將數組強制轉換為readObject時發生ClassCastException

Question

我必須運行對象Student的數組（有3個學生）：

        try
    {
        out = new ObjectOutputStream (new BufferedOutputStream (new FileOutputStream ("Students.dat")));
        out.writeObject(s[0]);
        out.writeObject(s[1]);
        out.writeObject(s[2]);
        out.close();
    }
    catch (IOException e)
    {
        System.out.println("Error writing student data to file.");
    }
}

並且每個Student對象必須設置如下：

for (int i = 0; i<3; i++)
    {
        System.out.println("The following information applies to student " + (i+1));
        System.out.println("What is the student's name?");
        String name = input.nextLine();
        if (i == 0)
        {
            System.out.println("Please enter the name again.");
        }
        name = input.nextLine();
        System.out.println("What is the Social Security Number of this student?");
        String ssn = input.next();
        System.out.println("How many courses has the student completed?");
        int numCourses = input.nextInt();
        int [] grades = new int [numCourses];
        int credits = (5*numCourses);

        double points = 0;
        for(int k = 0; k<numCourses; k++)
        {
            System.out.println("Type a number to represent the letter grade of course " + (k+1) + ". Type 1 for an A. Type 2 for a B. Type 3 for a C. Type 4 for a D. Type 5 for an F.");
            grades[k] = input.nextInt();
            switch (grades[k])
            {
                case 1:
                    points += 4;
                    break;
                case 2:
                    points += 3;
                    break;
                case 3:
                    points += 2;
                    break;
                case 4:
                    points += 1;
                    break;
                case 5:
                    break;
            }

            s[i] = new Student (name, ssn, numCourses, grades, credits);
        }
    }

運行這些行時，我不斷收到ClassCastException：

Object obj = new Object();
obj = in.readObject();
Student[] ns = (Student[])obj;

異常如下所示：

java.lang.ClassCastException: UnitSeven.Student cannot be cast to [LUnitSeven.Student;
at UnitSeven.StudentGPA.main(StudentGPA.java:21)

第21行是上述代碼中的最后一行。 有誰知道如何解決此問題，以便我可以正確投放？ 在此先感謝您的幫助。

Answer 1

您正在閱讀一個Student但試圖將其轉換為Student[] 。 只需刪除數組引用：

Student s = (Student)obj;

這是因為您將數組的每個元素ObjectOutputStream存儲在ObjectOutputStream ，如下所示：

out = new ObjectOutputStream (new BufferedOutputStream (new FileOutputStream ("Students.dat")));
//you write each reference of Student
out.writeObject(s[0]);
out.writeObject(s[1]);
out.writeObject(s[2]);
out.close();

如果您希望/需要將其讀取為數組，則也將其存儲為數組：

out = new ObjectOutputStream (new BufferedOutputStream (new FileOutputStream ("Students.dat")));
out.writeObject(s);
out.close();

因此，您可以正確閱讀它：

Object obj = new Object();
obj = in.readObject();
Student[] ns = (Student[])obj;

或一行：

Student[] ns = (Student[])in.readObject();