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Question

为什么老是抛出异常 ，那儿错了么？？？
import java.util.*;
import java.util.Collection;
import java.util.TreeSet;public class as implements Comparable{
public static void main(String[] agrs){

Collection<Student> treeset = new TreeSet<Student>();
    Student s1=new Student(12345,"John","Smith");
Student s2=new Student(23456,"Jane","Smith");
Student s3=new Student(34567,"Adam","Smith");
Student s4=new Student(45678,"Tom","Jones");
treeset.add(s1);
treeset.add(s2);
treeset.add(s3);
treeset.add(s4);
for(Student student:treeset){
System.out.println(student.getFullname());
}
}

//实现Comparable接口的方法
public int compareTo(Object o){
assert((o instanceof Student)&&(o!=null));
Student s=(Student)o;
int relativeValue=s.LastName.compareTo(s.getLastName());
if(relativeValue==0){
relativeValue=s.FirstName.compareTo(s.getFirstName());
}
return relativeValue;

}}
//student类public class Student {
private long ID;
String FirstName;
    String LastName;
    public Student(long id,String fn,String ln){
     ID=id;
     FirstName=fn;
     LastName=ln;
     }
    public String getFirstName(){
     return FirstName;
    }
    public String getLastName(){
     return LastName;
    }
    public String getFullname(){
     return FirstName+" "+LastName;
    }
}

Answer 1

要实现Comparable接口的是类Student， 而不是as, 修改了一下：import java.util.*;
import java.util.Collection;
import java.util.TreeSet;public class as {    public static void main(String[] agrs) {        Collection<Student> treeset = new TreeSet<Student>();
        Student s1 = new Student(12345, "John", "Smith");
        Student s2 = new Student(23456, "Jane", "Smith");
        Student s3 = new Student(34567, "Adam", "Smith");
        Student s4 = new Student(45678, "Tom", "Jones");
        treeset.add(s1);
        treeset.add(s2);
        treeset.add(s3);
        treeset.add(s4);
        for (Student student : treeset) {
            System.out.println(student.getFullname());
        }
    }}// student类class Student implements Comparable {
    private long ID;
    String       FirstName;
    String       LastName;    public Student(long id, String fn, String ln) {
        ID = id;
        FirstName = fn;
        LastName = ln;
    }    public String getFirstName() {
        return FirstName;
    }    public String getLastName() {
        return LastName;
    }    public String getFullname() {
        return FirstName + " " + LastName;
    }    // 实现Comparable接口的方法
    public int compareTo(Object o) {
        assert ((o instanceof Student) && (o != null));        Student s = (Student) o;
        int relativeValue = this.LastName.compareTo(s.getLastName());
        if (relativeValue == 0) {
            relativeValue = this.FirstName.compareTo(s.getFirstName());
        }        return relativeValue;
    }
}