java-concurrency

What this skill does

# Java Concurrency

Master Java's concurrency utilities including ExecutorService,
CompletableFuture, locks, and modern virtual threads for building
high-performance concurrent applications.

## Thread Basics

Understanding Java threads is fundamental to concurrency.

**Creating and running threads:**

```java
public class ThreadBasics {
    public static void main(String[] args) {
        // Using Thread class
        Thread thread1 = new Thread(() -> {
            System.out.println("Running in thread: " +
                Thread.currentThread().getName());
        });
        thread1.start();

        // Using Runnable
        Runnable task = () -> {
            for (int i = 0; i < 5; i++) {
                System.out.println("Task iteration: " + i);
            }
        };
        Thread thread2 = new Thread(task);
        thread2.start();

        // Join threads
        try {
            thread1.join();
            thread2.join();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
}
```

## ExecutorService

ExecutorService provides thread pool management and task scheduling.

**Basic executor usage:**

```java
import java.util.concurrent.*;

public class ExecutorBasics {
    public static void main(String[] args) {
        // Fixed thread pool
        ExecutorService executor = Executors.newFixedThreadPool(3);

        // Submit tasks
        for (int i = 0; i < 5; i++) {
            final int taskId = i;
            executor.submit(() -> {
                System.out.println("Task " + taskId + " on " +
                    Thread.currentThread().getName());
                return taskId * 2;
            });
        }

        // Shutdown executor
        executor.shutdown();
        try {
            if (!executor.awaitTermination(60, TimeUnit.SECONDS)) {
                executor.shutdownNow();
            }
        } catch (InterruptedException e) {
            executor.shutdownNow();
            Thread.currentThread().interrupt();
        }
    }
}
```

**Different executor types:**

```java
public class ExecutorTypes {
    public static void main(String[] args) {
        // Single thread executor
        ExecutorService single = Executors.newSingleThreadExecutor();

        // Fixed thread pool
        ExecutorService fixed = Executors.newFixedThreadPool(4);

        // Cached thread pool (creates threads as needed)
        ExecutorService cached = Executors.newCachedThreadPool();

        // Scheduled executor
        ScheduledExecutorService scheduled =
            Executors.newScheduledThreadPool(2);

        // Schedule task with delay
        scheduled.schedule(() -> {
            System.out.println("Delayed task");
        }, 5, TimeUnit.SECONDS);

        // Schedule periodic task
        scheduled.scheduleAtFixedRate(() -> {
            System.out.println("Periodic task");
        }, 0, 1, TimeUnit.SECONDS);

        // Work stealing pool (uses available processors)
        ExecutorService workStealing =
            Executors.newWorkStealingPool();
    }
}
```

**Future pattern:**

```java
public class FutureExample {
    public static void main(String[] args) throws Exception {
        ExecutorService executor = Executors.newFixedThreadPool(2);

        // Submit callable
        Future<Integer> future = executor.submit(() -> {
            Thread.sleep(1000);
            return 42;
        });

        // Do other work
        System.out.println("Waiting for result...");

        // Get result (blocks until ready)
        Integer result = future.get();
        System.out.println("Result: " + result);

        // With timeout
        try {
            Integer result2 = future.get(500, TimeUnit.MILLISECONDS);
        } catch (TimeoutException e) {
            System.out.println("Timed out");
            future.cancel(true);
        }

        // Check status
        boolean isDone = future.isDone();
        boolean isCancelled = future.isCancelled();

        executor.shutdown();
    }
}
```

## CompletableFuture

CompletableFuture enables composable asynchronous programming.

**Basic CompletableFuture:**

```java
import java.util.concurrent.CompletableFuture;

public class CompletableFutureBasics {
    public static void main(String[] args) {
        // Create completed future
        CompletableFuture<String> future =
            CompletableFuture.completedFuture("Hello");

        // Async computation
        CompletableFuture<Integer> asyncFuture =
            CompletableFuture.supplyAsync(() -> {
                sleep(1000);
                return 42;
            });

        // Run async without return value
        CompletableFuture<Void> runAsync =
            CompletableFuture.runAsync(() -> {
                System.out.println("Running async");
            });

        // Get result (blocking)
        try {
            Integer result = asyncFuture.get();
            System.out.println("Result: " + result);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static void sleep(long millis) {
        try {
            Thread.sleep(millis);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
}
```

**Chaining operations:**

```java
public class CompletableFutureChaining {
    public static void main(String[] args) {
        // thenApply - transform result
        CompletableFuture<String> future =
            CompletableFuture.supplyAsync(() -> "Hello")
                .thenApply(s -> s + " World")
                .thenApply(String::toUpperCase);

        System.out.println(future.join()); // HELLO WORLD

        // thenAccept - consume result
        CompletableFuture.supplyAsync(() -> 42)
            .thenAccept(result ->
                System.out.println("Result: " + result));

        // thenRun - run after completion
        CompletableFuture.supplyAsync(() -> "Done")
            .thenRun(() -> System.out.println("Finished"));

        // thenCompose - flatten nested futures
        CompletableFuture<String> composed =
            CompletableFuture.supplyAsync(() -> "User123")
                .thenCompose(userId -> fetchUserDetails(userId));
    }

    static CompletableFuture<String> fetchUserDetails(String userId) {
        return CompletableFuture.supplyAsync(() ->
            "Details for " + userId);
    }
}
```

**Combining futures:**

```java
public class CombiningFutures {
    public static void main(String[] args) {
        CompletableFuture<Integer> future1 =
            CompletableFuture.supplyAsync(() -> 10);
        CompletableFuture<Integer> future2 =
            CompletableFuture.supplyAsync(() -> 20);

        // Combine two futures
        CompletableFuture<Integer> combined = future1.thenCombine(
            future2,
            (a, b) -> a + b
        );
        System.out.println(combined.join()); // 30

        // Accept both results
        future1.thenAcceptBoth(future2, (a, b) ->
            System.out.println("Sum: " + (a + b)));

        // Run after both complete
        future1.runAfterBoth(future2, () ->
            System.out.println("Both completed"));

        // Either - whichever completes first
        CompletableFuture<String> either =
            future1.applyToEither(future2, result ->
                "First result: " + result);

        // All of - wait for all
        CompletableFuture<Void> allOf =
            CompletableFuture.allOf(future1, future2);

        // Any of - wait for any
        CompletableFuture<Object> anyOf =
            CompletableFuture.anyOf(future1, future2);
    }
}
```

**Error handling:**

```java
public class FutureErrorHandling {
    public static void main(String[] args) {
        // exceptionally - handle error
        CompletableFuture<Integer> future1 =
            CompletableFuture.supplyAsync(() -> {
                if (Math.random() > 0.5) {
                    throw new RuntimeException("Error!")