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Getting started with rx-java

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This section provides a basic overview and superficial introduction to rx-java.

RxJava is a Java VM implementation of Reactive Extensions: a library for composing asynchronous and event-based programs by using observable sequences.

Learn more about RxJava on the Wiki Home.


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An introduction to RxJava

The core concepts of RxJava are its Observables and Subscribers. An Observable emits objects, while a Subscriber consumes them.


Observable is a class that implements the reactive design pattern. These Observables provide methods that allow consumers to subscribe to event changes. The event changes are triggered by the observable. There is no restriction to the number of subscribers that an Observable can have, or the number of objects that an Observable can emit.

Take for example:

Observable<Integer> integerObservable = Observable.just(1, 2, 3); // Integer observable
Observable<String> stringObservable = Observable.just("Hello, ", "World", "!"); // String observable

Here, an observable object called integerObservable and stringObservable are created from the factory method just provided by the Rx library. Notice that Observable is generic and can thus can emit any object.


A Subscriber is the consumer. A Subscriber can subscribe to only one observable. The Observable calls the onNext(), onCompleted(), and onError() methods of the Subscriber.

Subscriber<Integer> mSubscriber = new Subscriber<Integer>() {
        public void onCompleted() {
            // called when all objects are emitted
            System.out.println("onCompleted called!");

        public void onError(Throwable throwable) {
            // called when an error occurs during emitting objects
            System.out.println("onError called!");

        public void onNext(Integer integer) {
            // called for each object that is emitted
            System.out.println("onNext called with: " + integer);

Notice that Subscriber is also generic and can support any object. A Subscriber must subscribe to the observable by calling the subscribe method on the observable.


The above, when run, will produce the following output:

onNext called with: 1
onNext called with: 2
onNext called with: 3
onCompleted called!

Hello, World!

The following prints the message Hello, World! to console

public void hello() {
  Observable.just("Hello, World!") // create new observable
    .subscribe(new Action1<String>() { // subscribe and perform action

       public void call(String st) {


Or using Java 8 lambda notation

public void hello() {
      Observable.just("Hello, World!") // create new observable
        .subscribe(onNext -> { // subscribe and perform action

Installation or Setup

rx-java set up

  1. Gradle

    compile 'io.reactivex:rxjava2:rxjava:2.1.1'
  2. Maven

  3. Ivy

    <dependency org="io.reactivex.rxjava2" name="rxjava" rev="2.1.1" />
  4. Snapshots from JFrog

    repositories {
    maven { url '' }
    dependencies {
        compile 'io.reactivex:rxjava:2.0.0-SNAPSHOT'
  5. If you need to download the jars instead of using a build system, create a Maven pom file like this with the desired version:

    <?xml version="1.0"?>
    <project xmlns="" 
        <name>Simple POM to download rxjava and dependencies</name>

Then execute:

$ mvn -f download-rxjava-pom.xml dependency:copy-dependencies

That command downloads rxjava-*.jar and its dependencies into ./target/dependency/.

You need Java 6 or later.

Understanding Marble Diagrams

An Observable can be thought of as just a stream of events. When you define an Observable, you have three listeners: onNext, onComplete and onError. onNext will be called every time the observable acquires a new value. onComplete will be called if the parent Observable notifies that it finished producing any more values. onError is called if an exception is thrown any time during the execution of the Observable chain. To show operators in Rx, the marble diagram is used to display what happens with a particular operation. Below is an example of a simple Observable operator "Just."

Example of a marble diagram

Marble diagrams have a horizontal block that represents the operation being performed, a vertical bar to represent the completed event, a X to represent an error, and any other shape represents a value. With that in mind, we can see that "Just" will just take our value and do an onNext and then finish with onComplete. There are a lot more operations then just "Just." You can see all the operations that are part of the ReactiveX project and there implementations in RxJava at the ReativeX site. There are also interactive marble diagrams via RxMarbles site.