How to cast ArrayList<> from List<>

When you do the second one, you're making a new arraylist, you're not trying to pretend the other list is an arraylist.

I mean, what if the original list is implemented as a linkedlist, or some custom list? You won't know. The second approach is preferred if you really need to make an arraylist from the result. But you can just leave it as a list, that's one of the best advantages of using Interfaces!

When you are using second approach you are initializing arraylist with its predefined values. Like generally we do ArrayList listofStrings = new ArrayList<>(); Let's say you have an array with values, now you want to convert this array into arraylist.

you need to first get the list from the array using Arrays utils. Because the ArrayList is concrete type that implement List interface. It is not guaranteed that method asList, will return this type of implementation.

List<String>  listofOptions = (List<String>) Arrays.asList(options);

then you can user constructoru of an arraylist to instantiate with predefined values.

ArrayList<String> arrlistofOptions = new ArrayList<String>(list);

So your second approach is working that you have passed values which will intantiate arraylist with the list elements.

More over

ArrayList that is returned from Arrays.asList is not an actual arraylist, it is just a wrapper which doesnt allows any modification in the list. If you try to add or remove over Arrays.asList it will give you

UnsupportedOperationException

Try running the following code:

List<String> listOfString = Arrays.asList("Hello", "World");
ArrayList<String> arrayListOfString = new ArrayList(listOfString);
System.out.println(listOfString.getClass());
System.out.println(arrayListOfString.getClass());

You'll get the following result:

class java.util.Arrays$ArrayList
class java.util.ArrayList

So, that means they're 2 different classes that aren't extending each other. java.util.Arrays$ArrayList signifies the private class named ArrayList (inner class of Arrays class) and java.util.ArrayList signifies the public class named ArrayList. Thus, casting from java.util.Arrays$ArrayList to java.util.ArrayList and vice versa are irrelevant/not available.

The second approach is clearly wrong if you want to cast. It instantiate a new ArrayList.

However the first approach should work just fine, if and only if getAllTasks return an ArrayList.

It is really needed for you to have an ArrayList ? isn't the List interface enough ? What you are doing can leads to Runtime Exception if the type isn't correct.

If getAllTasks() return an ArrayList you should change the return type in the class definition and then you won't need a cast and if it's returning something else, you can't cast to ArrayList.