Exception in thread "main" java.lang.ClassCastException:


package TreeSet;

import java.util.TreeSet;


public class mytreeset{
    public static void main(String[] args) {
        Students stu1 = new Students("aaa",25);
        Students stu2 = new Students("ccc",34);
        Students stu3 = new Students("bbb", 23);
        TreeSet ts =new TreeSet<>();
        ts.add(stu1);
        ts.add(stu2);
        ts.add(stu3);
        System.out.println(ts);



    }
}
class Students{
    private String name;
    private int age;

    public int compareTo(Students o){
        return this.getAge()-o.getAge();
    }
    public Students(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;
    }
}

E:\java8\bin\java.exe "-javaagent:D:\IntelliJ IDEA 2021.1\lib\idea_rt.jar=13522:D:\IntelliJ IDEA 2021.1\bin" -Dfile.encoding=UTF-8 -classpath E:\java8\jre\lib\charsets.jar;E:\java8\jre\lib\deploy.jar;E:\java8\jre\lib\ext\access-bridge-64.jar;E:\java8\jre\lib\ext\cldrdata.jar;E:\java8\jre\lib\ext\dnsns.jar;E:\java8\jre\lib\ext\jaccess.jar;E:\java8\jre\lib\ext\jfxrt.jar;E:\java8\jre\lib\ext\localedata.jar;E:\java8\jre\lib\ext\nashorn.jar;E:\java8\jre\lib\ext\sunec.jar;E:\java8\jre\lib\ext\sunjce_provider.jar;E:\java8\jre\lib\ext\sunmscapi.jar;E:\java8\jre\lib\ext\sunpkcs11.jar;E:\java8\jre\lib\ext\zipfs.jar;E:\java8\jre\lib\javaws.jar;E:\java8\jre\lib\jce.jar;E:\java8\jre\lib\jfr.jar;E:\java8\jre\lib\jfxswt.jar;E:\java8\jre\lib\jsse.jar;E:\java8\jre\lib\management-agent.jar;E:\java8\jre\lib\plugin.jar;E:\java8\jre\lib\resources.jar;E:\java8\jre\lib\rt.jar;E:\javacode\IDEA_java_Project\Regex\out\production\Regex TreeSet.mytreeset
Exception in thread "main" java.lang.ClassCastException: TreeSet.Students cannot be cast to java.lang.Comparable
    at java.util.TreeMap.compare(TreeMap.java:1294)
    at java.util.TreeMap.put(TreeMap.java:538)
    at java.util.TreeSet.add(TreeSet.java:255)
    at TreeSet.mytreeset.main(mytreeset.java:15)

为什么会报错？怎么改？

需要在 Students 类中实现 Comparable 接口，重写其中的 compareTo 方法即可

class Students implements Comparable<Students>

你Students类重写了compareTo方法，但你未在类上实现Comparable接口，Students 类定义修改为：
class Students implements Comparable

没有实现 Comparable 接口

  • 命名类名大写

  • 没实现toString

  • 集合遍历打印

public class Mytreeset{
    public static void main(String[] args) {
        Student stu1 = new Student("aaa",25);
        Student stu2 = new Student("ccc",34);
        Student stu3 = new Student("bbb", 23);
        TreeSet<Student> ts =new TreeSet<>();
        ts.add(stu1);
        ts.add(stu2);
        ts.add(stu3);
        ts.forEach(System.out::println);

    }
}
class Student implements Comparable{
    private String name;
    private int age;

    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 int compareTo(Object o) {
         if (o instanceof Student){
             return this.getAge()-((Student) o).getAge();
         }
         throw new RuntimeException("cannot cast");
    }

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