Interface Nuances

Recall in Java we cannot have multiple inheritance. In other words, a class (regular or abstract) cannot be a direct child of more than one (regular or abstract) class.

On the other hand, a class may implement more than one interface.

public class SomeClass implements InterfaceA, InterfaceB {

}

The example above shows that the syntax involves a comma-separated list of interfaces that the respective class implements. This construct is the closest thing Java has to Multiple Inheritance.

A class can extend another (regular or abstract) class and implement one or more interfaces:

public class SomeClass extends OtherClass implements InterfaceA, InterfaceB {

}

When a class implements an interface, it must implement all its methods unless it is an abstract class. In this case, it can defer the implementation of those methods to its sub-classes.

An interface itself can be a subtype of another interface in which case it is said that it extends the other interface:

public interface SomeInterface extends OtherInterface {

}