Question

ArrayList<Object> list = new ArrayList<Object>();
list.add(1);
list.add("Java");
list.add(3.14);
System.out.println(list.toString());

I tried:

ArrayList<String> list2 = (String)list; 

But it gave me a compile error.

Answer 1

Since this is actually not a list of strings, the easiest way is to loop over it and convert each item into a new list of strings yourself:

List<String> strings = list.stream()
   .map(object -> Objects.toString(object, null))
   .toList();

Or when you’re not on Java 16 yet:

List<String> strings = list.stream()
   .map(object -> Objects.toString(object, null))
   .collect(Collectors.toList());

Or when you’re not on Java 8 yet:

List<String> strings = new ArrayList<>(list.size());
for (Object object : list) {
    strings.add(Objects.toString(object, null));
}

Or when you’re not on Java 7 yet:

List<String> strings = new ArrayList<String>(list.size());
for (Object object : list) {
    strings.add(object != null ? object.toString() : null);
}

Note that you should be declaring against the interface (java.util.List in this case), not the implementation.