当子类对象存储在超类数组中时，如何调用子类方法？

Question

{
    Ship ships = new Ship();
    CargoShip cargoShips = new CargoShip();
    CruiseShip cruiseShips = new CruiseShip();

    Ship[] allShips = {ships, cargoShips, cruiseShips};

    allShips[0].setShipName("Boom");
    allShips[0].setYearBuilt("1900");
    allShips[1].setShipName("Bang");
    allShips[1].setCargoCapaicty(200);
    allShips[2].setShipName("Bam");
    allShips[2].setMaxPassengers(500);


    for (int i = 0; i < allShips.length; i++)
    {
        System.out.println(allShips[i]);
    }
}

So the Ship class is the super class while CargoShip and CruiseShip both extend the Ship class. I've stored the 3 objects into a Ship array. setCargoCapacity and setMaxPassengers are methods that only appear in the subclasses. For some reason I cannot access them. I can't figure out how to make it so that I can also access the subclass methods.

Answer 1

You can't call setCargoCapacity of a Ship you picked out of a Ship[] because that ship might not be a CargoShip . You must either provide some method in the Ship class (and therefore for all Ships) that does what you need it to do, or check whether ship instanceof CargoShip , and if so you can cast it ( CargoShip cargoShip = (CargoShip)ship ) and call the setCargoCapacity of that.

Answer 2

You could initialize your objects before storing them in the array:

Ship ships = new Ship();
ships.setShipName("Boom");
ships.setYearBuilt("1900");

CargoShip cargoShips = new CargoShip();
cargoShips.setShipName("Bang");
cargoShips.setCargoCapaicty(200);

CruiseShip cruiseShips = new CruiseShip();
cruiseShips.setShipName("Bam");
cruiseShips.setMaxPassengers(500);

Ship[] allShips = {ships, cargoShips, cruiseShips};

Answer 3

You cant do it. In java reference variables have two types: reference type and actual object type. So, when you write something like Object o = "12345" , o has type Object (not String ), and you cant work with o like with string without typecasting: o = ((String)o).substring(1); .

In your case all references in your array allShips have type Ship . If you know that actual type of some array element is a Ship subclass, you can cast actual type like above. But it's a bad practice and shouldn't be used in real world.

Answer 4

allShips[0].setShipName("Boom");
allShips[0].setYearBuilt("1900");
allShips[1].setShipName("Bang");
((CargoShip)allShips[1]).setCargoCapaicty(200);
allShips[2].setShipName("Bam");
((CruiseShip)allShips[2]).setMaxPassengers(500);

try this. It will work.