RxJava - BehaviorSubject

BehaviorSubject subject emits the most recent item it has observed and then all subsequent observed items to each subscribed Observer.

Class Declaration

Following is the declaration for io.reactivex.rxjava3.subjects.BehaviorSubject<T> class −

public final class BehaviorSubject<T>
   extends Subject<T>

Example - Usage of BehaviorSubject

ObservableTester.java

package com.tutorialspoint;

import io.reactivex.rxjava3.subjects.BehaviorSubject;

public class ObservableTester  {
   public static void main(String[] args) {   
      final StringBuilder result1 = new StringBuilder();
      final StringBuilder result2 = new StringBuilder();         
      BehaviorSubject<String> subject =  BehaviorSubject.create(); 
      subject.subscribe(value -> result1.append(value) ); 
      subject.onNext("a"); 
      subject.onNext("b"); 
      subject.onNext("c"); 
      subject.subscribe(value -> result2.append(value)); 
      subject.onNext("d"); 
      subject.onComplete();
      //Output will be abcd
      System.out.println(result1);
      //Output will be cd being BehaviorSubject 
      //(c is last item emitted before subscribe)
      System.out.println(result2);
   }
}

Output

Compile and Run the code to verify the following output −

abcd
cd

Example - Usage of BehaviorSubject events

ObservableTester.java

package com.tutorialspoint;

import io.reactivex.rxjava3.annotations.NonNull;
import io.reactivex.rxjava3.core.Observer;
import io.reactivex.rxjava3.disposables.Disposable;
import io.reactivex.rxjava3.subjects.BehaviorSubject;

public class ObservableTester  {
   public static void main(String[] args) {   
      BehaviorSubject<String> subject = BehaviorSubject.create(); 

      subject.subscribe(new Observer<Object>() {

         @Override
         public void onComplete() {
            System.out.println("Observer: Completed");
         }

         @Override
         public void onError(@NonNull Throwable error) {
            System.out.println("Observer: Error" + error.getMessage());
         }

         @Override
         public void onNext(@NonNull Object item) {
            System.out.println("Observer received: " + item);
         }

         @Override
         public void onSubscribe(@NonNull Disposable arg0) {
            System.out.println("Observer: Subscribed");
         }
      });

      subject.onNext("A"); 
      subject.onNext("B");

      subject.onComplete();
   }
}

Output

Compile and Run the code to verify the following output −

Observer: Subscribed
Observer received: A
Observer received: B
Observer: Completed