java - 将数组转换为 readObject 时出现 ClassCastException

Question

我必须运行这个对象 Student 数组(其中有 3 个学生):

        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.");
    }
}

每个 Student 对象必须像这样设置:

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);
        }
    }

运行这些行时，我不断收到 ClassCastException:

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

异常看起来像这样:

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

第 21 行是上述代码中的最后一行。有谁知道如何解决这个问题以便我可以正确地转换它？预先感谢您的帮助。

Answer 1

您正在阅读一个 Student，但试图将其转换为 Student[]。只需删除数组引用即可:

Student s = (Student)obj;

这是因为您将数组的每个元素单独存储在 ObjectOutputStream 中，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();

如果您想要/需要将其作为数组读取，则也将其存储为数组:

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

这样你就可以正确阅读它:

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

或者在一行中:

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