Converting List to Map

I’m trying to convert a list to map with this code and I’ve got an error which is

Type mismatch: cannot convert from Map<Object,Object> to Map<String,String>

In class Node I have 2 properties which are Strings, why should I put them like object and not like String??

public static void main (String ...strings){
    List<Node> test = new ArrayList<>();
    test.add(new Node("test1","test1"));
    test.add(new Node("test3","test4"));
    test.add(new Node("test5","test6"));
    test.add(new Node("test7","test8"));
    Map <String, String> map2 = -> x.getIndex() , x-> x.getValue()));