java list集合存储学生类，之后去重，我的set方法为什么没有去掉重复的元素

[IT_小斯]

package homework;

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.io.PrintWriter;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;

public class Test01 {
	public static void main(String[] args) throws IOException {
		// 入到ArrayList集合中
		ArrayList<Student> al = getStuInfo();
		weiterInfoToFile(al);
		readInfo();
		TreeSet<Student> ts = sortStu(al);
		printInfoToFile(ts);

	}

	public static TreeSet<Student> sortStu(ArrayList<Student> al) {
		// (5)对ArrayList集合中的6个学生对象进行去重并按照年龄从小到大的顺序排序
		Set<Student> set = new HashSet<Student>();
		set.addAll(al);

		TreeSet<Student> ts = new TreeSet<>(new Comparator<Student>() {
			@Override
			public int compare(Student o1, Student o2) {
				int i = o1.getAge() - o2.getAge();
				return i == 0 ? 1 : i;
			}
		});

		for (Student student : set) {
			ts.add(student);
		}

		return ts;
	}

	// (6)将ArrayList集合中排序后的结果利用PrintWriter流写入到E:\\StudentInfo.txt文件中(
	// 写入格式：张三-男-25)
	public static void printInfoToFile(TreeSet<Student> ts)
			throws FileNotFoundException {
		PrintWriter pw = new PrintWriter(new File("EStudentInfo.txt"));
		for (Student student : ts) {
			pw.println(student.getName() + "-" + student.getGender() + "-"
					+ student.getAge());
		}
		pw.close();
	}

	public static void weiterInfoToFile(ArrayList<Student> al)
			throws IOException, FileNotFoundException {
		// (3)将存有6个学员信息的ArrayList集合对象写入到D:\\StudentInfo.txt文件中
		BufferedWriter bfw = new BufferedWriter(new FileWriter(
				"DStudentInfo.txt"));
		for (Student student : al) {
			bfw.write(student.toString());
		}
		bfw.close();
	}

	// (2)键盘录入6个学员信息(录入格式:张三,男,25),要求有两个相同的信息,将6个学员信息存
	public static ArrayList<Student> getStuInfo() {
		Scanner sc = new Scanner(System.in);
		ArrayList<Student> al = new ArrayList<>();
		for (int i = 0; i < 6; i++) {
			System.out.println("请输入第" + (i + 1) + "个学员信息：");
			// 获取输入的信息，并去除首尾空格
			String s = sc.nextLine().trim();
			// 将接收的字符串以 , 截取 存入字符串数组中
			String[] strings = s.split(",");
			// 将学生信息添加到ArrayList集合中
			al.add(new Student(strings[0], strings[1], Integer
					.parseInt(strings[2])));
		}
		return al;
	}

	// (4)读取D:\\StudentInfo.txt文件中的ArrayList对象
	public static void readInfo() throws FileNotFoundException, IOException {
		BufferedReader bfr = new BufferedReader(new FileReader(
				"DStudentInfo.txt"));
		String line = null;
		while ((line = bfr.readLine()) != null) {
			System.out.println(line);
		}
	}
}

[IT_小斯]

package homework;

public class Student {
	// 1)定义学生类,包含姓名(String name),性别(String gender),年龄(int age)三个属性,生
	// 成空参有参构造,set和get方法,toString方法
	private String name;
	private String gender;
	private int age;

	public String getName() {
		return name;
	}

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

	public String getGender() {
		return gender;
	}

	public void setGender(String gender) {
		this.gender = gender;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	@Override
	public String toString() {
		return "学生姓名=" + name + ", 性别=" + gender + ", 年龄=" + age + "\r\n";
	}

	public Student(String name, String gender, int age) {
		super();
		this.name = name;
		this.gender = gender;
		this.age = age;
	}

	public Student() {
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + age;
		result = prime * result + ((gender == null) ? 0 : gender.hashCode());
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Student other = (Student) obj;
		if (age != other.age)
			return false;
		if (gender == null) {
			if (other.gender != null)
				return false;
		} else if (!gender.equals(other.gender))
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}
	
	
}

在Student类中忘了写equals和hashCode