<dependency> 
 <groupId>org.springframework.boot</groupId> 
 <artifactId>spring-boot-devtools</artifactId> 
 <scope>runtime</scope> 
</dependency>

public class TreeSetTest
{
  public static void main(String[] args)
  {
    MyComparator comparator = new MyComparator(); 

  // TreeSet<Student> set = new TreeSet<Student>(comparator);
  // 錯誤的代碼，少了比較器，運(yùn)行則報下面的異常。
    TreeSet<Student> set = new TreeSet<Student>();

    Student s1 = new Student(50);
    Student s2 = new Student(70);
    Student s3 = new Student(40);

    set.add(s1);
    set.add(s2);
    set.add(s3);

    System.out.println(set);
  }
}

class Student 
{
  int score;

  public Student(int score)
  {
    this.score = score;
  }
  @Override
  public String toString()
  {
    // TODO Auto-generated method stub

    return String.valueOf(this.score);
  }
}
class MyComparator implements Comparator<Student>
{

  @Override
  //按分?jǐn)?shù)高低比較，int為返回負(fù)數(shù)、零、整數(shù)，這里我寫的不咋好，但意思一樣
  public int compare(Student o1, Student o2)
  {
    // TODO Auto-generated method stub
    int result = 0;
    if(o1.score > o2.score)
    {
      result = 1;
    }else
    {
      result = -1;
    }

    return result;
  }
}

Exception in thread "main" java.lang.ClassCastException: com.shengsiyuan2.Student 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 com.shengsiyuan2.TreeSetTest.main(TreeSetTest.java:17)