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The CompletableFuture class in Java provides the complete() method to manually complete a CompletableFuture with a specified value.
Introduction
The CompletableFuture.complete() method is used to manually complete a CompletableFuture with a specified value. This can be useful when you need to programmatically control the completion of a future, such as when a result is obtained from an external source.
complete Method Syntax
The syntax for the complete method is as follows:
public boolean complete(T value)
- The method takes a single parameter
valueof typeT, which is the value to complete theCompletableFuturewith. - The method returns a boolean value:
trueif theCompletableFuturewas completed as a result of this call,falseotherwise.
Examples
Example 1: Completing a CompletableFuture with a Result
In a scenario where a result is obtained from an external source, you might want to manually complete the CompletableFuture when the result is available.
import java.util.concurrent.CompletableFuture;
public class CompleteExample {
public static void main(String[] args) {
// Create a CompletableFuture
CompletableFuture<String> future = new CompletableFuture<>();
// Simulate an external source providing a result
new Thread(() -> {
try {
Thread.sleep(1000); // Simulate delay
} catch (InterruptedException e) {
e.printStackTrace();
}
future.complete("Result from external source");
}).start();
// Process the result when available
future.thenAccept(result -> System.out.println("Received result: " + result));
// Wait for the future to complete
future.join();
}
}
Output:
Received result: Result from external source
Example 2: Completing a CompletableFuture in a Task Management System
In a task management system, you might want to manually complete a CompletableFuture when a task is marked as completed.
import java.util.concurrent.CompletableFuture;
public class TaskManagementSystem {
public static void main(String[] args) {
// Create a CompletableFuture for a task
CompletableFuture<Task> taskFuture = new CompletableFuture<>();
// Simulate marking the task as completed
new Thread(() -> {
try {
Thread.sleep(1500); // Simulate task completion delay
} catch (InterruptedException e) {
e.printStackTrace();
}
taskFuture.complete(new Task("Complete project report", 2));
}).start();
// Process the completed task
taskFuture.thenAccept(task -> System.out.println("Task completed: " + task));
// Wait for the task to be completed
taskFuture.join();
}
}
class Task {
private String description;
private int priority;
public Task(String description, int priority) {
this.description = description;
this.priority = priority;
}
@Override
public String toString() {
return description + " (Priority: " + priority + ")";
}
}
Output:
Task completed: Complete project report (Priority: 2)
Example 3: Completing a CompletableFuture with a Default Value
In some cases, you might want to complete a CompletableFuture with a default value if the actual value is not available within a certain time frame.
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.TimeUnit;
public class DefaultCompleteExample {
public static void main(String[] args) {
// Create a CompletableFuture
CompletableFuture<String> future = new CompletableFuture<>();
// Simulate an external source providing a result with a delay
new Thread(() -> {
try {
Thread.sleep(3000); // Simulate delay
} catch (InterruptedException e) {
e.printStackTrace();
}
future.complete("Delayed result from external source");
}).start();
// Complete the future with a default value if not completed within 2 seconds
CompletableFuture<String> resultFuture = future.completeOnTimeout("Default value", 2, TimeUnit.SECONDS);
// Process the result when available
resultFuture.thenAccept(result -> System.out.println("Received result: " + result));
// Wait for the future to complete
resultFuture.join();
}
}
Output:
Received result: Default value
Conclusion
The CompletableFuture.complete() method in Java is used for manually completing a future with a specified value. It is particularly useful in scenarios where the result is obtained from an external source, when a task is completed, or when a default value is needed. Understanding how to use this method allows for greater control and flexibility in asynchronous programming.
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