Streams - Java 8 Lambdas - page 22

collect(toList())
NOTE
collect(toList()) is an eager operation that generates a list from the values in a
Stream .
The values in the Stream that are operated on are derived from the initial values and the re-
cipe produced by the sequence of Stream calls. In fact, collect is a very general and power-
ful construct, and we'll look into its other uses in more detail in Chapter 5 . Here's an ex-
ample of this operation:
List < String > collected = Stream . of ( "a" , "b" , "c" )
. collect ( Collectors . toList ());
assertEquals ( Arrays . asList ( "a" , "b" , "c" ), collected );
This example shows how collect(toList()) can be used to build a result list out of a
Stream . It's important to remember, as discussed in the previous section, that because many
Stream functions are lazy, you do need to use an eager operation such as collect at the end
of a sequence of chained method calls.
This example also shows the general format for all the examples in this section. It starts by
taking a Stream from a List . There is some operation, followed by collecting into a list
. Finally, we perform an assert to show you what the results are equal to .
You can think of the opening call to stream and the closing call to a collect or other ter-
minal method as bun methods. They aren't the actual filling of our stream burger, but they do
help us see where the operations begin and end.