What is multithreading in Java? How do you create threads?

NEW → RUNNABLE → RUNNING → BLOCKED/WAITING → DEAD

class MyThread extends Thread {
    private String threadName;

    MyThread(String name) {
        this.threadName = name;
    }

    @Override
    public void run() {
        for (int i = 1; i <= 5; i++) {
            System.out.println(threadName + " - Count: " + i);
            try { Thread.sleep(500); } catch (InterruptedException e) {}
        }
    }
}

// Create and start threads
MyThread t1 = new MyThread("Thread-1");
MyThread t2 = new MyThread("Thread-2");
t1.start();  // starts a new thread — calls run() in new thread
t2.start();

class MyTask implements Runnable {
    private String taskName;

    MyTask(String name) { this.taskName = name; }

    @Override
    public void run() {
        System.out.println(taskName + " executing on: " + Thread.currentThread().getName());
    }
}

Thread t1 = new Thread(new MyTask("Login Test"));
Thread t2 = new Thread(new MyTask("Search Test"));
t1.start();
t2.start();

// With Lambda (Java 8+)
Thread t3 = new Thread(() -> System.out.println("Lambda thread"));
t3.start();

import java.util.concurrent.*;

// Fixed thread pool — reuses 3 threads
ExecutorService executor = Executors.newFixedThreadPool(3);

// Submit tasks
executor.submit(() -> System.out.println("Task 1 running"));
executor.submit(() -> System.out.println("Task 2 running"));
executor.submit(() -> {
    System.out.println("Task 3 running");
    return "result";
});

executor.shutdown();  // no new tasks, finish existing
executor.awaitTermination(30, TimeUnit.SECONDS);

Thread t = new Thread(() -> {
    System.out.println("Running: " + Thread.currentThread().getName());
});

t.setName("MyThread");
t.setPriority(Thread.MAX_PRIORITY); // 1-10, default 5
t.setDaemon(true);   // daemon thread — dies when main thread dies
t.start();
t.join();            // wait for this thread to finish
t.interrupt();       // request interruption
Thread.sleep(1000);  // current thread sleeps 1 second
Thread.currentThread().getName(); // get current thread name

class Counter {
    private int count = 0;

    // synchronized — only one thread can execute this at a time
    public synchronized void increment() {
        count++;
    }

    public int getCount() { return count; }
}

Counter counter = new Counter();
Thread t1 = new Thread(() -> { for (int i=0; i<1000; i++) counter.increment(); });
Thread t2 = new Thread(() -> { for (int i=0; i<1000; i++) counter.increment(); });
t1.start(); t2.start();
t1.join();  t2.join();
System.out.println(counter.getCount()); // 2000 — correct with sync

// TestNG runs tests in parallel across browsers using multithreading
// ThreadLocal ensures each thread has its own WebDriver instance
ThreadLocal<WebDriver> driverThreadLocal = new ThreadLocal<>();

public WebDriver getDriver() {
    return driverThreadLocal.get();
}

@BeforeMethod
public void setup() {
    driverThreadLocal.set(new ChromeDriver());
}

@AfterMethod
public void teardown() {
    getDriver().quit();
    driverThreadLocal.remove();
}