Interfaces vs. Abstract Classes

Interfaces, like classes, define a set of properties, methods, and
events

Interfaces, like classes, define a set of
properties, methods, and events. But unlike classes, interfaces do not provide
implementation. They are implemented by classes, and defined as separate
entities from classes. Even though class inheritance allows your classes to
inherit implementation from a base class, it also forces you to make most of
your design decisions when the class is first published.

Abstract classes are useful when
creating components because they allow you specify an invariant level of
functionality in some methods, but leave the implementation of other methods
until a specific implementation of that class is needed. They
also version well, because if additional functionality is needed in derived
classes, it can be added to the base class without breaking code.

Interfaces vs. Abstract Classes

Feature

Interface

Abstract
class

Multiple inheritance

A class
may implement several interfaces.

A class
may extend only one abstract class.

Default implementation

An interface cannot
provide any code at all, much less default code.

An abstract class can
provide complete code, default code, and/or just stubs that have to be
overridden.

Constants

Static final constants
only, can use them without qualification in classes that implement the
interface. On the other paw, these unqualified names pollute the namespace.
You can use them and it is not obvious where they are coming from since the
qualification is optional.

Both instance and
static constants are possible. Both static and instance intialiser

code are also possible to compute the constants.

Third party convenience

An interface
implementation may be added to any existing third party class.

A third party class
must be rewritten to extend only from the abstract class.

is-a vs -able or can-do

Interfaces are often
used to describe the peripheral abilities of a class, not its central
identity, e.g. an Automobile class might implement the Recyclable interface,
which could apply to many otherwise totally unrelated objects.

An abstract class
defines the core identity of its descendants. If you defined a Dog abstract
class then Damamation descendants are Dogs, they
are not merely dogable. Implemented interfaces
enumerate the general things a class can do, not the things a class is.

Plug-in

You can write a new
replacement module for an interface that contains not one stick of code in
common with the existing implementations. When you implement the interface,
you start from scratch without any default implementation. You have to obtain
your tools from other classes; nothing comes with the interface other than a
few constants. This gives you freedom to implement a radically different
internal design.

You must use the
abstract class as-is for the code base, with all its attendant baggage, good
or bad. The abstract class author has imposed structure on you. Depending on
the cleverness of the author of the abstract class, this may be good or bad.
Another issue that’s important is what I call "heterogeneous vs.
homogeneous." If implementors/subclasses are
homogeneous, tend towards an abstract base class. If they are heterogeneous,
use an interface. (Now all I have to do is come up with a good definition of
hetero/homogeneous in this context.) If the various objects are all
of-a-kind, and share a common state and behavior, then tend towards a common
base class. If all they share is a set of method signatures, then tend
towards an interface.

Homogeneity

If all
the various implementations share is the method signatures, then an
interface works best.

If the various implementations
are all of a kind and share a common status and behavior, usually an abstract
class works best.

Maintenance

If your client code
talks only in terms of an interface, you can easily change the concrete
implementation behind it, using a factory method.

Just like an interface,
if your client code talks only in terms of an abstract class, you can easily
change the concrete implementation behind it, using a factory method.

Speed

Slow, requires extra
indirection to find the corresponding method in the actual class. Modern JVMs are discovering ways to reduce this speed penalty.

Fast

Terseness

The constant
declarations in an interface are all presumed public static final, so you may
leave that part out. You can’t call any methods to compute the initial values
of your constants. You need not declare individual methods of an interface
abstract. They are all presumed so.

You can put shared code
into an abstract class, where you cannot into an interface. If interfaces
want to share code, you will have to write other bubblegum to arrange that.
You may use methods to compute the initial values of your constants and
variables, both instance and static. You must declare all the individual
methods of an abstract class abstract.

Adding functionality

If you add a new method
to an interface, you must track down all implementations of that interface in
the universe and provide them with a concrete implementation of that method.

If you add a new method
to an abstract class, you have the option of providing a default
implementation of it. Then all existing code will continue to work without
change.

 

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