- __multiple inheritance__: the ability to inherit more than one type.
- Java onnly allows multiple inheritance for interfaces. If java allowed multiple inheritance for classes, the instance variables and methods could become ambiguous.
- some languages allow multiple inheritance from concrete classes. Inheriting from multiple classes called __mixins__.
- These mixin classes aren't meant to be instantiated, only inherited from.
- java approximates this with interfaces
- interfaces __extend__ other interfaces, but classes __implement__ interfaces.
- According to the book, "Java interfaces that approximate teh mixin include:"
-`java.lang.Cloneable`
-`java.lang.Comparable`
-`java.util.Observer` which adds an update feature to a class that whishes to be notified when certain "ovservable" objects change state.
- The book doesn't offer much information on how these methods are implemented, just that mixins implement them.
### 2.3.3 Abstract Classes
- abstract classes include teh __abstract__ modifier in their class definition.
- Abstract classes can't be instantiated themselves. They must be overridden to be instanteiated.
- Abstract classes can have abstract methods that also have the __abstract__ modifier, don't have a method body, and must me instantiated by their subclasses.
- Although abstract methods have no body, they can be called from other methods within the abstract class because their subclasses *must* implement them. This is a design pattern known as __template method pattern__
