Scala中带有构造函数参数的抽象类

Question

I have the following class model:

sealed abstract class Tile(val coordinate: Int, val isOccupied: Boolean) {
  def isEmpty() : Boolean
  def getPiece() : Option[Piece]
}

case class EmptyTile(coordinate: Int) extends Tile(coordinate, false) {
  override def toString: String = "" +coordinate
  override def isEmpty() = true
  override def getPiece() = None
}

case class OccupiedTile(coordinate: Int, val piece: Piece) extends Tile(coordinate, true) {
  override def toString = piece.toString
  override def isEmpty = false
  override def getPiece = Some(piece)
}

and I get the following error:

Error:(6, 22) overriding value coordinate in class Tile of type Int;
 value coordinate needs `override' modifier
case class EmptyTile(coordinate: Int) extends Tile(coordinate, false) {
                 ^

What am I doing wrong?

EDIT: Request to see Piece class, adding here:

import Alliance.Alliance
import PieceType.PieceType

abstract class Piece(val piecePosition: Int, val pieceType : PieceType, val alliance: Alliance) extends Movable {
}

object PieceType extends Enumeration {
  type PieceType = Value
  val PAWN, KNIGHT, BISHOP, ROOK, QUEEN, KING = Value
}

Answer 1

Your abstract class Tile declares a val coordinate , making this value publicly accessible. Your case class EmptyTile implicitly declares coordinate as a val as well (case class "magic"). Basically, your case class is effectively trying to override a value already provided by your abstract class.

You can either remove the val in your abstract class declaration, or not make EmptyTile and OccupiedTile case classes.

Edit: proposed alternative after comment:

trait Tile {
  def coordinate: Int
  def isOccupied: Boolean
  def isEmpty() : Boolean = !isOccupied
  def getPiece() : Option[Piece]
}

case class EmptyTile(coordinate: Int) extends Tile {
  override def toString: String = "" +coordinate
  val isOccupied = false
  def getPiece() = None
}

case class OccupiedTile(coordinate: Int, val piece: Piece) extends Tile {
  override def toString = piece.toString
  val isOccupied = true
  def getPiece = Some(piece)
}

Answer 2

case class automatically makes its arguments val s. This is why the argument coordinate to EmptyTile , understood as val coordinate , conflicts with the abstract class's val coordinate .

One way to fix this is to have coordinate and isOccupied be defined as abstract in Tile . Tile can even be a trait instead of an abstract class:

sealed trait Tile {
  def coordinate: Int
  def isOccupied: Boolean
  ...
}

case class EmptyTile(coordinate: Int) extends Tile {
  def isOccupied = false
  ...
}

case class OccupiedTile(coordinate: Int, piece: Piece) extends Tile {
  def isOccupied = true
  ...
}