To begin with this story, let’s first have a look at how to creat a List from Stream in Java
List<String> sublist = list .stream() .filter(...) .collect(Collectors.toList());
This works perfectly fine but what if we want the list to be immutable? We could do this
List<String> immutableSubList = Collections.unmodifiableList(sublist);
or if we would like to use Guava ImmutableList, we could do
ImmutableList<String> immutableSubList = ImmutableList.copyOf(sublist);
However this is a bit awkward to use since the list will be copied one more time. If we want to do this in a lot of places throughout the code base, it is not fluid. Instead, what we want is
ImmutableList<String> sublist = list .stream() .filter(...) .collect(ImmutableCollectors.toList());
This post will discuss how to create the Collector of ImmutableList.
Collector
To create a Collector, we will use the static method of.
public static<t, A, R> Collector<T, A, R> of( Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner, Function<A, R> finisher, Characteristics... characteristics);