在添加ArrayList的时候出现 NullPointerException 应该怎么解决

Question

import java.io.*;
import java.util.*;
public class FileSystem
{
private ArrayList students;

public void FileSystem()
{
students = new ArrayList();
}

public void readFile()
{
int index = 0;
String fileName = ("student.txt");
try
{
FileReader fr = new FileReader(fileName);
try
{
if (fr.equals(""))
index = 0;
else
{
Scanner parser = new Scanner(fr);
Student studentDetail = new Student();
studentDetail.setStudentDetail(parser.nextLine());
students.add(studentDetail); （此时出现报错。。）
System.out.println(students.get(index));
System.out.println("");
。。。。后面还有程序。。。。