- (testDrives ::: purchases).asInstanceOf[List[{def date: Date}]].sortBy( {
	// Now if we want to show fields other than "date", we can easily check the type of each object, and cast it appropriately
	if (something.isInstanceOf[TestDrive]) {
		val testDrive = something.asInstanceOf[TestDrive]
		println("Test Drive: %s\tQuoted price: %d \t Date: %s".format(, testDrive.priceQuoted, dateFormat.format(
	else {
		val purchase = something.asInstanceOf[Purchase]
		println("Purchase: %s\tSold price: %d \t Date: %s".format(, purchase.priceSold, dateFormat.format(

The program creates the following output, all test drives and purchases combined into a single timeline sorted descending by date.

>scala example.scala
Purchase: Infiniti G35x Sold price: 19500        Date: 2011/12/23 14:48:41
Test Drive: Charger     Quoted price: 22000      Date: 2011/12/23 14:40:21
Test Drive: Camero      Quoted price: 250000     Date: 2011/12/23 13:50:21
Purchase: Infiniti G37x Sold price: 28500        Date: 2011/12/23 13:17:01
Purchase: Altima        Sold price: 35000        Date: 2011/12/23 12:27:01
Test Drive: Pony        Quoted price: 2500       Date: 2011/12/23 12:10:21

That is all!


1 Comment »

  1. Reblogged this on solutions to the algorithm design manual.

    Comment by rajeevprasanna — October 23, 2014 @ 8:24 pm | Reply

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