What is a Thread in Java?

A thread is a lightweight subprocess, the smallest unit of processing. Java supports multithreading, which means you can run multiple threads concurrently — increasing the efficiency and performance of your program.

Why Use Threads?

• Perform tasks in parallel (e.g., downloading, file I/O, UI responsiveness).
• Improve application speed and responsiveness.
• Handle background tasks (like animations, network calls).

Ways to Create a Thread in Java

1. By extending the Thread class
2. By implementing the Runnable interface
3. (Java 8+) Using Lambda Expressions

Extending Thread Class

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

public class ThreadExample1 {
    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        t1.start();  // starts the thread and calls run()
    }
}

Don't use .run() directly; use .start() to create a new thread.

Implementing Runnable Interface

class MyRunnable implements Runnable {
    public void run() {
        System.out.println("Thread is running using Runnable interface.");
    }
}

public class ThreadExample2 {
    public static void main(String[] args) {
        Thread t1 = new Thread(new MyRunnable());
        t1.start();
    }
}

This approach is better when your class needs to extend another class, as Java doesn't support multiple inheritance.

Using Lambda Expression (Java 8+)

public class LambdaThreadExample {
    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            System.out.println("Thread is running using Lambda.");
        });
        t1.start();
    }
}

Example: Running Multiple Threads

class Task1 extends Thread {
    public void run() {
        for (int i = 1; i <= 5; i++) {
            System.out.println("Task1 - " + i);
        }
    }
}

class Task2 extends Thread {
    public void run() {
        for (int i = 1; i <= 5; i++) {
            System.out.println("Task2 - " + i);
        }
    }
}

public class MultiThreadDemo {
    public static void main(String[] args) {
        Task1 t1 = new Task1();
        Task2 t2 = new Task2();

        t1.start();
        t2.start();
    }
}

Output (order may vary):

Task1 - 1
Task2 - 1
Task1 - 2
Task2 - 2
...

Threads run asynchronously, so output order may change every time.

Thread Methods

Thread Synchronization

When multiple threads access shared resources (e.g., a bank account), you must use synchronization to prevent data inconsistency.

class Counter {
    int count = 0;

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

public class SyncExample {
    public static void main(String[] args) throws InterruptedException {
        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("Final Count: " + counter.count);
    }
}