我们知道TreeSet可以实现Set元素的排序功能，但是要求元素的类型是可比较的(Comparable)，也就是元素类型必须实现Comparable接口。

例如：

class Person {

    private String name;
    private String gender;
    private int age;

    public Person() {
    }

    public Person(String name, String gender, int age) {
        this.name = name;
        this.gender = gender;
        this.age = 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 boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Person person = (Person) o;
        return age == person.age &&
                Objects.equals(name, person.name) &&
                Objects.equals(gender, person.gender);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, gender, age);
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", gender='" + gender + '\'' +
                ", age=" + age +
                '}';
    }


}

public class Test {

    public static void main(String[] args) {
        Set set = new TreeSet();
        set.add(new Person("张三","男",18));
        set.add(new Person("李四","女",20));
        set.add(new Person("王五","男",19));
        set.add(new Person("张三","男",18));


        Iterator it = set.iterator();
        while(it.hasNext()){
            System.out.println(it.next());
        }

    }
}

运行时抛出以下异常：
Exception in thread "main" java.lang.ClassCastException: setdemo.Person cannot be cast to java.lang.Comparable

改写Person类，实现Comparable接口，如下所示：

class Person implements Comparable<Person>{

    private String name;
    private String gender;
    private int age;

    public Person() {
    }

    public Person(String name, String gender, int age) {
        this.name = name;
        this.gender = gender;
        this.age = 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 boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Person person = (Person) o;
        return age == person.age &&
                Objects.equals(name, person.name) &&
                Objects.equals(gender, person.gender);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, gender, age);
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", gender='" + gender + '\'' +
                ", age=" + age +
                '}';
    }

    @Override
    public int compareTo(Person p) {

        return this.getAge()-p.getAge(); //按照年龄实现升序排序
    }
}

public class Test {

    public static void main(String[] args) {
        Set set = new TreeSet();
        set.add(new Person("张三","男",18));
        set.add(new Person("李四","女",20));
        set.add(new Person("王五","男",19));
        set.add(new Person("张三","男",18));


        Iterator it = set.iterator();
        while(it.hasNext()){
            System.out.println(it.next());
        }

    }
}

运行结果：
Person{name='张三', gender='男', age=18}
Person{name='王五', gender='男', age=19}
Person{name='李四', gender='女', age=20}

小结：

1)Set集合可以通过TreeSet实现类实现内部元素的排序,但是要求元素类型必须是可比较的，也就是必须实现Comparable接口。

2)因为Collections.sort()要求参数类型必须是List,因此Set不能使用Collections.sort()排序。