java - ArrayList 未按预期方式添加对象

Question

我正在尝试通过 if/else 语句向 ArrayList 添加多个值。变量 numberOfShips 将数量声明为字段。然而，当我打印出 .size() 时，大小仅为 1，其中最新添加的对象就是其中的一个。我不明白我做错了什么。我知道 ArrayList 的大小随着元素的添加而隐式增加，所以不可能是这样。

public ArrayList<Ship> createFleet(int choice) {
    ArrayList<Ship> fleet = new ArrayList<Ship>();
    if (count < numberOfShips && choice > 0 && choice < 5) {
        if (choice == 1) {
            Ship ac = new Ship("Aircraft carrier", 5, false);
            fleet.add(ac);
            count++;
            System.out.println("Aircraft carrier has been added to fleet.");
        } else if (choice == 2) {
            Ship bs = new Ship("Battleship", 4, false);
            fleet.add(bs);
            count++;
            System.out.println("Battleship has been added to fleet.");
        } else if (choice == 3) {
            Ship sm = new Ship("Submarine", 3, false);
            fleet.add(sm);
            count++;
            System.out.println("Submarine has been added to fleet.");
        } else if (choice == 4) {
            Ship ds = new Ship("Destroyer", 3, false);
            fleet.add(ds);
            count++;
            System.out.println("Destroyer has been added to fleet.");
        } else if (choice == 5) {
            Ship sp = new Ship("Patrol Boat", 2, false);
            fleet.add(sp);
            count++;
            System.out.println("Patrol boat has been added to fleet.");
        }
    } else {
        System.out.println("Not an option.");
    }
    return fleet;
}

Answer 1

每次调用该方法时，您都会创建一个新的ArrayList。您需要在方法之外维护一个ArrayList 船舶。

public ArrayList<Ship> createFleet(int choice) {
    ArrayList<Ship> fleet = new ArrayList<Ship>(); //Here you create a new ArrayList and return it with a single Ship in it.

您需要一个全局范围的ArrayList变量:

private List<Ship> fleet = new ArrayList<Ship>();

public ArrayList<Ship> createFleet(int choice) {
    if (count < numberOfShips && choice > 0 && choice < 5) {
        if (choice == 1) {
            Ship ac = new Ship("Aircraft carrier", 5, false);
            fleet.add(ac);
            count++;
            System.out.println("Aircraft carrier has been added to fleet.");
        } else if (choice == 2) {
            Ship bs = new Ship("Battleship", 4, false);
            fleet.add(bs);
            count++;
            System.out.println("Battleship has been added to fleet.");
        } else if (choice == 3) {
            Ship sm = new Ship("Submarine", 3, false);
            fleet.add(sm);
            count++;
            System.out.println("Submarine has been added to fleet.");
        } else if (choice == 4) {
            Ship ds = new Ship("Destroyer", 3, false);
            fleet.add(ds);
            count++;
            System.out.println("Destroyer has been added to fleet.");
        } else if (choice == 5) {
            Ship sp = new Ship("Patrol Boat", 2, false);
            fleet.add(sp);
            count++;
            System.out.println("Patrol boat has been added to fleet.");
        }
    } else {
        System.out.println("Not an option.");
    }
    return fleet;
}

我的建议是创建一个 Fleet 类，其中包含 ArrayList 的引用:

public class Fleet {
    private List<Ship> internalFleet = new ArrayList<Ship>();
    private static int MAX_SHIPS = 10;

    public void addShip(int choice){
        if (internalFleet.size() < MAX_SHIPS) {
            Ship ship;
            switch(choice){
                case 1: ship = new Ship("Aircraft carrier", 5, false);
                        break;
                case 2: ship = new Ship("Battleship", 4, false);
                        break;
                case 3: ship = new Ship("Submarine", 3, false);
                        break;
                case 4: ship = new Ship("Destroyer", 3, false);
                        break;
                case 5: ship = new Ship("Patrol Boat", 2, false);
                        break;
                default: System.out.println("Not an option.");
            }
            if(ship!=null){
                internalFleet.add(ship);
                System.out.println(ship.getName() + " has been added to fleet.");
            }
        }
    }
    public ArrayList<Ship> getFleet(){
        return internalFleet;
    }
}