How do you apply the OOP concept of polymorphism to create flexible and reusable concurrent classes in Java?

1 Interfaces and concurrency

An interface is a contract that specifies the methods that a class must implement. You can use interfaces to create concurrent classes that share a common behavior, but have different implementations. For example, you can define an interface called Runnable that has a run() method, and then create different classes that implement Runnable and override the run() method to perform different tasks. You can then use a thread pool to execute these classes in parallel, as shown in the following code snippet:

// Define an interface
public interface Runnable {
  void run();
}

// Create two classes that implement Runnable
public class Task1 implements Runnable {
  @Override
  public void run() {
    // Do something
  }
}

public class Task2 implements Runnable {
  @Override
  public void run() {
    // Do something else
  }
}

// Create a thread pool and execute the tasks
ExecutorService executor = Executors.newFixedThreadPool(2);
executor.execute(new Task1());
executor.execute(new Task2());
executor.shutdown();        

By using interfaces, you can achieve polymorphism and concurrency at the same time, as the thread pool can execute any class that implements Runnable, regardless of its specific details.

2 Abstract classes and concurrency

An abstract class is a class that cannot be instantiated, but can have abstract or concrete methods. You can use abstract classes to create concurrent classes that have some common functionality, but also some abstract methods that must be overridden by subclasses. For example, you can define an abstract class called Worker that has a start() and a stop() method, and then create different subclasses that extend Worker and override the abstract work() method to perform different jobs. You can then use a concurrent collection to store and manage these workers, as shown in the following code snippet:

// Define an abstract class
public abstract class Worker {
  private boolean running;

  // Start the worker
  public void start() {
    running = true;
    work();
  }

  // Stop the worker
  public void stop() {
    running = false;
  }

  // Check if the worker is running
  public boolean isRunning() {
    return running;
  }

  // Define an abstract method
  public abstract void work();
}

// Create two subclasses that extend Worker
public class Worker1 extends Worker {
  @Override
  public void work() {
    // Do something
  }
}

public class Worker2 extends Worker {
  @Override
  public void work() {
    // Do something else
  }
}

// Create a concurrent collection and add the workers
ConcurrentLinkedQueue<Worker> queue = new ConcurrentLinkedQueue<>();
queue.add(new Worker1());
queue.add(new Worker2());

// Start and stop the workers
for (Worker worker : queue) {
  worker.start();
}

for (Worker worker : queue) {
  worker.stop();
}        

By using abstract classes, you can achieve polymorphism and concurrency at the same time, as the concurrent collection can store and manipulate any class that extends Worker, regardless of its specific details.

3 Method overriding and concurrency

Method overriding is a technique that allows you to redefine a method in a subclass that has the same name and signature as a method in a superclass. You can use method overriding to create concurrent classes that inherit some behavior from a superclass, but also modify or extend it in a subclass. For example, you can define a class called Processor that has a process() method that synchronizes on a shared object, and then create different subclasses that override the process() method to perform different operations on the shared object. You can then use an executor service to run these subclasses in parallel, as shown in the following code snippet:

// Define a class
public class Processor {
  private Object lock;

  // Constructor
  public Processor(Object lock) {
    this.lock = lock;
  }

  // Process method
  public void process() {
    synchronized (lock) {
      // Do something
    }
  }
}

// Create two subclasses that override the process method
public class Processor1 extends Processor {
  // Constructor
  public Processor1(Object lock) {
    super(lock);
  }

  // Override the process method
  @Override
  public void process() {
    synchronized (lock) {
      // Do something else
    }
  }
}

public class Processor2 extends Processor {
  // Constructor
  public Processor2(Object lock) {
    super(lock);
  }

  // Override the process method
  @Override
  public void process() {
    synchronized (lock) {
      // Do something different
    }
  }
}

// Create a shared object and an executor service
Object lock = new Object();
ExecutorService executor = Executors.newFixedThreadPool(2);

// Execute the subclasses
executor.execute(new Processor1(lock));
executor.execute(new Processor2(lock));
executor.shutdown();        

By using method overriding, you can achieve polymorphism and concurrency at the same time, as the executor service can run any class that extends Processor, regardless of its specific details.

4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?