ClassCastException異常鑄造陣列的readObject

Question

當我要運行這個數組對象學生（其中有3名學生）：

 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."); 
    } 
} 

並且每個學生對象必須設定這樣的：

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); 
     } 
    } 

我繼續運行，這些線路時得到ClassCastException異常：

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

唯一的例外是這樣的：

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

而第21行是上述代碼中的最後一行。有誰知道如何解決這個問題，以便我可以正確施放它？預先感謝您的幫助。

Answer 1

您正在閱讀的是單個Student，但試圖將其轉換爲Student[]。只是刪除陣列基準：

Student s = (Student)obj; 

這是因爲你本身的存儲陣列中的每個元素在你的ObjectOutputStream，注意這裏：

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(); 

如果你想/需要它讀成一個數組，然後將其存儲作爲數組，以及：

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

所以，你可以正確讀出它：

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

或者在單行：

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