How can I convert ArrayList<Object> to ArrayList?

Answer 1

With Java Generics Takes a list of X and returns a list of T that extends or implements X, Sweet!

    // the cast is is actually checked via the method API
@SuppressWarnings("unchecked")
public static <T extends X, X> ArrayList<T> convertToClazz(ArrayList<X> from, Class<X> inClazz, Class<T> outClazz) {
    ArrayList<T> to = new ArrayList<T>();
    for (X data : from) {
        to.add((T) data);
    }
    return to;
}

Answer 2

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.

Answer 3

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.

Answer 4

If you want to do it the dirty way, try this.

@SuppressWarnings("unchecked")
public ArrayList<String> convert(ArrayList<Object> a) {
   return (ArrayList) a;
}

Advantage: here you save time by not iterating over all objects.

Disadvantage: may produce a hole in your foot.

Answer 5

Here is another alternative using Guava

List<Object> lst ...    
List<String> ls = Lists.transform(lst, Functions.toStringFunction());

Answer 6

Using Java 8 lambda:

ArrayList<Object> obj = new ArrayList<>();
obj.add(1);
obj.add("Java");
obj.add(3.14);

ArrayList<String> list = new ArrayList<>();
obj.forEach((xx) -> list.add(String.valueOf(xx)));