Java CompletableFuture 实现了 CompletionStage 和 Future 接口。 CompletableFuture.applyToEither 继承自 CompletionStage。 applyToEither 方法返回一个新的 CompletionStage，当这个或另一个给定阶段正常完成时，将使用相应的结果作为所提供函数的参数执行。
从 Java 文档中找到 applyToEither 方法的方法声明。

<U> CompletionStage<U> applyToEither(CompletionStage<? extends T> other, Function<? super T,U> fn)

类型参数U是函数的返回类型
参数other 是另一个CompletionStage。
参数 fn 是用于计算返回的 CompletionStage 值的函数。
applyToEither 返回新的 CompletionStage。

查找示例。
示例 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());
  }
}

输出

Mohan - Varanasi

applyToEither 方法将此完成阶段的结果或其他通常较早完成的完成阶段的结果应用于给定函数并返回一个新的完成阶段。在上面的例子中，有时我们会得到 primaryFuture 的结果，有时会得到 secondaryFuture 的结果。
示例 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");
  }
}

输出

Default name - Default city

这里 defaultFuture 将比 mainFuture 更早完成。所以 applyToEither 会将 defaultFuture 的结果应用于给定的函数。
Example-3：使用 acceptEither 方法查找 applyToEither 的示例。

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");
  }
}

输出

Namo - Gujraat