Java ArrayList, access method to return ArrayList of abstract class

Question

I have an ArrayList containing objects derived from an abstract class, is there a way to return the an array list of the abstract class type then, using polymorphism, be able to reference the origin object?

The only way I can see of doing this is to create an ArrayList of the abstract class type, then copy the elements from the original ArrayList into this, but I don't want copies.

Post your code, and tell us precisely what you would like to do that is not possible with what you have. — JB Nizet
– JB Nizet, Commented Nov 6, 2015 at 15:21

Jeff Morin · Accepted Answer · 2015-11-06 15:33:32Z

2

Let's suppose your concrete class is Integer and your abstract class, Number.

If you are willing to receive a result cast as a List<Number> instead of an ArrayList<Number> (a good practice anyway), you can do something like the following:

List<Integer> listOfConcrete = new ArrayList<>(); // your actual list
List<Number> listOfAbstract = listOfConcrete.stream()
  .map(n -> (Number) n)
  .collect(Collectors.toList());

You need the explicit cast in the map() call so that the casting is performed properly.

edited Nov 6, 2015 at 15:33

answered Nov 6, 2015 at 15:22

Jeff Morin

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3 Comments

SPlatten Over a year ago

Wow, that was quick, I used your example and it works great, my method looks like this: public ArrayList<clsPoll> alobjGetPollSchedule() { retrurn (ArrayList<clsPoll>)malobjPollSchedule.stream().map(n -> (clsPoll)n).collect(Collectors.toList()); }

Holger Over a year ago

No, you don’t need the explicit cast in map. Actually, you don’t need the map step at all. listOfAbstract = listOfConcrete.stream().collect(Collectors.toList()); is sufficient. But you don’t even need Java 8 to copy a list: List<Number> listOfAbstract = new ArrayList<>(listOfConcrete); works as well.

JB Nizet Over a year ago

And if the goal is to consume the List<Integer> as numbers, maybe all is needed is List<? extends Number> listOfAbstract = listOfConcrete;, which doesn't need any copy. But the OP should really clarify, so that we know.

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