Java中的ArrayLists和多态性

Question

我正在尝试在 Java 中实现一个垄断风格的游戏，作为学校项目的一部分，但我遇到了一个我无法解决的问题，或者似乎找到了我所遇到问题的答案。问题是我的代码中出现了 classCastException，我想我已经弄清楚了为什么会得到它（我最初试图将类型 Property 转换为下面的 newPosition 变量）但现在我需要找到一种方法来实现代码，这样我就可以避免它。

问题是我有一个超类Square，它有3个可能的子类Property、Go和FreeParking。我将我的电路板存储为 ArrayList，但我需要访问与 Property 对象相关的方法来计算租金等。如果我从 ArrayList 访问其类型为 Square 的属性，我如何访问这些方法。

如果这没有太大意义，请提前道歉。

public abstract class Square {
    private String name;
    private boolean isAvailable;
    private Player owner;

    //getters and setters
}

public class Property extends Square{
    private String colour;
    private int buyHouset;
    private int rent1House;
    private int rent2House;
    private int rent3House;
    private int rentHotel;
    private int numHouses;
    private int rent;
    private int price;

    //getters and setters
}

public static void takeTurn(Player player, Dice die, ArrayList<Square> board) {
    System.out.println(player.getGamertag() + "'s turn");

    //get value from roll of die
    int move =  askToRoll();

    String gamertag = player.getGamertag();

    //if player does not quit continue with turn
    if(move > 0) {
        //get player current position and add roll of die
        int newSpaceIndex = player.getPosition() + move;

        //if players new position exceeds the array of squares, return to 0 index and add remainder 
        //add credits for passing Go
        //else update position
        if (newSpaceIndex > 11) {
            player.setPosition(newSpaceIndex - 12);
            Go.passGoAction(player);
        }else {
            player.setPosition(newSpaceIndex);
        }


        if(player.getPosition() == 1) {
            Square newPosition = board.get(player.getPosition());
            System.out.println(gamertag + " new position " + newPosition.getName());
        }else if(player.getPosition() == 6) {
            FreeParking.freeParkingAction();
        } else {
            Square newPosition = board.get(player.getPosition());
            System.out.println("____________________");
            System.out.println("Land on " + newPosition.getName());

            if(newPosition.getOwner() != null) {
                Player owner = newPosition.getOwner();

/* *****THIS IS WHERE THE ERROR IS STEMING FROM*****
*THE  newPosition.getRent() method can not be accessed as newPosition 
*is defines as Square
*/
                System.out.println(owner.getGamertag() + " demands you pay " + newPosition.getRent() + " credits!");





            }else {
                System.out.println("What would you like to do now? ...\n"
                        + "1. Buy property\n"
                        + "2. End turn");
                int input = in.nextInt();
                boolean valid = false;
                do {
                    switch(input) {
                    case 1:
                        System.out.println("You now own controlling stock of " + newPosition.getName());
                        player.setOwnsProperty(newPosition);
                        newPosition.setOwner(player);
                        valid = true;
                        break;
                    case 2:
                        System.out.println("Ending turn");
                        valid=true;
                        break;
                        default:
                            System.out.println("Invalid input, please select one of the options 1-2");
                            valid = false;
                            break;
                    }
                }while(!valid);
            }
        }

    }
    System.out.println("END TURN: " + player);
    System.out.println();
}       

Answer 1

如果您知道特定的 Square 是一个属性，请将其转换并分配给适当的类型。

Property newPosition = (Property)board.get(player.getPosition());

编辑：将来，您可能还想考虑将特定类型的 Square 的逻辑放入子类中。换句话说，像square.process(player) 这样的东西，其中Property、Go 和FreeParking 都以适当的逻辑实现process(player) 方法。这涉及到面向对象设计的更详细的概念，现在可能与您无关。

Answer 2

希望这个sn-p能帮助你理解这个概念

public static void main(String[] args) {
        Square newPosition = new Property();
        if (newPosition instanceof Property) {
            System.out.println("newPosition cast as Property");
            Property newPositionAsProperty = ((Property) newPosition);
            int newRent = newPositionAsProperty.getRent();
            // or
            newRent = ((Property) newPosition).getRent();
        }
        // To understand better
        if (newPosition instanceof Square) {
            System.out.println("newPosition instanceof Square");
            ((Square)newPosition).getName();
        }
        if (Square.class.isAssignableFrom(Property.class)) {
            System.out.println("Square.class.isAssignableFrom(Property.class)");
            ((Square)newPosition).getName();
        }
        if (Property.class.isInstance(newPosition)) {
            System.out.println("Property.class.isInstance(newPosition)");
            ((Property)newPosition).getName();
        }
        if (Square.class.isInstance(newPosition)) {
            System.out.println("Square.class.isInstance(newPosition)");
            ((Square)newPosition).getName();
        }
    }

结果：

newPosition cast as Property
newPosition instanceof Square
Square.class.isAssignableFrom(Property.class)
Property.class.isInstance(newPosition)
Square.class.isInstance(newPosition)