我有下面的函数，它应该只在对象不在studentlist集中时才写入文件。
我正在使用contains方法。我已经重写了student类中的hashcode和equals方法。集合工作正常，没有打印重复项，但是当我转到文本文件时，同一记录的多个值会出现。
我的主代码：

package fileproject;
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.HashSet;
import java.util.Set;

public class FileManipulator {

    Set <Student> studentList = new HashSet<Student>();
    File file; 

    public void readData(String f) {
        try {
            //create a file
            file = new File(f);
            //instantiate the buffered 
            BufferedReader reader = new BufferedReader(new FileReader(file));
            String line;

            while((line = reader.readLine()) != null) {
                parseText(line);
            }
            reader.close();
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } 
    }

    public void parseText(String line) {
        String [] student = line.split(",");
        Student s = new Student(Integer.parseInt(student[0]), student[1], student[2]);
        studentList.add(s);
    }

    public void display() {
        studentList.forEach(s -> System.out.println(s));
//      studentList.clear();
    }

    public void appendData(String f, Student s) {
        file = new File(f);
        if (studentList.contains(s)) return; //I expect this value to be true

        try {
            BufferedWriter writer = new BufferedWriter(new FileWriter(file, true));
            writer.newLine();
            writer.append(s.toString());
            writer.close();

        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        FileManipulator myFile = new FileManipulator();
        myFile.readData("students.txt");

        Student st = new Student(4, "Mo Dna", "Genetics");
        myFile.appendData("students.txt", st);
        myFile.readData("students.txt");

        myFile.display();

        System.out.println(myFile.studentList.contains(st)); //this should return true but returns false
        myFile.studentList.forEach(s -> System.out.println(s));
    }
}

我做错什么了？