import java.util.*;
class Test
{
public static void diedai(Collection c)
{
  Iterator i=c.iterator();
  while(i.hasNext())
  {
   System.out.println(i.next());
  }
}
public static void main(String[] args)
{
  Student s1=new Student(2,"zhangsan");
  Student s2=new Student(3,"lisi");
  Student s3=new Student(1,"wangwu");
  ArrayList<Object> a=new ArrayList<Object>();
  a.add(s1);
  a.add(s2);
  a.add(s3);
  Collections.sort(a);
  diedai(a);  

  }
}

class Student implements Comparable
{
int num;
String name;
Student(int num,String name)
{
  this.num=num;
  this.name=name;
}
public int compareTo(Object o)
{
  Student s=(Student)o;
  return num>s.num ? 1 : (num==s.num ? 0 : -1);
}
public String toString()
{
  return "num="+num+","+"name="+name;
}
}
运行时出现出现了Collections.sort(a)找不到符号
把ArrayList<Object>a1=new ArrayList<Object>();
这句换成
ArrayList<Student>a1=new ArrayList<Student>();就出现了使用了未经检查或不安全的操作

1. import java.util.*;
   
2. 
   
3. class Test {
   
4. public static void diedai(Collection<Student> c) {
   
5. Iterator<Student> i = c.iterator();
   
6. while (i.hasNext()) {
   
7. System.out.println(i.next());
   
8. }
   
9. }
   
10. 
    
11. public static void main(String[] args) {
    
12. Student s1 = new Student(2, "zhangsan");
    
13. Student s2 = new Student(3, "lisi");
    
14. Student s3 = new Student(1, "wangwu");
    
15. ArrayList<Student> a = new ArrayList<Student>();
    
16. a.add(s1);
    
17. a.add(s2);
    
18. a.add(s3);
    
19. Collections.sort(a);
    
20. diedai(a);
    
21. 
    
22. }
    
23. }
    
24. 
    
25. class Student implements Comparable<Student> {
    
26. int num;
    
27. String name;
    
28. 
    
29. Student(int num, String name) {
    
30. this.num = num;
    
31. this.name = name;
    
32. }
    
33. 
    
34. public int compareTo(Student o) {
    
35. Student s = o;
    
36. return num > s.num ? 1 : (num == s.num ? 0 : -1);
    
37. }
    
38. 
    
39. public String toString() {
    
40. return "num=" + num + "," + "name=" + name;
    
41. }
    
42. }
    

复制代码

你的泛型写少了.

第一个问题：因为类型参数是Object，而Object类没有实现Comparable接口，自身没有可比较性，所以Collections.sort(a)会出错
第二个问题：class Student implements Comparable未定义参数类型，需写成 class Student implements Comparable<Student>,然后compareTo方法中的参数Object换成Student