Which One to Choose

From the previous chapters, you might have noticed that abstract classes and interfaces are quite similar. However, there are differences between them. To cut to the chase, it can be said that in practice, interfaces are often preferred over abstract classes. But let's understand why this is the case.

What’s the Difference

To begin with, we need to understand the differences between an abstract class and an interface. Let's go through the list:

• Syntax: abstract class when declaring an abstract class and interface when declaring an interface;
• Inheritance keyword: extends for inheriting an abstract class, and implements for inheriting an interface;
• Names for subclasses: A subclass of an abstract class is referred to as a subclass or inheriting class, while a subclass of an interface is called a class - implementation;
• Number of inheritances: You can inherit from only one abstract class, while you can inherit from multiple interfaces;
• An abstract class can have both implemented and abstract methods. Starting from Java 8, interfaces can also have default methods, which we will discuss later in this section;
• Methods in an abstract class can have any access modifier, whereas, in interfaces, only the public access modifier is allowed.

Note

We can also simultaneously inherit from an abstract class and implement an interface. First, we use the keyword extends, and then we use implements. The syntax looks like this: class ClassName extends AbstractClass implements Interface {}

In simple terms, think of an abstract class as a class that provides some functionality and enforces certain rules but leaves some parts for its subclasses to complete. An interface, on the other hand, is like a checklist that a class needs to fulfill, specifying what methods it must have, without providing any actual code.