Print Treeset 返回奇怪的打印？

Question

所以我有三节课：

• 实验室
• 学生
• 考试

我需要输入学生的ID，将Exam类的考试添加到带有Exam类型的Treeset中，所有这些都在Student类中。

完成此操作后，我需要使用Iterator打印此树集。但是我得到的只是有些奇怪：java.util.TreeMap$KeyIterator@1b6d3586。

我创建了迭代器并通过了树集，但是代码无法正常工作。

这里是文件。

Lab.java

public class Lab {

    public static void main(String[] args) {

        Student stu = new Student();

        Student.insert();
        System.out.println("Insert ended.\n");

        stu.print();
        System.out.println("\nEnd of the program.");

    }

}

Student.java

import java.util.Iterator;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;

public class Student implements Comparable<Student>{

    private static int ID;

    private static Set<Exam> exams = new TreeSet<Exam>();

    public static void insert() {

        Scanner input  = new Scanner(System.in);
        Scanner input1 = new Scanner(System.in);
        Scanner input2 = new Scanner(System.in);
        Scanner input3 = new Scanner(System.in);

        int exam_code = 1;
        String exam_name;
        int exam_grade;

        System.out.println("Student's ID: ");
        ID = input.nextInt();

        System.out.println("--- EXAMS INSERT ---");

        while (exam_code != 0) {

            System.out.println("Exam's code: ");
            exam_code = input1.nextInt();

            if (exam_code == 0) {
                break;
            }

            else {

                System.out.println("Exam's name: ");
                exam_name = input2.nextLine();

                System.out.println("Exam's grade: ");
                exam_grade = input3.nextInt();

                exams.add(new Exam(exam_code, exam_name, exam_grade));

            }

        }

        input.close();
        input1.close();
        input2.close();
        input3.close();

    }

    public void print() {

        System.out.println("ID: " + ID);

        System.out.println("\nExams done: ");

        Iterator<Exam> es = exams.iterator();

        boolean hasnext = false;

        while (hasnext != true) {

            if (es.hasNext()) {

                System.out.println(es);

                hasnext = true;
            }

            else {
                hasnext = false;
            }
        }

    }

    @Override
    public int compareTo(Student o) {
        // TODO Auto-generated method stub
        return 0;
    }

}

Exam.java

public class Exam implements Comparable<Exam>{

    public int exam_code;
    private String exam_name;
    private int exam_grade;

    public Exam(int code, String name, int grade) {

        exam_code = code;
        exam_name = name;
        exam_grade = grade;

    }

    @Override
    public int compareTo(Exam o) {
        // TODO Auto-generated method stub
        return 0;
    }

}

Answer 1

替换

System.out.println(es);

to

System.out.println(es.next());