Hi there. In this video, we'll talk about the Marker and Functional Interfaces in Java. If an interface does not contain any methods or a field, it's called a marker interface. There are some interfaces that are ready to find in Java, which we can call marker interfaces. These are the Serializable interface in the java.io package, the quotable interface in the java.lang package, and the remote interface defined in the java.rmi package. For example, on the slide, you can see the Serializable interface. If you notice, it does not contain any methods or fields. So, what is this used for?

For example, the process of making an object ready to be saved in a file, thanks to the Serializable interface, is called serialization. We'll learn about this in the Java IO section. In this case, a class that does not implement the Serializable interface cannot do this. So, rather than using any method or property of these interfaces, we use the interfaces as a marker. That's why such interfaces are called marker interfaces. Now, let's look at the functional interface. An interface which has only one abstract method is called the functional interface. For example, the Runnable interface in the java.lang package can be given as an example. As you can see from the slide, this interface only has an abstract method called run. 

In functional interfaces, there is no need to use the abstract keyword as it's optional to use the abstract keyword. Because by default, the method defined inside the interface is abstract only. As described earlier, functional interfaces can contain only one abstract method. However, they can include any quantity of default and static methods. Functional interface is additionally recognized as Single Abstract Method interfaces. In short, they are also known as SAM interfaces. Functional interfaces in Java are the new feature that provides users with the approach of fundamental programming. Functional interfaces are used and executed by representing the interface with an annotation called @FunctionalInterface. With this annotation, we declare the interface as functional interface. Functional interfaces are included in Java SE 8 with lambda expressions and method references in order to make code more readable, clean, and straightforward. In the next lesson, we'll learn to use the lambda expression. Let's take a short break here. See you in the next lesson.