Java CompletableFuture thenApply()
May 21, 2019
CompletableFuture inherits CompletionStage and Future interfaces. CompletableFuture.thenApply is inherited from CompletionStage. The thenApply returns a new CompletionStage that, when this stage completes normally, is executed with this stage's result as the argument to the supplied function.
Find the method declaration of
thenApply from Java doc.
<U> CompletionStage<U> thenApply(Function<? super T,? extends U> fn)
The fn parameter is the function to use to compute the value of the returned
CompletionStage.
The
thenApply method returns a CompletionStage.thenApply() can be used to perform some extra task on the result of another task. Now find the examples.
Example-1: We create a
CompletionStage to perform some task and then on the result of this stage we apply a function to perform other task using thenApply. Find the example.
ThenApplyDemo1.java
package com.concretepage;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
public class ThenApplyDemo1 {
public static void main(String[] args) throws InterruptedException, ExecutionException {
CompletableFuture<String> cfuture =
CompletableFuture.supplyAsync(() -> "Krishna").thenApply(data -> "Shri "+ data);
String msg = cfuture.get();
System.out.println(msg);
}
}
Shri Krishna
Example-2:
ThenApplyDemo2.java
package com.concretepage;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
public class ThenApplyDemo2 {
public static void main(String[] args) throws InterruptedException, ExecutionException {
CompletableFuture<String> cfuture =
CompletableFuture.supplyAsync(() -> computeArea(20, 30)).thenApply(data -> prettyPrint(data));
String msg = cfuture.get();
System.out.println(msg);
}
private static int computeArea(int a, int b) {
return a * b;
}
private static String prettyPrint(int area) {
return "Area: "+ area;
}
}
Area: 600
Example-3:
ThenApplyDemo3.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.CompletableFuture;
public class ThenApplyDemo3 {
public static void main(String[] args) throws InterruptedException {
List<Integer> list = Arrays.asList(10, 20, 30, 40);
list.stream().map(num -> CompletableFuture.supplyAsync(() -> num * num))
.map(cfuture -> cfuture.thenApply(res -> "Square: " + res)).map(t -> t.join())
.forEach(s -> System.out.println(s));
}
}
Square: 100 Square: 400 Square: 900 Square: 1600
References
Java Doc: Class CompletableFutureJava CompletableFuture supplyAsync()
