Item 18: Prefer interfaces to abstract classes

interface
Steps to add a new interface for several classes:
add an

“implements interfaceName” in each class
add the required methods for each class

What about abstract classes?
Two classes should extend the same abstract class –>
Place the abstract class high up in the type hierarchy after an ancestor of both classes –>
Great damage to the type hierarchy.

example Comparable)
Class can implement several mixins as interfaces
Class can have

only one superclass – there is no place in hierarchy for mixins as abstract classes

+ interface Songwriter
= interface SingerSongwriter extends Singer, Songwriter

The alternative

is a bloated class hierarchy containing a separate class for every supported combination of attributes (2n possible combinations).

interface – enable to use composition

- Using an abstract class

– no alternative but to use inheritance

class
Skeletal implementations are called AbstractInterface (AbstractCollection, AbstractSet, AbstractList, AbstractMap)
Simulated multiple inheritance
Simple

implementation

abstract class than an interface

+ All existing implementations of the

abstract class will then provide the new method

- Once an interface is released and implemented, it is almost impossible to change

interfaces, it’s antipattern.
Use utility classes for constants and static import.
Constant

interfaces in the Java platform libraries, such as java.io.ObjectStreamConstants - should be regarded as anomalies