Convert List<Object[]> to list<String> Cast exception

Question

I have a list with an array of objects (strings) :

 List<Object[]> params  = ...

I want to cast this list to a list of strings List

  List<Object> results = params.stream()
                            .flatMap(arr -> Arrays.stream(arr))
                            .collect(Collectors.toList());

Then I want to do something like that :

results.forEach(System.out::println);

I get this exception :

java.lang.ClassCastException: class java.lang.String cannot be cast to class [Ljava.lang.Object; (java.lang.String and [Ljava.lang.Object; are in module java.base of loader 'bootstrap')

This how I create the params :

 Query query = entityManager.createNativeQuery(sql);
 List<Object[]> params = query.getResultList();;

Answer 1

• Stream the arrays
• flatten the arrays into a stream of objects.
• filter out non strings.
• cast via a map remaining objects to strings
• collect into a List

List<Object> params = List.of(1, 2,
                   "hello",
                   "computer",  new Object(), 4.32,
                   new Object[]{1,2});

List<String> strings = params.stream()
        .filter(ob -> ob instanceof String)
        .map(ob -> (String) ob)
        .collect(Collectors.toList(), "java");

System.out.println(strings);

Prints

[hello, computer, java]