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The CompletableFuture class in Java provides the isDone() method to check if a CompletableFuture has completed.
Introduction
The CompletableFuture.isDone() method is used to check if a CompletableFuture has completed, whether normally or exceptionally. This is useful for polling the status of an asynchronous computation without blocking the calling thread.
isDone Method Syntax
The syntax for the isDone method is as follows:
public boolean isDone()
- The method does not take any parameters.
- The method returns a boolean value:
trueif theCompletableFuturehas completed,falseotherwise.
Examples
Example 1: Basic Usage
In a scenario where you have a CompletableFuture that completes with a result, you might want to check its status using the isDone() method.
import java.util.concurrent.CompletableFuture;
public class IsDoneExample {
public static void main(String[] args) {
// Create a CompletableFuture that completes with a string
CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
try {
Thread.sleep(2000); // Simulate delay
} catch (InterruptedException e) {
e.printStackTrace();
}
return "Hello, world!";
});
// Check the status of the future
while (!future.isDone()) {
System.out.println("Future is not done yet...");
try {
Thread.sleep(500); // Polling delay
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// Retrieve and print the result
future.thenAccept(result -> System.out.println("Result: " + result));
}
}
Output:
Future is not done yet...
Future is not done yet...
Future is not done yet...
Future is not done yet...
Result: Hello, world!
Example 2: Task Management System
In a task management system, you might want to check if a task has completed before performing a follow-up action.
import java.util.concurrent.CompletableFuture;
public class TaskManagementSystem {
public static void main(String[] args) {
// Create a CompletableFuture that completes with a task
CompletableFuture<Task> taskFuture = CompletableFuture.supplyAsync(() -> {
try {
Thread.sleep(3000); // Simulate task delay
} catch (InterruptedException e) {
e.printStackTrace();
}
return new Task("Complete project report", 2);
});
// Check the status of the task
while (!taskFuture.isDone()) {
System.out.println("Task is not done yet...");
try {
Thread.sleep(1000); // Polling delay
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// Retrieve and print the task result
taskFuture.thenAccept(task -> System.out.println("Task completed: " + task));
}
}
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 is not done yet...
Task is not done yet...
Task is not done yet...
Task completed: Complete project report (Priority: 2)
Example 3: Handling Exceptions
In an application where an exception might be thrown during the asynchronous computation, you can check if the future is done and handle the exception appropriately.
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
public class ExceptionHandlingExample {
public static void main(String[] args) {
// Create a CompletableFuture that throws an exception
CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
throw new RuntimeException("Something went wrong");
});
// Check the status of the future
while (!future.isDone()) {
System.out.println("Future is not done yet...");
try {
Thread.sleep(500); // Polling delay
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// Handle the exception
try {
String result = future.get();
System.out.println("Result: " + result);
} catch (InterruptedException e) {
System.out.println("Thread was interrupted");
} catch (ExecutionException e) {
System.out.println("Exception: " + e.getCause().getMessage());
}
}
}
Output:
Future is not done yet...
Future is not done yet...
Exception: Something went wrong
Conclusion
The CompletableFuture.isDone() method in Java is used for checking if a CompletableFuture has completed without blocking the calling thread. It is particularly useful in scenarios where you need to poll the status of an asynchronous computation, such as checking task completions, performing follow-up actions, or handling exceptions.
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