Lambda-Enabled Concurrency - Java 8 Lambdas - page 164

of declarative programming that lets us program in terms of changes and data flows that get
automatically propagated.

You can think of a spreadsheet as a commonly used example of reactive programming. If you
enter =B1+5 in cell C1 , it tells the spreadsheet to add 5 to the contents of cell B1 and put the
result in C1 . In addition, the spreadsheet reacts to any future changes in B1 and updates the
value in C1 .

The RxJava library is a port of these reactive ideas onto the JVM. We won't be going into the
library in huge amounts of depth here, just covering the key concepts.

RxJava introduces a class called Observable that represents a sequence of events that you
can react to. It's an IOU for a sequence. There is a strong connection between an Observ-
able and the Stream interface that we encountered in Chapter 3 .

In both cases we build up recipes for performing work by chaining higher-order functions
and use lambda expressions in order to associate behavior with these general operations. In
fact, many of the operations defined on an Observable are the same as on a Stream : map ,
filter , reduce .

The big difference between the approaches is the use case. Streams are designed to build up
computational workflows over in-memory collections. RxJava, on the other hand, is de-
signed to compose and sequence asynchronous and event-based systems. Instead of pulling
data out, it gets pushed in. Another way of thinking about RxJava is that it is to a sequence of
values what a CompletableFuture is to a single value.

Our concrete example this time around is searching for an artist and is shown in
Example 9-15 . The search method filters the results by name and nationality. It keeps a loc-
al cache of artist names but must look up other artist information, such as nationality, from
external services.

Example 9-15. Searching for an artist by name and nationality
public

public Observable < Artist > search ( String searchedName ,
String searchedNationality ,
int int maxResults ) {

return return getSavedArtists ()

. filter ( name -> name . contains ( searchedName ))
. flatMap ( this

this :: lookupArtist )
. filter ( artist -> artist . getNationality ()
. contains ( searchedNationality ))