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Multithreading and concurrency are at the core of building high-performance, responsive, and scalable Java applications. Java provides a powerful and flexible set of APIs to create and manage multiple threads and coordinate their execution.

1. What Is Multithreading?

Multithreading allows a program to execute multiple parts (threads) concurrently. Each thread is a lightweight subprocess sharing the same memory space, ideal for tasks like I/O operations, background jobs, and UI responsiveness.

2. Creating Threads in Java

Using the Thread class:

java

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running");
    }
}

Using the Runnable interface:

java

Runnable task = () -> System.out.println("Runnable is running");
Thread t = new Thread(task);
t.start();

Using Callable and Future:

java

Callable<Integer> task = () -> 42;
ExecutorService executor = Executors.newSingleThreadExecutor();
Future<Integer> result = executor.submit(task);

3. Thread Lifecycle

Java threads go through the following states:

• New
• Runnable
• Blocked/Waiting
• Timed Waiting
• Terminated

Use Thread.getState() to check a thread's current state.

4. Synchronization and Thread Safety

Threads share memory — which can lead to issues like race conditions, deadlocks, or inconsistent data.

Use synchronized to prevent conflicts:

java

public synchronized void increment() {
    count++;
}

Or synchronize a block:

java

synchronized (this) {
    // critical section
}

5. Executor Framework

Java 5 introduced the Executor Framework to manage thread pools efficiently.

Example:

java

ExecutorService pool = Executors.newFixedThreadPool(5);
pool.execute(() -> System.out.println("Task executed"));

Shut down the executor:

java

pool.shutdown();

6. Concurrent Collections

For safe operations in multithreaded environments, use:

• ConcurrentHashMap
• CopyOnWriteArrayList
• BlockingQueue

These are part of java.util.concurrent.

7. Common Pitfalls

• Starting the same thread object multiple times
• Forgetting to join() threads if synchronization is needed
• Using non-thread-safe collections without external locking
• Deadlocks due to nested synchronized blocks

8. Best Practices

• Use ExecutorService over manual thread creation
• Minimize the use of synchronized; prefer higher-level concurrency tools
• Avoid shared mutable state when possible
• Use thread-safe collections and atomic variables (AtomicInteger, AtomicBoolean)

Java multithreading is a powerful but complex topic. Understanding the underlying thread model and mastering synchronization techniques are essential for writing reliable and scalable Java applications.