Updated April 5, 2023
Introduction to RxJava Interview Questions
RxJava interview questions will consist of the top 15 questions, which will be helpful while attending interviews. RxJava is VM implementation of the ReactiveX library in Java used for event-based and asynchronous programs. RxJava has 2 building blocks, i.e., Observables and Subscribers. One is used for emitting items, and the other is used to consume the emitted items. The base of RxJava is Reactive programming, which is a programming language that’s concerned with data streams and propagation. Reactive is nothing but reacting to changes in the state instead of actually having state changes. Here we shall see some of the important questions to tackle interviews and grab opportunities.
Top 14 Interview Questions of RxJava
Top Interview Questions of Rx Java are given below:
1. When does Observable Start to Emit Items?
In Observable, there are two types: Cold and Hot Observables. Cold Observables will perform work and subsequently emit items only once is someone has subscribed, whereas Hot Observables will perform work and emit items regardless of observers or not.
2. Define Scheduler Explain Why RxJava Uses Schedulers?
Schedulers are used to switch execution to a different thread. RxJava is single-threaded by default, i.e., all operations are executed on a single thread. Also used as an abstraction overtime concept for time-sensitive operations such as delay(), buffer(), timeout(), window(), etc.
3. Differentiate flatMap(), switchMap() and concatMap()
flatMap() is used to split chain to multiple intermediary streams, and results of which are flattened to a single stream. Emissions of these intermediary streams are directly propagated to the main stream in any order. switchMap() is similar to flatMap(), but whenever a new item is being emitted from a source, all the intermediary streams generated are terminated, and hence only the latest intermediary stream remains active. concatMap() also works similar to flatMap(), with an exception such that intermediary streams are activated based on the order appearance.
4. How many times can onNext(), onError() and onComplete() be called?
onNext() can be called from zero to an infinite number of times, onError() can be called at maximum once per stream; similarly onComplete() is called at maximum once per stream.
5. Define Observable chain
List of operations or transformations performed in between the source and end subscriber. One of the examples, it to emit User objects, filtering out the admin users and checking for authentication of users, and finally map full name.
6. Explain the Difference between Reactive Programming and Imperative Programming
In Reactive, observables will emit data and send it to subscribers, which means data streams are being pushed, whereas in Imperative, data streams are being pulled, i.e., the user explicitly requests data from the collection or any Database, etc.
7. Can users create custom operators in RxJava?
Users can create custom operators in RxJava, and this is highly recommended to reuse existing or any combination. It is tricky to implement a new operator as it would lead to too many of errors, such as thread safety, API breaking, etc.
8. List out some error handling operators in RxJava.
We have two categories of such operators, one for side effects only and the other for handle error and continue. doOnError(…), onErrorReturn(…), onErrorResumeNext(…) are some of the error handling operators in RxJava.
9. Define Marble Diagram
It is a graphical representation of how the RxJava operators work. In most of the cases, RxJava has a source stream, operator, and resulting stream. Each of the streams is represented by a timeline with all emissions, known as marbles and terminating events.
10. Define Backpressure. How To Deal with Backpressure?
Backpressure is the inability of a subscriber to handle incoming events in time. Backpressure can occur when the producer of events is much faster than consumers; if not will error stream.
11. What is Subject? List out 4 Types of Subject in RxJava
Subject means both subscriber and observer at the same time. With subjects in RxJava, users can transform cold observables to hot ones. They are also used to introduce some type of local and temporary caching of the stream. Also, help in transforming non-reactive code to reactive if the user does not find any operator for use case creation.
Types of Subjects in RxJava:
- PublishSubject: It passes incoming events to all subscribers. New subscribers will receive events only from the point of subscription.
- BehaviourSubject: Similar to publish subject, but each new subscriber will receive the latest value of the stream, i.e., the default value. Here, the default value of the stream provides a good user experience.
- AsyncSubject: It emits only the last value of Observable, and that too only after the source observable completes emitting.
- ReplaySubject: Each subscriber will receive all the events emitted by the source, regardless of at which point it is subscribed. If Observable emits too many items, they need to be in-memory cache.
12. Will RxJava Support Parallelism? How is it Achieved?
RxJava will support parallelism, and this is achieved in two ways; using flatMap() operator, each stream inside flatMap() should subscribe background thread. Using ParallelFlowable, it provides easier and explicit API to achieve parallelism.
13. Define Transformer
It is a convenient way to encapsulate operations common to users in a reusable way. This logic can be tested in isolation, easier, and simplifies testing of all chains that use.
13. Define term Non-Blocking
An algorithm is considered to be non-blocking if threads competing for resources do not have execution postponed due to mutual exclusion of protecting the resource.
14. Define Elasticity in RxJava
It means that the throughput of the system scales up or down automatically to meet demand as a resource is proportionally added/ removed. Elasticity, therefore, builds up Scalability and expands by adding the notion of automatic resource management. With this, we shall conclude the topic “RxJava Interview questions.” We have seen the top 15 questions on RxJava, which will be helpful for cracking interviews. Sometimes, the interviewer can even ask you the syntax or how is any particular part coded. Having theoretical knowledge is of much importance to at least access your skills.
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