Unchecked or insafe operation? compilation error

https://online.codingblocks.com/app/player/147318/content/141389/4873
what is the complilation error?
import java.util.*;

public class Main {
public static void main(String[] args) {
Scanner s=new Scanner(System.in);
System.out.println(“enter:”);
int res=0;
String str=s.nextLine();
List myList = new ArrayList<>();
for(int i=0;i<str.length();i++) {
myList.add(i,str.charAt(i));
}

	 for(int i=0;i<myList.size()-1;i++) {
		   
			 if(myList.get(i)==myList.get(i+1))
			 {    
				  res=i;
				  myList.add(res+1,"*");
				
			 }
			 
	 }   
	System.out.println(myList);
	 
	
		       
	 
	
		
}