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Modern Java in Action: learn all the things Java can do


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The release of Java 9 builds on what made Java 8 so exciting. In addition to Java 8's lambdas and streams, Java 9 adds a host of new features of its own.
Manning's bestselling Java 8 book has been revised for Java 9! In Java 8 and 9 in Action, you'll build on your existing Java language skills with the newest features and techniques.

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Modern Java in Action: learn all the things Java can do

  1. 1. Creating and using an Observable with RxJava. Save 42% off Modern Java in Action with code slurma at
  2. 2. Both the Observable and Flowable classes come with tons of convenient factory methods to create many different types of reactive streams. The simplest Observable that you may want to create is one that is made of a fixed number of predetermined elements. An example of an observable that emits a fixed number of predetermined elements: Observables
  3. 3. The just factory method converts one or more elements into an Observable that emits those elements. Another Observable that is quite common, especially when time is a relevant factor in your application, is one that emits events at a fixed time rate. An example of an Observable that emits a sequence of ascending longs, one per second: Observables
  4. 4. The interval factory method returns in fact an Observable that emits an infinite sequence of ascending longs, with a constant interval of time of your choosing (one second In the former example) between emissions. We can try to use the Observable defined in the previous slide as the basis to create another Observable that emits the temperature detected in a given town each second. As an intermediate step to reach this goal, let's start with the simpler task of printing those temperatures each second. Observables
  5. 5. To track our temperatures by the second, we need to subscribe to an Observable in order to be notified by it every time a second is passed and then fetch and print the temperature of the town of our interest. In RxJava, the Observable plays the role of the Publisher in the Flow API, so the Observer corresponds to the Flow's Subscriber interface. The Observer Interface
  6. 6. With RxJava’s API it is possible to subscribe an Observable with an Observer that implements only the onSubscribe method with a consumer of the received event, leaving the others undefined. By leveraging this feature it is possible to subscribe to the Observable defined in the listing and print the temperatures in New York once a second. Here’s what it looks like: Subscribing to an Observable
  7. 7. There is an issue with the statement on the previous slide - the Observable emits one event per second and then the subscriber fetches the temperature in New York every second and prints it. Indeed, if you put this statement in a main and try to execute it you will see … nothing. This is because the Observable publishing an event per second is executed in a thread of RxJava's computation thread pool, and the main thread then terminates immediately. Subscribing to an Observable
  8. 8. We can prevent this immediate termination simply by putting a thread sleep after this statement from slide 6: Doing so might give us the following output: Troubleshooting our Observable
  9. 9. Now, since our Observer only implements the happy path and doesn't have any sort of error management, this will eventually blow up in our user's face. So let’s raise the bar a bit by adding error management, and generalizing what we have: we don't want to immediately print the temperatures, but provide to our user a factory method that returns an Observable emitting those temperatures once a second for at most 5 times before completing. Improving our Observable
  10. 10. We can do this by using another factory method, named create, that instances an Observable from a lambda taking another Observer as its argument and returning void. This will make our Observable function better – i.e. the way we want it to! On the next slide you can see our new Observable. Improving our Observable
  11. 11. Improving our Observable
  12. 12. In the previous slide we created the returned an Observable from a function that consumes an Observer sending to it the desired events. Internally we are subscribing on an Observable publishing an infinite sequence of ascending longs, one per second, in order to have a timer that emits an event each second. At this point it is easy to implement a complete Observer that subscribes to the Observable returned by this method and printing the temperatures published by it. Refining our Observable
  13. 13. Here’s our Observable printing out received temperatures: Our Observable in Action
  14. 14. We are now ready to create a main where we subscribe this Observer to the Observable returned by the method of slide 10 in order to print the temperatures in New York. Creating a main
  15. 15. It turned out to be necessary to add a sleep at the end of the main method because the Observable works in a thread of RxJava's pool and we need to give it time to perform its job. As long as there isn’t an error while fetching the temperatures, the main will print a line per second for 5 times and then the Observable will emit the complete signal, so we might get an output like the following: Creating a main
  16. 16. Let’s take our RxJava example a bit further to show how this library allows manipulation of one or more reactive streams. One of the main advantages of RxJava and other reactive libraries, while working with reactive streams (compared with what is offered by the native Java Flow API), is that they provide a rich toolbox of functions to combine, create, and filter any of those streams. Manipulating reactive streams
  17. 17. We’ve seen a stream getting used as an input to another one, but it is also possible to filter a stream to get another one that has only those elements you are interested in, transform those elements with a given mapping function, or even merge or combine two streams in many different ways. To overcome the problem of explaining function behavior in plain words –which often comes out rather cryptic– the reactive streams community documents functions using marble diagrams. Manipulating reactive streams
  18. 18. A marble diagram
  19. 19. Using this type of notation it's easy to provide a visual representation of the features of all of RxJava's functions, like map and merge (shown below) or a typical Observer like on the previous slide. A marble diagram
  20. 20. So how can we put these two functions to use with our thermometer example? First, let’s put map to work converting from Fahrenheit to Celsius, previously implemented using the Flow API's Processor. This simple method takes the Observable returned by our getTemperature from slide 10 and converts the temperatures to Celsius. Putting it all together
  21. 21. Now, let’s imagine that we need to emit temps for a group of towns, rather than one. We can perform the method from the previous slide for each town and combine the Observables together using the merge function. This method takes a varargs, containing the set of towns for which we are emitting temps, as an argument. Putting it all together
  22. 22. The varargs gets put into a Stream of String, and then each String is passed to the method from slide 18. Basically, each town is transformed into an Observable emitting a temperature every second. In the end, the Stream of Observables is collected into a List and the List is passed to the merge static factory method provided by the Observable class itself. This method takes an Iterable of Observables and combines their output so they act like a single Observable. Pretty cool! Putting it all together
  23. 23. Let's also try to put this last method at work by using it in one last main class. Putting it all together
  24. 24. You might have noticed that this last main class is nearly identical to the one from slide 13, but this one is now subscribing to the Observable returned by the method in slide 19 and printing the temperatures registered in 3 different towns. When run, it should give an output like this: Putting it all together
  25. 25. As you can see, each second it prints the temperatures of each of the requested towns until one of the temperature fetching operations raises an error that is propagated to the Observer, thus interrupting the stream of data. Now, let’s use what we’ve learned for a little quiz… Let’s see how we can manipulate the elements emitted by an Observable using another transforming function. Putting it all together
  26. 26. QUIZ -Filtering only negative temperatures The filter method of the Observable class takes a Predicate as argument and produces a second Observable that emits only the elements that pass the test defined by that Predicate. Supposing that you have been asked to develop a warning system that alerts the user when there is risk of snow, how can you use this operator to create an Observable that emits the temperature in degrees Celsius registered in a town, only when it is below zero? Quiz time!
  27. 27. Answer: It's enough to take the Observable returned by the method in slide 18 and apply to it the filter operator with a Predicate function that accepts only negative temperature as it follows: Quiz time!
  28. 28. That’s all for now! Hopefully, you’ve learned a bit and had some fun doing it. Don’t forget to save 42% off Modern Java in Action with code slurma at Also see: