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

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