一个简单的学生信息管理系统，类中报错 java.lang.NullPointerException

Question

/*

• 学生类

• 姓名，学号，性别，年级，班级，总分
  */
  public class Student implements Comparable {

  private String name;
  private String num;
  private String sex;
  private String grade;
  private String banji;
  private String score;

  public Student(String name, String num, String sex, String grade, String banji, String score)
  {
  this.name = name;
  this.num = num;
  this.sex = sex;
  this.grade = grade;
  this.banji = banji;
  this.score = score;
  }

  public Student()
  {
  this.name = "未知";
  this.num = "未知";
  this.sex = "未知";
  this.grade = "未知";
  this.banji = "未知";
  this.score = "未知";
  }

  public String getName() {
  return name;
  }

  public void setName(String name) {
  this.name = name;
  }

  public String getNum() {
  return num;
  }

  public void setNum(String num) {
  this.num = num;
  }

  public String getSex() {
  return sex;
  }

  public void setSex(String sex) {
  this.sex = sex;
  }

  public String getGrade() {
  return grade;
  }

  public void setGrade(String grade) {
  this.grade = grade;
  }

  public String getBanji() {
  return banji;
  }

  public void setBanji(String banji) {
  this.banji = banji;
  }

  public String getScore() {
  return score;
  }

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

  public String getStudent() {
  return name+" "+num+" "+sex+" "+grade+" "+banji+" "+score+"\r\n";
  }
  public int compareTo(Student other) {
  
  if (Integer.parseInt(num) return - 1 ;
  if (Integer.parseInt(num)>Integer.parseInt(other.num))
  
  return 1 ;
  
  return 0 ;
  
  }
  }

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class Sort {

private File f;

public Sort(File f) {
    this.f = f;
}

public Sort() {     
}

public List<Student> filereader() throws IOException{//对原TXT文件的读取
    FileReader fr = new FileReader(f);
    BufferedReader cin=new BufferedReader(fr);
    List<Student> l=new ArrayList<Student>();
    Student[] student=new Student[1000];
    String s="";int i=0;
    while((s=cin.readLine())!=null)
    {
        student[i]=new Student();
        String[] a=new String[6];
        a=s.split(" ");
        student[i].setName(a[0]);
        student[i].setNum(a[1]);
        student[i].setSex(a[2]);;
        student[i].setGrade(a[3]);
        student[i].setBanji(a[4]);
        student[i].setScore(a[5]);
        i++;
        l.add(student[i]);
    }
    cin.close();
    fr.close();
    return l;
}


public void Sortsex(String sex,File f1) throws IOException{//指定性別后按学号排序
    FileWriter fw = new FileWriter(f1);
    @SuppressWarnings("resource")
    BufferedWriter cout=new BufferedWriter(fw);
    List<Student> a=new ArrayList<Student>();
    List<Student> b=new ArrayList<Student>();
    a=filereader();
    int i=0;    
    Student[] student=new Student[1000];
    for(int t=0;t<a.size();t++){
        if(a.get(t).getSex().equals(sex)){  /*******这行报错*********/
            student[i]=a.get(t);
            i++;
        }
    }
    Arrays.sort(student);
    for(int m=0;m<i-1;m++)
    {
    cout.write(student[i].getStudent());
    }
    System.out.println("已生成文件");
}

}

Exception in thread "main" java.lang.NullPointerException
at Info.main(Info.java:31)

Answer 1

类Info第31行空指针异常，检查下吧