Java Multithreading

Method 1: Extending Thread Class

Too create a thread by extending the Thread class, override the run() method and call start() to begin execution.

public class CustomThread extends Thread {
    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            System.out.println("Executing thread task: " + i);
        }
    }

    public static void main(String[] args) {
        CustomThread thread = new CustomThread();
        thread.start();
        
        for (int i = 0; i < 200; i++) {
            System.out.println("Main thread executing: " + i);
        }
    }
}

Calling start() creates a new thread alongside the main thread. Directly calling run() executes in the main thread only.

Example: Image Downloader

import org.apache.commons.io.FileUtils;
import java.io.File;
import java.io.IOException;
import java.net.URL;

public class DownloadThread extends Thread {
    private String fileUrl;
    private String fileName;

    public DownloadThread(String fileUrl, String fileName) {
        this.fileUrl = fileUrl;
        this.fileName = fileName;
    }

    @Override
    public void run() {
        FileDownloader downloader = new FileDownloader();
        downloader.download(fileUrl, fileName);
    }

    public static void main(String[] args) {
        new DownloadThread("https://example.com/image1.jpg", "1.jpg").start();
        new DownloadThread("https://example.com/image2.jpg", "2.jpg").start();
    }
}

class FileDownloader {
    public void download(String url, String name) {
        try {
            FileUtils.copyURLToFile(new URL(url), new File(name));
            System.out.println(name + " downloaded successfully");
        } catch (IOException e) {
            System.out.println("Download failed");
        }
    }
}

Method 2: Implementing Runnable Interface

public class RunnableExample implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            System.out.println("Runnable thread: " + i);
        }
    }

    public static void main(String[] args) {
        RunnableExample example = new RunnableExample();
        new Thread(example).start();
    }
}

Concurrency Issues

When multiple threads access shared resources:

public class TicketSystem implements Runnable {
    private int tickets = 10;

    @Override
    public void run() {
        while (tickets > 0) {
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + " purchased ticket " + tickets--);
        }
    }

    public static void main(String[] args) {
        TicketSystem system = new TicketSystem();
        new Thread(system, "Customer 1").start();
        new Thread(system, "Customer 2").start();
    }
}

Method 3: Implementing Callable Interface

(Implementation example omitted for brevity)