In the previous exercise, you learned that you can convert a class to run as a thread by extending the Thread class. Java allows us to make classes threaded in another way as well, by implementing the Runnable interface.

This approach is often the preferred one because you are only allowed to extend one class, and wasting it on Thread might not be beneficial to your program. Here, rather than extending the capability of the built-in Thread class, we just want to use its threading capability. Because of this, implementing the Runnable interface, which is what the Thread class does anyways, and passing the object into a new Thread object is the preferred way of implementing them. Here’s how we would implement our Factorial example by implementing Runnable instead of extending Thread.

public class Factorial implements Runnable {
 private int n;

 public Factorial(int n) {
   this.n = n;
 }

 public int compute(int n) {
   // ... the code to compute factorials
 }

 public void run() {
   System.out.print("Factorial of " + String.valueOf(this.n) + ":")
   System.out.println(this.compute(this.n));
 }

 public static void main(String[] args) {
   Factorial f = new Factorial(25);
   Factorial g = new Factorial(10);
   Thread t1 = new Thread(f);
   Thread t2 = new Thread(f);
   t1.start();
   t2.start();
 }
}

Another way of using the Runnable interface, which is even more succinct, is to use lambda expressions. This is a more modern syntax that allows you to define the run method you want to use inline, without requiring the class to implement Runnable or extend Thread. For the above example, this syntax looks like this:

public class Factorial {
 public int compute(int n) {
   // ... the code to compute factorials
 }

 public static void main(String[] args) {
   Factorial f = new Factorial();

   // the lambda function replacing the run method
   new Thread(() -> {
     System.out.println(f.compute(25));
   }).start();

   // the lambda function replacing the run method
   new Thread(() -> {
     System.out.println(f.compute(10));
   }).start();
 }
}

Behind the scenes, this syntax is still using the Runnable interface. This is because Java will translate this lambda syntax into something like:

new Thread(new Runnable() {
 void run() {
   System.out.println(f.compute(25));
 }
}).start();

Using the lambda expression syntax for starting threads, you can create a threaded version of your class in only a few lines!

This syntax has several benefits:

• Your class no longer needs to extend Thread OR implement Runnable. You can make a thread from anything!
• Your class no longer needs a constructor to store arguments! Since you can pass an argument directly into the compute function when you create your thread, we no longer need to create a separate instance of our object any time we want to perform a threaded task with it.
• Your class is easier to read! The lambda syntax makes it so that people reading your code can immediately identify what task is being performed in your thread, without having to read your class first to find the run method.

Now, let’s try both of these methods of implementing Runnable with our FortuneTeller class.

Note: Sometimes you will see developers specifically import the Thread and Runnable classes from java.lang. This is not necessary since all Java programs naturally import the java.lang package anyways. It is often used to help with readability or remind an author to add a specific feature.