放入TreeSet集合中的元素必须实现Comparable接口,不然会报错,因为这个集合中的元素会自动按元素的大小顺序排序,所以不是实现比较的接口就会出现ClassCastException。
还要注意一点的是Set集合中的元素是不可重读的,我们写的Comparable接口的实现方法如果返回的是0,就会认为这两个元素是相等的,也就是只能放进去一个,另一个放不进去。
测试代码
public static void main(String[] args) throws FileNotFoundException {
TreeSet set = new TreeSet();
set.add(new Student("zhangsan",12));
set.add(new Student("lisi",85));
set.add(new Student("lisi2",85));
set.add(new Student("wangwu",42));
set.add(new Student("zhaoliu",45));
for (Object o : set) {
System.out.println(o);
}
}
实体类
class Student implements Comparable<Student>{
String name;
int age;
@Override
public String toString() {
return "Student{" +
"name='" + name + '\'' +
", age=" + age +
'}';
}
public Student() {
}
public Student(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
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;
Student student = (Student) o;
return age == student.age && Objects.equals(name, student.name);
}
@Override
public int hashCode() {
return Objects.hash(name, age);
}
@Override
public int compareTo(Student o) {
return o.age-this.age;
}
}