How can I convert ArrayList<Object> to ArrayList<String>?

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))
   .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.

It's not safe to do that!
Imagine if you had:

ArrayList<Object> list = new ArrayList<Object>();
list.add(new Employee("Jonh"));
list.add(new Car("BMW","M3"));
list.add(new Chocolate("Twix"));

It wouldn't make sense to convert the list of those Objects to any type.

You can use wildcard to do this as following

ArrayList<String> strList = (ArrayList<String>)(ArrayList<?>)(list);

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