ArrayList implementation methods

Question

I have a function which prints the strings which are anagrams of each other using the List interface in Java. This function return type cannot be changed(its from a coding interview archive).

public List<List<String>> Anagrams(String[] string_list) {
        // Code here
        ArrayList<ArrayList<String>> anagrams=new ArrayList<ArrayList<String>>();
        String l[]=new String[string_list.length];
        for(int i=0;i<l.length;i++)
        {
            char c[]=string_list[i].toCharArray();
            Arrays.sort(c);
            l[i]=new String(c);
        }
        HashMap<String,ArrayList<String>> hash=new HashMap<String,ArrayList<String>>();
        for(int i=0;i<l.length;i++)
        {
            //check if anagram list exists
            ArrayList<String> list=hash.get(l[i]);
            //anagram present
            if(list!=null)
            {
                list.add(string_list[i]);
            }
            else
            {
                ArrayList<String> x=new ArrayList<String>();
                x.add(string_list[i]);
                hash.put(l[i],x);
            }
        }
        for(Map.Entry element:hash.entrySet())
        {
            ArrayList<String> value=(ArrayList<String>)element.getValue();//unchecked cast
            anagrams.add(value);
        }
        return anagrams;
    }

The code is not able to run because it not able to implement the List interface using ArrayList. I face the following error:

error: incompatible types: ArrayList<ArrayList<String>> cannot be converted to List<List<String>>
        return anagrams;

What am I doing incorrectly in this program? I must return adhere to the function template.