How to get the size of a Stream after applying a filter by lambda expression?

Question

Consider the following code:

List<Locale> locales = Arrays.asList(     new Locale("en", "US"),     new Locale("ar"),     new Locale("en", "GB") );   locales.stream().filter(l -> l.getLanguage() == "en"); 

How do I get the size of the locales ArrayList after applying filter, given that locales.size() gives me the size before applying filter?

Answer 1

When you get a stream from the list, it doesn't modify the list. If you want to get the size of the stream after the filtering, you call count() on it.

long sizeAfterFilter =      locales.stream().filter(l -> l.getLanguage().equals("en")).count(); 

If you want to get a new list, just call .collect(toList()) on the resulting stream. If you are not worried about modifying the list in place, you can simply use removeIf on the List.

locales.removeIf(l -> !l.getLanguage().equals("en")); 

Note that Arrays.asList returns a fixed-size list so it'll throw an exception but you can wrap it in an ArrayList, or simply collect the content of the filtered stream in a List (resp. ArrayList) using Collectors.toList()(resp. Collectors.toCollection(ArrayList::new)).