Collect list of Long from Double stream in Java 8

Question

I have the following code:

List<Long> list = new ArrayList<>(); list.add(4L); list.add(92L); list.add(100L); List<Long> newList = list.stream().map(i -> i * 2.5)                                   .mapToLong(Double::doubleToRawLongBits)                                   .collect(Collectors.toList()); 

This code doesn't work and the compilation error is:

method collect in interface java.util.stream.LongStream cannot be applied to given types;
required: java.util.function.Supplier<R>,java.util.function.ObjLongConsumer<R>,java.util.function.BiConsumer<R,R>
found: java.util.stream.Collector<java.lang.Object,capture#1 of ?,java.util.List<java.lang.Object>>
reason: cannot infer type-variable(s) R (actual and formal argument lists differ in length)

I have tried many usages of Collectors but I still can't make it to work. What am I doing wrong?

Answer 1

mapToLong gives you a LongStream which is not able to be collect-ed by Collectors.toList.

This is because LongStream is

A sequence of primitive long-valued elements

We can't have a List<long>, we need a List<Long>. Therefore to be able to collect them we first need to box these primitive longs into Long objects:

list.stream().map(i -> i * 2.5)     .mapToLong(Double::doubleToRawLongBits)     .boxed()                                //< I added this line     .collect(Collectors.toList()); 

The boxed method gives us a Stream<Long> which we're able to collect to a list.

Using map rather than mapToLong will also work because that will result in a Steam<Long> where the values are automatically boxed:

list.stream().map(i -> i * 2.5)     .map(Double::doubleToRawLongBits)     .collect(Collectors.toList());