What is multithreading in Java and how do you implement it?

Multithreading allows a Java program to execute multiple threads simultaneously, improving performance for concurrent tasks.

**Two ways to create threads:**

**1. Extend Thread class:**
```java
class MyThread extends Thread {
public void run() {
System.out.println("Thread running: " + getName());
}
}
new MyThread().start();
```

**2. Implement Runnable (preferred):**
```java
Runnable task = () -> System.out.println("Running in: " + Thread.currentThread().getName());
new Thread(task).start();
```

**Using ExecutorService (best practice):**
```java
ExecutorService pool = Executors.newFixedThreadPool(4);
pool.submit(() -> System.out.println("Task 1"));
pool.submit(() -> System.out.println("Task 2"));
pool.shutdown();
```

**Common issues:** Race conditions, deadlocks — use `synchronized` blocks or `ReentrantLock` to handle shared state.