Scala objects not changing their internal state

Question

I am seeing a problem with some Scala 2.7.7 code I'm working on, that should not happen if it the equivalent was written in Java. Loosely, the code goes creates a bunch of card players and assigns them to tables.

class Player(val playerNumber : Int)

class Table (val tableNumber : Int) {
    var players : List[Player]  = List()

    def registerPlayer(player : Player) {
        println("Registering player " + player.playerNumber + " on table " + tableNumber)
        players = player :: players
    }
}

object PlayerRegistrar  {
    def assignPlayersToTables(playSamplesToExecute : Int, playersPerTable:Int) = {
        val numTables = playSamplesToExecute / playersPerTable
        val tables = (1 to numTables).map(new Table(_))
        assert(tables.size == numTables)

        (0 until playSamplesToExecute).foreach {playSample =>
            val tableNumber : Int = playSample % numTables
            tables(tableNumber).registerPlayer(new Player(playSample))
        }
        tables
    }
}

The PlayerRegistrar assigns a number of players between tables. First, it works out how many tables it will need to break up the players between and creates a List of them.

Then in the second part of the code, it works out which table a player should be assigned to, pulls that table from the list and registers a new player on that table.

The list of players on a table is a var, and is overwritten each time registerPlayer() is called. I have checked that this works correctly through a simple TestNG test:

@Test def testRegisterPlayer_multiplePlayers() {
    val table = new Table(1)
    (1 to 10).foreach { playerNumber =>
        val player = new Player(playerNumber)
        table.registerPlayer(player)
        assert(table.players.contains(player))
        assert(table.players.length == playerNumber)
    }
}

I then test the table assignment:

  @Test def testAssignPlayerToTables_1table() = {
    val tables = PlayerRegistrar.assignPlayersToTables(10, 10)
    assertEquals(tables.length, 1)
    assertEquals(tables(0).players.length, 10)
}

The test fails with "expected:<10> but was:<0>". I've been scratching my head, but can't work out why registerPlayer() isn't mutating the table in the list. Any help would be appreciated.

Answer 1

val tables = (1 to numTables).map(new Table(_))

This line seems to be causing all the trouble - mapping over 1 to n gives you a RandomAccessSeq.Projection, and to be honest, I don't know how exactly they work, but a bit less clever initialising technique does the job.

var tables: Array[Table] = new Array(numTables)
for (i <- 0 to numTables) tables(i) = new Table(i)

Using the first initialisation method I wasn't able to change the objects (just like you), but using a simple array everything seems to be working.

Answer 2

The reason is that in the assignPlayersToTables method, you are creating a new Table object. You can confirm this by adding some debugging into the loop:

val tableNumber : Int = playSample % numTables
println(tables(tableNumber))
tables(tableNumber).registerPlayer(new Player(playSample))

Yielding something like:

Main$$anon$1$Table@5c73a7ab
Registering player 0 on table 1
Main$$anon$1$Table@21f8c6df
Registering player 1 on table 1
Main$$anon$1$Table@53c86be5
Registering player 2 on table 1

Note how the memory address of the table is different for each call.

The reason for this behaviour is that a Range is non-strict in Scala (until Scala 2.8, anyway). This means that the call to the range is not evaluated until it's needed. So you think you're getting back a list of Table objects, but actually you're getting back a range which is evaluated (instantiating a new Table object) each time you call it. Again, you can confirm this by adding some debugging:

val tables = (1 to numTables).map(new Table(_))
println(tables)

Which gives you:

RangeM(Main$$anon$1$Table@5492bbba)

To do what you want, add a toList to the end:

val tables = (1 to numTables).map(new Table(_)).toList