Java: Threads

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

Creating a Thread

There are two ways to create a thread.

It can be created 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");
  }
}

Another way to create a thread is to implement 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 run by creating an instance of the class and call its start() method:

public class Main extends Thread {
  public static void main(String[] args) {
    Main thread = new Main();
    thread.start(); -> run()
    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 the Runnable interface, the thread can be run by passing an instance of the class to a Thread object's constructor and then calling the thread's start() method:

public class Main implements Runnable {
  public static void main(String[] args) {
    Main obj = new Main();
    Thread thread = new Thread(obj);
    thread.start(); -> run()
    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

Because threads run at the same time as other parts of the program, there is no way to know in which order the code will run. When the threads and main program are reading and writing the same variables, the values are unpredictable. The problems that result from this are called concurrency problems.

To avoid concurrency problems, it is best to share as few attributes between threads as possible. If attributes need to be shared, one possible solution is to use the isAlive() method of the thread to check whether the thread has finished running before using any attributes that the thread can change.

public class Main extends Thread {
  public static int amount = 0;

  public static void main(String[] args) {
    Main thread = new Main();
    thread.start(); -> run()
    // Wait for the thread to finish
    while(thread.isAlive()) {
      System.out.println("Waiting...");
    }
    // Update amount and print its value
    System.out.println("Main: " + amount);
    amount++;
    System.out.println("Main: " + amount);
  }
  public void run() {
    amount++;
  }
}

// Outputs:
Waiting...
Waiting...
Waiting...
Waiting...
Waiting...
Waiting...
Waiting...
Waiting...
Waiting...
Waiting...
Waiting...
Main: 1
Main: 2