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Java CompletableFuture applyToEither()

By Arvind Rai, May 29, 2019
Java CompletableFuture implements CompletionStage and Future interfaces. CompletableFuture.applyToEither is inherited from CompletionStage. The applyToEither method returns a new CompletionStage that, when either this or the other given stage complete normally, is executed with the corresponding result as argument to the supplied function.
Find the method declaration of applyToEither method from Java doc.
<U> CompletionStage<U> applyToEither(CompletionStage<? extends T> other, Function<? super T,U> fn) 
The type parameter U is the function's return type
The parameter other is the other CompletionStage.
The parameter fn is the function to use to compute the value of the returned CompletionStage.
applyToEither returns new CompletionStage.


Find the examples.
Example-1:
ApplyToEitherDemo1.java
package com.concretepage;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
public class ApplyToEitherDemo1 {
  public static void main(String[] args) throws InterruptedException, ExecutionException {
	  CompletableFuture<Person> primaryFuture = CompletableFuture.completedFuture(new Person("Mohan", "Varanasi"));
	  
	  CompletableFuture<Person> secondaryFuture = CompletableFuture.completedFuture(new Person("Shyam", "Prayagraj"));
	  
	  CompletableFuture<String> future =
	      primaryFuture.applyToEither(secondaryFuture, person -> person.getName() + " - " +person.getCity());
	  
	  System.out.println(future.get());
  }
} 
Output
Mohan - Varanasi 
applyToEither method applies the result of this completion stage or other completion stage whichever completes normally earlier, to the given function and returns a new completion stage. In the above example, sometimes we will get result for primaryFuture and sometimes for secondaryFuture.
Example-2:
ApplyToEitherDemo2.java
package com.concretepage;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
public class ApplyToEitherDemo2 {
  public static void main(String[] args) throws InterruptedException, ExecutionException {
	CompletableFuture<Person> mainFuture = CompletableFuture.supplyAsync(() -> getPerson());

	CompletableFuture<Person> defaultFuture = CompletableFuture.supplyAsync(() -> getDefaultFuture());

	CompletableFuture<String> future = mainFuture.applyToEither(defaultFuture,
		person -> person.getName() + " - " + person.getCity());

	System.out.println(future.join());
  }

  private static Person getPerson() {
	try {
	  Thread.sleep(500);
	} catch (InterruptedException e) {
	  e.printStackTrace();
	}
	return new Person("Krishna", "Delhi");

  }

  private static Person getDefaultFuture() {
	return new Person("Default name", "Default city");
  }
} 
Output
Default name - Default city 
Here defaultFuture will complete earlier than mainFuture. So applyToEither will apply the result of defaultFuture to the given function.
Example-3: Find the example of applyToEither with acceptEither method.
package com.concretepage;
import java.util.concurrent.CompletableFuture;
public class ApplyToEitherDemo3 {
  public static void main(String[] args) {
	CompletableFuture<Person> mainFuture = CompletableFuture.supplyAsync(() -> getPerson());

	CompletableFuture<Person> defaultFuture = CompletableFuture.supplyAsync(() -> getDefaultFuture());

	CompletableFuture<String> cfuture = mainFuture.applyToEither(defaultFuture,
		person -> person.getName() + " - " + person.getCity());

	CompletableFuture<String> otherCFuture = CompletableFuture.supplyAsync(() -> getMsg());

	CompletableFuture<Void> cf = cfuture.acceptEither(otherCFuture, s -> System.out.println(s));

	cf.join();
  }

  private static String getMsg() {
	try {
	  Thread.sleep(400);
	  // Thread.sleep(1000);
	} catch (InterruptedException e) {
	  System.err.println(e);
	}
	return "Namo - Gujraat";
  }

  private static Person getPerson() {
	try {
	  Thread.sleep(500);
	} catch (InterruptedException e) {
	  e.printStackTrace();
	}
	return new Person("Krishna", "Delhi");
  }

  private static Person getDefaultFuture() {
	try {
	  Thread.sleep(600);
	} catch (InterruptedException e) {
	  e.printStackTrace();
	}
	return new Person("Default name", "Default city");
  }
} 
Output
Namo - Gujraat 

References

Class CompletableFuture
Java CompletableFuture acceptEither()
POSTED BY
ARVIND RAI
ARVIND RAI
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