Question

我是数组列表的新手。我有一个程序，我想找到独特的城市。我使用for循环来使用它，但它似乎没有显示我想要它。我可以知道我哪里出错吗？

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,

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,

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类必须覆盖方法equals()以获得所需的equals()行为（默认只测试对象引用。此行为不是您想要的）。

顺便说一句，您甚至不需要进行此重复检测。只需切换到使用接口Set，它就可以保证集合中的所有元素都是唯一的。如果您不关心订单，那么您可以使用实现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<>();

Answer 2

如果您希望保持原始列表不变，并构建一个具有不同值的新列表，以下是使用java8流的方法：

<img>

正如已经说过和解释的那样，您的List<City> allCities = s1.returnListOfCities(); List<City> distinctCities = allCities.stream().distinct().collect(Collectors.toList());必须覆盖City。

使用流，你也可以像这样打印你的列表：

equals()

或者，如果你不能用System.out.println(allCities.stream().distinct().map(City::getName).collect(Collectors.joining(",")));覆盖City（假设它已经定义为在邮政编码上进行比较，但你想在名称上进行比较），你可以这样做：

equals()

在这种情况下，名称将直接比较为String。

在arraylist错误中查找唯一元素

2 个答案: