Multiple inheritance

In a derived class, you aren’t forced to have a base class that is either abstract or "concrete" (one with no abstract methods). If you do inherit from a non-interface, you can inherit from only one. All the rest of the base elements must be interfaces. You place all the interface names after the implements keyword and separate them with commas. You can have as many interfaces as you want. You can upcast to each interface, because each interface is an independent type.

When you combine a concrete class with interfaces this way, the concrete class must come first, then the interfaces. (The compiler gives an error otherwise.)

Keep in mind that one of the core reasons for interfaces is to upcast to more than one base type (and the flexibility that this provides). However, a second reason for using interfaces is the same as using an abstract base class: to prevent the client programmer from making an object of this class and to establish that it is only an interface.

This brings up a question: Should you use an interface or an abstract class? If it’s possible to create your base class without any method definitions or member variables, you should always prefer interfaces to abstract classes. In fact, if you know something is going to be a base class, you can consider making it an interface