Java Threads

Introduction to Threads

Threads allow a program to operate more efficiently by doing multiple things at the same time.

Threads can be used to perform complicated tasks in the background without interrupting the main program.

Creating a Thread

There are two ways to create a thread:

By extending the Thread class and overriding its run() method:

public class Main extends Thread {
  public void run() {
    System.out.println("This code is running in a thread");
  }
}

By implementing the Runnable interface:

public class Main implements Runnable {
  public void run() {
    System.out.println("This code is running in a thread");
  }
}

Running Threads

If the class extends the Thread class, the thread can be started using start():

public class Main extends Thread {
  public static void main(String[] args) {
    Main thread = new Main();
    thread.start();
    System.out.println("This code is outside of the thread");
  }
  public void run() {
    System.out.println("This code is running in a thread");
  }
}

If the class implements Runnable, the thread is started by passing an instance to a Thread object:

public class Main implements Runnable {
  public static void main(String[] args) {
    Main obj = new Main();
    Thread thread = new Thread(obj);
    thread.start();
    System.out.println("This code is outside of the thread");
  }
  public void run() {
    System.out.println("This code is running in a thread");
  }
}

Concurrency Problems

Since threads run simultaneously, execution order is unpredictable. This can cause concurrency problems when multiple threads modify shared data.

Example of concurrency issue:

public class Main extends Thread {
  public static int amount = 0;
 
  public static void main(String[] args) {
    Main thread = new Main();
    thread.start();
    System.out.println(amount);
    amount++;
    System.out.println(amount);
  }
  public void run() {
    amount++;
  }
}

To avoid such issues, use isAlive() to check if a thread has finished execution:

public class Main extends Thread {
  public static int amount = 0;
 
  public static void main(String[] args) {
    Main thread = new Main();
    thread.start();
    while(thread.isAlive()) {
      System.out.println("Waiting...");
    }
    System.out.println("Main: " + amount);
    amount++;
    System.out.println("Main: " + amount);
  }
  public void run() {
    amount++;
  }
}

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