Instantiate ArrayList with map from another ArrayList

Question

I have a current ArrayList of type A and I want to create a new list of all the object in this list that is of type B where B is a sub-class of A. Is there a way to do this using a map()?

ArrayList<B> allBs = allAs.stream().map( b -> where b instanceof B)

That would maybe look something like this?

GBlodgett · Accepted Answer · 2019-01-29 18:25:28Z

3

You can do this with the filter function:

List<B> allBs = allAs.stream()
                     .filter(B.class::isInstance)  
                     .map(B.class::cast)
                     .collect(Collectors.toList());

Which will filter out any elements that don't match the given predicate, cast them to B objects, and then collect it to a new List.

Also note that I changed ArrayList<B> allBs to List<B> allBs, because it is good practice to program to the interface

edited Jan 29, 2019 at 18:25

answered Jan 26, 2019 at 17:51

GBlodgett

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2 Comments

Hdot Over a year ago

Im unable to create allBs as type B, it only works if I make the list of type A. The error I get is: Required List B, but 'collect' was inferred to R

GBlodgett Over a year ago

@Hdot Sorry fixed

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