TreeSet

Posted 2020-11-21 inspred

一.TreeSet可以对元素进行排序

前提是元素要具备比较功能

方式一：元素自己具备比较功能，实现Comparable接口，重写compareTo()方法

/**
 * 排序：
 * 先按年龄排序，年龄相同按姓名排序
 * TreeSet可以对Set集合中元素进行排序，
 * TreeSet判断元素唯一的方式：根据比较方法的返回结果是否是0。是0代表相同，就不存
 */
public class Student implements Comparable{

    private String name;

    private Integer age;

    public Student(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

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

    public Integer getAge() {
        return age;
    }

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

    @Override
    public int compareTo(Object o) {

        if (!(o instanceof Person)){
            //如果类型不能转换,不用比较了
            throw new ClassCastException("类型错误");
        }
        Student student = (Student) o;

        if (this.age < student.age){
            return 1;
        }else if(this.age < student.age){
            return -1;
        }else {
           return this.name.compareTo(student.name);
        }
    }
}

测试：

List<Student> list = new ArrayList<>();
        list.add(new Student("ac",12));
        list.add(new Student("ab",13));
        Collections.sort(list);

 

方式二：外部比较器，让集合具备排序功能：实现comparator接口，重写compare()方法

 

/**
 *
 * 创建一个根据person类的name进行排序的比较器
 */
public class ComparatorByName implements Comparator{
    @Override
    public int compare(Object o1, Object o2) {
        Student person1 = (Student) o1;
        Student person2 = (Student) o2;
        int temp = person1.getName().compareTo(person2.getName());

        return temp ==0?person1.getAge()-person2.getAge():temp;
    }
}

 

 

 

测试：

List<Student> list = new ArrayList<>();
        list.add(new Student("ac",12));
        list.add(new Student("ab",13));
        Collections.sort(list,new ComparatorByName());