Java NullPointerException 排序数组

Question

我应该能够按姓氏对students 数组进行排序，然后是名字。我的 compareTo 方法适用于它，直到最后一次迭代。然后它抛出一个NullPointerException，我不知道为什么。我已经在我的书和互联网上搜索了将近 12 个小时。我已经尝试了我找到的所有东西，但仍然没有骰子。

这是程序的代码：

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package ch11pr112;

import java.io.*;
import java.util.Arrays;
/**
 *
 * @author Tytus
 */
public class CH11PR112{

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws IOException {
        // TODO code application logic here
        BufferedReader in = new BufferedReader(new FileReader("src//ch11pr112//Students.txt"));
        Student[] students = new Student[100];
        int i = 0;
        double sum = 0;
        String line = in.readLine();
        while (line != null)
        {
            String[] studentParts = line.split(" ");
            String firstName = studentParts[1];
            String lastName = studentParts[0];
            Double score = Double.parseDouble(studentParts[2]);
            students[i] = new Student(firstName, lastName, score);
            sum += score;
            i++;
            line = in.readLine();
        }
        double average = sum / i;
        double x = i;
        Arrays.sort(students);
        for (i = 0; i < x; i++)
        {
            String studentList = students[i].getLastName() + " " + students[i].getFirstName() + " " + students[i].getScore();
            if (students[i].getScore() < (average - 10))
            {
                System.out.println(studentList + " BELOW AVERAGE");
            }
            else
            {
                System.out.println(studentList);
            }
        }
        System.out.println();
        System.out.println("Average:\t" + average);
    }
}

这是我的 Students.txt 文件中的数据：

Gator Ali 85
Vator Ella 75
Beam John 60
Beam James 95
Class Lastin 55
Steelman Andrea 95
Murach Joel 92
Lowe Doug 82
Murach Mike 93

这是我的 Student.java 文件中的代码：

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package ch11pr112;

/**
 *
 * @author Tytus
 */
public class Student implements Comparable<Student>{
    private String firstName;
    private String lastName;
    private double score;

    public Student(String firstName, String lastName, double score)
    {
        this.firstName = firstName;
        this.lastName = lastName;
        this.score = score;
    }

    public String getFirstName()
    {
        return firstName;
    }

    public void setFirstName(String firstName)
    {
        this.firstName = firstName;
    }

    public String getLastName()
    {
        return lastName;
    }

    public void setLastName(String lastName)
    {
        this.lastName = lastName;
    }

    public double getScore()
    {
        return score;
    }

    public void setScore(double score)
    {
        this.score = score;
    }

    @Override
    public int compareTo(Student x) {
        int lastNameCompare = this.lastName.compareToIgnoreCase(x.getLastName()); 
        if (this.lastName != null && x.lastName != null)
        {
            if (lastNameCompare == 0)
            {
                int firstNameCompare =     this.firstName.compareToIgnoreCase(x.getFirstName()); 
                if (this.firstName != null && x.firstName != null)
                { 
                    if (firstNameCompare == 0)
                        {
                        return 0;
                    }
                    else if (firstNameCompare > 0)
                    {
                        return 1;
                    }
                    else if (firstNameCompare < 0)
                    {
                        return - 1;
                    }
                }
            }
            else if (lastNameCompare > 0)
            {
                return 1;
            }
            else if (lastNameCompare < 0)
            {
                return - 1;
            }
        }
        return 0;
    }
}

由于某种原因，它在 ComparableTimSort.java 文件的第 232 行 (if (pivot.compareTo(a[mid]) < 0)) 的最后一次迭代期间创建了一个 NullPointerException。

问题是如何防止NullPointerException，以及如果lastName 或firstName 变量是null，为什么不应该运行代码时会抛出它。

Answer 1

我不相信您可以使用 Arrays.sort 对包含空值的数组进行排序，因为最终它会遇到您拥有pivot == null 的情况，因此您尝试进行比较

null.compareTo(Object) < 0

查看Arrays API顶部的警告

如果指定的数组引用为空，则此类中的方法都会抛出 NullPointerException，除非另有说明。

Answer 2

public static void main(String[] args) throws Exception {
     BufferedReader in = new BufferedReader(new FileReader("src//ch11pr112//Students.txt"));
     String strLine;
     int count = 0;
     while ((strLine = in.readLine()) != null)   {
       count++;
     }
      Student[] students = new Student[count];
     int i = 0;
     double sum = 0;
     in = new BufferedReader(new FileReader("src//ch11pr112//Students.txt"));
     String line = in.readLine();
     System.out.println(line);
     while (line != null)
     {
         String[] studentParts = line.split(" ");
         String firstName = studentParts[1];
         String lastName = studentParts[0];
         Double score = Double.parseDouble(studentParts[2]);
         students[i] = new Student(firstName, lastName, score);
         sum += score;
         i++;
         line = in.readLine();
     }
     double average = sum / i;
     double x = i;
     for(int w=0;w<students.length;w++)
     System.out.println(students[w]);
     Arrays.sort(students);
     for (i = 0; i < x; i++)
     {
         String studentList = students[i].getLastName() + " " + students[i].getFirstName() + " " + students[i].getScore();
         if (students[i].getScore() < (average - 10))
         {
             System.out.println(studentList + " BELOW AVERAGE");
         }
         else
         {
             System.out.println(studentList);
         }
     }
     System.out.println();
     System.out.println("Average:\t" + average);
 }