java - 在 ArrayList 中的指定索引处添加元素时出错 [Java]

Question

在 ArrayList 中的指定索引处添加元素时出现错误，我正在使用循环和变量，每次循环时它都会增加

1. 我尝试使用命令 arrayListName.add(index,thing) 但这在第二次分配时给我一个错误问题==> https://ibb.co/ZSQvywY .
2. 我尝试在没有索引的情况下循环，但始终在索引 [0] 中分配

public class CollegeSystem {

    public static void printOptions() {
        System.out.println("Welcome to our university!");
        System.out.println("Operations:");
        System.out.println("1- College");System.out.println("a) Number of Departments");System.out.println("b) Number of Courses");System.out.println("c) Number of Professors");System.out.println("d) Number of Students");System.out.println("e) Report");
        System.out.println("2- Department");System.out.println("a) New");System.out.println("b) Number of Courses");System.out.println("c) Number of Students");System.out.println("d) Is Full");System.out.println("e) Enroll");System.out.println("f) Report");
        System.out.println("3- Course");System.out.println("a) New");System.out.println("b) Number of Students");System.out.println("c) Assign");System.out.println("d) Is assigned");System.out.println("e) Professor Name");System.out.println("f) Is Full");System.out.println("g) Enroll");System.out.println("h) Report");
        System.out.println("4- Professor");System.out.println("a) New");System.out.println("b) Display Salary");System.out.println("c) Get Raise");System.out.println("d) Report");
        System.out.println("5- Student");System.out.println("a) New");System.out.println("b) Report");
        System.out.println("6- Quit");
    }

    public static void main(String[] args) {
        // TODO code application logic here

        printOptions() ;
        Scanner in = new Scanner(System.in) ; 
        int d = 0 , c = 0  , p = 0 , s=0 ; 
        College AinShams = new College() ; 
        while (true){
            String option = in.nextLine() ; 
            if(!"6".equals(option)) {
                if ("2a".equals(option)) { // Define new department
                    System.out.println("Department Name:");
                    String depName = in.nextLine() ; 
                    System.out.println("Department Description:");
                    String depDescripe = in.nextLine() ;
                    System.out.println("Department Max Students:");
                    int max_num = in.nextInt() ;
                    in.nextLine() ; 
                    Department Department_Name = 
                    new Department(depName,   depDescripe, max_num); 
                    List<Department> departmentList;
                   //here create a new arrayList
                   departmentList = new ArrayList<>();
                 //try to add element without index
                   //  departmentList.add(Department_Name);
                 //try to add element with index
                   departmentList.add(d, Department_Name);
                    d++ ;
                    AinShams.setDepart(departmentList);      
                }
            }   
        }   
    } 
 }

我需要在每次循环时添加一个元素而不删除旧数据。

Answer 1

之所以总是在索引0，与是否传递索引无关。

while (true){
        String option = in.nextLine() ; 
        if(!"6".equals(option)) {
            if ("2a".equals(option)) { // Define new department
                ...
                List<Department> departmentList;
               //here create a new arrayList
               departmentList = new ArrayList<>();
             //try to add element without index
               //  departmentList.add(Department_Name);
             //try to add element with index
               departmentList.add(d, Department_Name);
                d++ ;
                ....

            }
        }

每次添加元素时，都会将其添加到新创建的列表中。您应该在循环之前创建列表，并将添加语句保留在其中:

List<Department> departmentList = new ArrayList<>();
while (true){
        String option = in.nextLine() ; 
        if(!"6".equals(option)) {
            if ("2a".equals(option)) { // Define new department
                ...
             //try to add element without index
               //  departmentList.add(Department_Name);
             //try to add element with index
               departmentList.add(d, Department_Name);
                d++ ;
                ....

            }
        }