Question

我必须运行这个对象学生数组（其中有3名学生）：

        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对象必须设置如下：

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);
        }
    }

运行这些行时，我不断收到ClassCastException：

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

例外情况如下：

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，以便我可以正确地投射它？提前感谢您的帮助。

Answer 1

您正在阅读单个Student但尝试将其转换为Student[]。只需删除数组引用：

Student s = (Student)obj;

这是因为您在ObjectOutputStream中自行存储数组的每个元素，请注明：

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/需要将其作为数组读取，请将其存储为数组：

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

所以你可以正确阅读：

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

或者在一行中：

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

将数组转换为readObject时的ClassCastException

1 个答案: