java - 在数组列表中查找唯一元素错误

Question

我是数组列表的新手。我有一个计划，我想寻找独特的城市。我使用 for 循环来使用它，但它似乎没有显示我想要的内容。我可以知道我哪里出错了吗？

run:
Enter name of city: Singapore
Enter size of city: Small
Enter postal code: 132115
___________________________________________
Enter name of city: Singapore
Enter size of city: Small
Enter postal code: 132115
___________________________________________
Enter name of city: Singapore
Enter size of city: Small
Enter postal code: 132115
___________________________________________
Enter name of city: Singapore
Enter size of city: Small
Enter postal code: 132115
___________________________________________
Enter name of city: Malaysia
Enter size of city: Medium
Enter postal code: 132222
___________________________________________
List Of Unique Cities: 
Malaysia,Singapore,Singapore,Singapore,Malaysia,Singapore,

<小时/>

我希望它像这样打印

Enter name of city: Singapore
Enter size of city: Small
Enter postal code: 132115
___________________________________________
Enter name of city: Singapore
Enter size of city: Small
Enter postal code: 132115
___________________________________________
Enter name of city: Singapore
Enter size of city: Small
Enter postal code: 132115
___________________________________________
Enter name of city: Singapore
Enter size of city: Small
Enter postal code: 132115
___________________________________________
Enter name of city: Malaysia
Enter size of city: Medium
Enter postal code: 132222
___________________________________________
List Of Unique Cities: 
Malaysia,Singapore,

<小时/>

public class TravellingApp {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        SalesPerson s1 = new SalesPerson();

        for(int j=1; j<=20; j++)
        {
            System.out.print("Enter name of city: ");
            String name = sc.nextLine();

            System.out.print("Enter size of city: ");
            String size = sc.nextLine();

            System.out.print("Enter postal code: ");
            int postalCode = sc.nextInt();
            sc.nextLine();

            System.out.println("___________________________________________");



            City c1 = new City(name, size, postalCode);
            s1.addCity(c1);

        }
        System.out.println("List Of Unique Cities: ");
        for (int i = 0; i < s1.returnListOfCities().size(); i++) 
        {
            for (int k = s1.returnListOfCities().size() - 1; k >= i; k--) 
            {
                if (s1.returnListOfCities().get(i).equals(s1.returnListOfCities().get(k)))
                {
                    s1.returnListOfCities().remove(s1.returnListOfCities().get(i));
                    break;
                }
                System.out.print(s1.returnListOfCities().get(k) + ",");

            }
        }

    }

    }

Answer 1

您的 City 类必须重写方法 equals() 才能获得您想要的 equals() 行为(默认情况下仅测试对象引用。这种行为不是您想要的)。

顺便说一句，您甚至不需要进行重复检测。只需改用Set接口(interface)，就可以保证集合中的所有元素都是唯一的。如果你不关心顺序，那么你可以使用实现HashSet作为实现。因此，实现 City#hashCode() 然后切换到使用此行:

Set<City> s1 = new HashSet<>();