java编译时出现使用了未经检查或不安全的操作解决方法

import java.util.*;
class collection{
    public static void main(String[] args) {
        Collection c1=new ArrayList(25);

        c1.add(new String("one"));
        c1.add(new String("two"));
        String s="three";
        c1.add(s);
        for (Iterator i=c1.iterator();i.hasNext();)
        {

            System.out.println(i.next());
        }
    }
}

List temp = new ArrayList ();
temp.add("1");
temp.add("2");

List <String> temp = new ArrayList <String> ();
temp.add("1");
temp.add("2");

import java.util.*;
class collection{
    public static void main(String[] args) {
        Collection<String> c1=new ArrayList<String>(25);

        c1.add(new String("one"));
        c1.add(new String("two"));
        String s="three";
        c1.add(s);
        for (Iterator i=c1.iterator();i.hasNext();)
        {

            System.out.println(i.next());
        }
    }
}