根据元素类实现了Comparable接口之后重写了compareTo方法的返回值是正数、负数、0三种值来排序：
如果是正数就代表非重复元素，升序添加到集合，如果将返回值设置为常数 1，就会按照添加顺序，可重复的添加到集合中；
如果是 0 就代表 重复元素，不会添加到集合，如果将返回值设置为常数 0，只会添加第一个元素；
如果是负数就代表非重复元素，降序添加到集合，如果将返回值设置为常数-1，就会按照添加逆序，可重复的添加到集合中。

        public int compareTo(Student student) {
            int num;
            //首要条件 根据年龄升序排列
            num = this.age - student.age;
            //次要条件 根据姓名升序排列
            int condition =  num == 0 ? this.name.compareTo(student.name):num;
            return condition;
        }
    }
    /**
    *   this 就是 当前的元素， student是集合里要比较的元素
    */

Eg:

    import java.util.Iterator;
    import java.util.TreeSet;
    
    public class Student implements Comparable<Student> {
        private int age;
        private String name;
    
        public Student(int age, String name) {
            this.age = age;
            this.name = name;
        }
    
        public int getAge() {
            return age;
        }
    
        public void setAge(int age) {
            this.age = age;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        @Override
        public boolean equals(Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }
    
            Student student = (Student) o;
    
            if (age != student.age) {
                return false;
            }
            return name.equals(student.name);
        }
    
        @Override
        public String toString() {
            return "Student{" +
                    "age=" + age +
                    ", name='" + name + '\'' +
                    '}';
        }
    
        @Override
        public int hashCode() {
            int result = age;
            result = 31 * result + name.hashCode();
            return result;
        }
    
        public int compareTo(Student student) {
            int num;
            //首要条件 根据年龄升序排列
            num = this.age - student.age;
            //次要条件 根据姓名升序排列
            int condition =  num == 0 ? this.name.compareTo(student.name):num;
            return condition;
        }
    }
    
    class Test {
        public static void main(String[] args) {
            TreeSet<Student> set = new TreeSet<Student>();
            set.add(new Student(16,"小芳"));
            set.add(new Student(16,"小芳"));
            set.add(new Student(18,"小微"));
    
            Iterator<Student> iterator = set.iterator();
            while(iterator.hasNext()){
                Student next = iterator.next();
                System.out.println("next = " + next);
            }
        }
    }