java - 初级 Java(使用数组；类分配)

Question

我觉得我好像正确地编写了这个作业的代码。我们获得了一个框架和 2 个必须导入的类(FileIOHelper 和 Student)。

 /*
 * Created: *** put the date here ***
 * 
 * Author: *** put your name here ***
 * 
 * The program will read information about students and their
 * scores from a file, and output the name of each student with
 * all his/her scores and the total score, plus the average score
 * of the class, and the name and total score of the students with
 * the highest and lowest total score.
 */

//
import java.util.Scanner;
import java.io.*;

// C:\Users\Adam\info.txt

    public class Lab6
{
    public static void main(String[] args) throws IOException
    {

    // Fill in the body according to the following comments
 Scanner key

boardFile = new Scanner(System.in);


     // Input file name
     String filename = getFileName(keyboardFile);

     //Open the file

     // Input number of students
      int numStudents = FileIOHelper.getNumberOfStudents(filename);
      Student students[] = new Student[numStudents];

     // Input all student records and create Student array and
     // integer array for total scores

      int totalScore[] = new int[students.length];
      for (int i = 0; i < students.length; i++){
       for(int j = 1; j < 4; j++){
                 totalScore[i] = totalScore[i] + students[i].getScore(j);
         }

      }



     // Compute total scores and find students with lowest and
     // highest total score

      int maxScore = 0;
      int minScore = 0;
      for(int i = 0; i < students.length; i++){
             if(totalScore[i] >= totalScore[maxScore]){
                     maxScore = i;
             }
             else if(totalScore[i] <= totalScore[minScore]){
                     minScore = i;
             }
     }

     // Compute average total score

      int allScores = 0;
      int average = 0;
      for (int i = 0; i < totalScore.length; i++){
       allScores = allScores + totalScore[i];
      }

      average = allScores / totalScore.length;


     // Output results
      outputResults(students, totalScore, maxScore, minScore, average);
    }

    // Given a Scanner in, this method prompts the user to enter
    // a file name, inputs it, and returns it.
    private static String getFileName(Scanner in)
    {
        // Fill in the body
     System.out.print("Enter the name of a file: ");
     String filename = in.next();
     return filename;

        // Do not declare the Scanner variable in this method.
        // You must use the value this method receives in the
        // argument (in).
   }


    // Given the number of students records n to input, this
    // method creates an array of Student of the appropriate size,
    // reads n student records using the FileIOHelper, and stores
    // them in the array, and finally returns the Student array.
    private static Student[] getStudents(int n)
    {
     Student[] myStudents = new Student[n];
     for(int i = 0; i <= n; i++){
      myStudents[i] = FileIOHelper.getNextStudent();
     }
  return myStudents;
    }

    // Given an array of Student records, an array with the total scores,
    // the indices in the arrays of the students with the highest and
    // lowest total scores, and the average total score for the class,
    // this method outputs a table of all the students appropriately
    // formatted, plus the total number of students, the average score
    // of the class, and the name and total score of the students with
    // the highest and lowest total score.
    private static void outputResults(
            Student[] students, int[] totalScores,
            int maxIndex, int minIndex, int average
        )
    {
        // Fill in the body
     System.out.println("\nName \t\tScore1 \tScore2 \tScore3 \tTotal");
        System.out.println("--------------------------------------------------------");
        for(int i = 0; i < students.length; i++){
                outputStudent(students[i], totalScores[i], average);
                System.out.println();
        }
        System.out.println("--------------------------------------------------------");
        outputNumberOfStudents(students.length);
        outputAverage(average);
        outputMaxStudent(students[maxIndex], totalScores[maxIndex]);
        outputMinStudent(students[minIndex], totalScores[minIndex]);
        System.out.println("--------------------------------------------------------");

    }

    // Given a Student record, the total score for the student,
    // and the average total score for all the students, this method
    // outputs one line in the result table appropriately formatted.
    private static void outputStudent(Student s, int total, int avg)
    {
            System.out.print(s.getName() + "\t");
            for(int i = 1; i < 4; i++){
                    System.out.print(s.getScore(i) + "\t");
            }
            System.out.print(total + "\t");
            if(total < avg){
                    System.out.print("-");
            }else if(total > avg){
                    System.out.print("+");
            }else{
                    System.out.print("=");
            }
    }

    // Given the number of students, this method outputs a message
    // stating what the total number of students in the class is.
    private static void outputNumberOfStudents(int n)
    {
            System.out.println("The total number of students in this class is: \t" + n);
    }

    // Given the average total score of all students, this method
    // outputs a message stating what the average total score of
    // the class is.
    private static void outputAverage(int average)
    {
            System.out.println("The average total score of the class is: \t" + average);
    }

    // Given the Student with highest total score and the student's
    // total score, this method outputs a message stating the name
    // of the student and the highest score.
    private static void outputMaxStudent(
                    Student student,
                    int score
    )
    {
            System.out.println(student.getName() + " got the maximum total score of: \t" + score);
    }

    // Given the Student with lowest total score and the student's
    // total score, this method outputs a message stating the name
    // of the student and the lowest score.
    private static void outputMinStudent(
                    Student student,
                    int score
    )
    {
            System.out.println(student.getName() + " got the minimum total score of: \t" + score);
    }
}

但是现在我在 TotalScore[i] = TotalScore[i] + Students[i].getScore(j); 行出现错误

Exception in thread "main" java.lang.NullPointerException

在 Lab6.main(Lab6.java:42)

Answer 1

您的学生数组未初始化。

学生[0]到学生[student.length]均为空

根据您的代码，您应该使用

Student student[] = getStudents(numStudents); 

而不是

Student student[] = new Student[numStudents];