Java Interfaces Explained – J040

by Dec 1, 2015

DeegeU Java Course

The “Java Interfaces Explained” video is part of a larger free online class called “Free Java Course Online”. You can find more information about this class on “Free Java Course Online” syllabus.

Transcript – Java Interfaces Explained

One advance I’m really hoping for in the future is driverless cars. I really hate driving, unless it’s at ludicrous speed! That’s just fun. My requirements for the driverless car are very simple. I need it to accept a location, drive me to the destination, and wake me when it gets there. I really don’t care what company does it first or how it gets done. The car just has to be capable of performing my three tasks.

You can think of these car tasks as a contract of three methods for a class. Set location, drive, and wake me up. That’s what an interface is. It’s a contract that states any class implementing this interface can be typed as the interface and it guarantees the methods listed in the interface will be implemented. So let’s talk about how interfaces work, and how we can use them in our applications.

How to define an interface

When defining an interface, we use the keyword “interface” instead of class. That’s pretty much the biggest difference. Interfaces must be public or package scope. If we don’t specify the scope of the interface, it’s protected.

public interface SelfDrivable {


We can have attributes in an interface, but the catch is all attributes must be public, static and final. That’s a constant. Even if we don’t declare the attribute scope, Java makes the attribute public, static and final. The reason is interfaces are contracts, they can’t hold state. If we need state, we should use classes.

public interface SelfDrivable {
    int numberOfPassengers = 4;

Sometimes we’ll see a constant interface pattern. The idea is we put a ton of constants in an interface, and we add the interface where we need it. It’s a bad idea, and it’s considered an anti-pattern. If you ever find yourself wanting to create a constant interface, put the constants in a final class.

public interface SelfDrivable {
    void setDestination(String dest);
    void drive();
    void wakeDJUp;

For the most part, interfaces are only method declarations, and we do not implement any of the methods. We’ll cover the exception later.

Interfaces in the class hierarchy

When we talk about interfaces, we’re not talking about the is-a relationship we see with inheritance. We say the class implements or does x. We use the keyword “implements” on our class.

If we use the interface as a type, it points to a class instance that implements the interface. We cannot create an instance of an interface. Sometimes we see interfaces used to describe an “is-a” relationship as well. It’s usually when the interface is used as the type for the class. I prefer to use the words “does”, “acts like”, or “implements” when describing the relationship between interface concepts.

We can create a hierarchy of interfaces. In this case, we’re extending the contract of methods with more methods. In this case, we’ll use the keyword “extends”.

In Java we usually add the word “-able” after the name. So if the interface’s purpose is to be self driving, we might name the interface SelfDriveable. It’s a weird convention, and you’ll see some very strange mental gymnastics to create an interface name that can end in “-able”. My recommendation is to try your best, but if the interface name would be clearer with out the “-able” suffix you can skip it. The other option is to prefix the interface with the word “Can”, as in CanSelfDrive.

Defender Methods

Recently the people at Java ran into a situation where they were painted into a corner. Most of their collection classes are defined using interfaces. They wanted to add functional programming to their interfaces, but if they changed the interface it would break every class using the interface. The interfaces are a fundamental part of Java, so it would break other people’s code.

To solve this problem, they added something called default or defender methods to interfaces. Defender methods enable us to add new functionality to interfaces and ensure binary compatibility with code written for older versions of those interfaces.

public interface SelfDrivable {
    void setDestination(String dest);
    void drive();
    void honkHorn();

    default void wakeDJUp() {
        for (int i=0; i<10; i++) {

This means we can provide default functionality for some methods. To do this, we use the keyword “default” when defining the method, and provide the default implementation of the method.

Defender methods can lead to a situation where two interfaces provide a defender method with the same name. In this case, the code will not compile and we cannot use the two interfaces together.

When we have an interface that extends another interface, we can call any super interface defender method. We do this using the interface name with the super keyword.

Marker Interfaces

Sometimes you’ll see an interface containing no methods at all. It’s just an empty interface. These are called marker interfaces. They inform the compiler the objects of the classes implementing the marker interface need to be treated differently.

An example of this in Java is the Serializable interface. Serializable says this object can be copied, stored to disk, and then recreated. There are conventions, but there are no methods we are forced to implement. Java won’t let you serialize an object unless you tell the compiler it’s ok. The marker interface does that by checking for the interface type.

Functional interfaces

There’s one other type of interface. It’s called a functional interface. This is an interface containing exactly one method. It’s also marked with something called an annotation. The annotation isn’t required. The annotation lets the compiler know it should enforce single abstract method inside the interface. These interfaces are important for functional programming and lambdas. We’ll cover those later, but you should be aware of what is a functional interface.

public interface IntToLongFunction {
    long applyAsLong(int value);

Classes vs interfaces

The first thing that might come to mind is, how are these different from abstract classes? An abstract class is a partially completed class. It’s up to us to complete the class when we derive from it. Since it’s a class, you can only inherit from a single class. If we want to add or change behavior, abstract classes force us to use inheritance.

Interfaces say we promise to implement the methods listed in the interface, but there’s no limit to the number of interfaces our class will implement. Our classes are free to use as many interfaces as needed. That’s why I say “does” is a better relationship word for interfaces. A class usually represents one concept, but it can “do” many actions.

The most common use for interfaces is to hide the details of a class. If we look at the List interface in Java, it contains many methods for manipulating a list. Behind the interface curtain, there are many types of lists. There’s an ArrayList, a LinkedList, a Vector and so on. With the list interface, we’re separating what each class does from how it does it. The actual class used doesn’t matter.

If we need to change the class behind the scenes, say a different class handles large lists better than the one we’re using, we can swap the class without needing to change much code. This is called programming to an interface.

In most cases, we want to avoid inheritance. We should use abstract classes only when we want to include functionality, but parts of the class’s purpose need to be implemented by the child class. With abstract classes, you are forcing other developers to implement child classes using inheritance. Interfaces are much easier to add to existing classes, and with defender methods there is little reason to not consider interfaces first.

OK, that was another very long tutorial. Thank you for making it to the end!

If you have any questions let me know in the comments. Liking the video helps me understand what I’m doing right, so if you liked the video… you know what to do.

And with that, I’ll see you in the next video!

Tools Used

  • Java
  • NetBeans

Media Credits

All media created and owned by DJ Spiess unless listed below.

  • No infringement intended

Hackbeat by Kevin MacLeod is licensed under a Creative Commons Attribution license (

Get the code

The source code for “Java Interfaces Explained” can be found on Github. If you have Git installed on your system, you can clone the repository by issuing the following command:

 git clone

Go to the Support > Getting the Code page for more help.

If you find any errors in the code, feel free to let me know or issue a pull request in Git.

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DJ Spiess

DJ Spiess

Your personal instructor

My name is DJ Spiess and I’m a developer with a Masters degree in Computer Science working in Colorado, USA. I primarily work with Java server applications. I started programming as a kid in the 1980s, and I’ve programmed professionally since 1996. My main focus are REST APIs, large-scale data, and mobile development. The last six years I’ve worked on large National Science Foundation projects. You can read more about my development experience on my LinkedIn account.

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