=== Substitution Principle ===

=== Substitution Principle ===

When writing an [[API]], the term ''subclass'' shouldn’t be used for someone who can ''participate

When writing an [[API]], the term ''subclass'' shouldn’t be used for someone who can ''participate

in a switch'', but for someone who ''behaves exactly like me'', plus adds some additional behavior.

in a switch'', but for someone who ''behaves exactly like me'', plus adds some additional behavior.

True, many would agree that a '''Human''' is a subclass (a specialization) of a '''Mammal'''. But how

True, many would agree that a '''Human''' is a subclass (a specialization) of a '''Mammal'''.

much can you use this fact in an API design?

Nice, but simply exposing a [[DeepHierarchy]] of classes is unlikely to improve an [[API]]’s usability magically.

Especially [[GUI]] toolkits got this all wrong.

Especially [[GUI]] toolkits (full of widgets inheriting from each other) got this all wrong.

They like to claim that a {{JDK|javax.swing|JFrame}} is a specialization of {{JDK|javax.swing|JComponent}},

They like to claim that a {{JDK|javax.swing|JFrame}} is a specialization of {{JDK|javax.swing|JComponent}},

but try to use {{JDK|javax.swing|JFrame}} where you can use {{JDK|javax.swing|JComponent}}! It is going to fail.

but try to use {{JDK|javax.swing|JFrame}} where you can use {{JDK|javax.swing|JComponent}}! It is going to fail.

That is not a failure of the usage, but a failure of the [[GUI]] toolkit [[API]] designer!

That is not a failure of the usage, but a failure of the [[API]] designer!

== Do Not Expose Deep Hierarchies! ==

== Do Not Expose Deep Hierarchies! ==

easy it is to describe nature, the object-oriented concept is also elevated to be the right programming

easy it is to describe nature, the object-oriented concept is also elevated to be the right programming

technique to describe the real world. [[OOP]] languages are supposed to be more suitable

technique to describe the real world. [[OOP]] languages are supposed to be more suitable

than traditional, procedural ones for coding and capturing real-world concepts.

than traditional ones for coding and capturing real-world concepts.

=== Just an Intricate Switch ===

=== Just an Intricate Switch ===

After maintaining a [[NetBeans|framework in Java]] for more than fifteen years, I don’t buy this. Honestly, ''virtual method dispatch''

After maintaining a [[NetBeans|framework in Java]] for more than fifteen years, I don’t buy this. Honestly, ''virtual method dispatch''

is nothing other than an

is nothing else than an

''intricate switch statement''! A method call isn’t a direct invocation of some code but rather a

''intricate switch statement''! A method call isn’t a direct invocation of some code but rather a

switch that chooses from multiple implementations. The switch is based on the list of subclasses

switch that chooses from multiple implementations. The switch is based on the list of subclasses

=== Just an Intricate Switch ===

=== Just an Intricate Switch ===

After maintaining a [[NetBeans|framework in Java]] for more than fifteen years, I don’t buy this. Honestly, _virtual method dispatch_

After maintaining a [[NetBeans|framework in Java]] for more than fifteen years, I don’t buy this. Honestly, ''virtual method dispatch''

is nothing other than an

is nothing other than an

''intricate switch statement''! A method call isn’t a direct invocation of some code but rather a

''intricate switch statement''! A method call isn’t a direct invocation of some code but rather a

it’s still a '''switch'''. Of course, explaining [[OOP]] as an ''enhanced switch'' is

it’s still a '''switch'''. Of course, explaining [[OOP]] as an ''enhanced switch'' is

much less fancy than presenting it as a technology that helps us model the real world. That’s

much less fancy than presenting it as a technology that helps us model the real world. That’s

why computer courses are likely to stick with the '''Mammal''' example.

why computer courses are likely to stick with the '''Mammal''' example.

=== Switch with no Limits! ===

=== Switch with no Limits! ===

[[OOP]] encourages code reuse and may lead to deep subclassing hierarchies. Beware of that when designing [[API]].

[[OOP]] encourages code reuse and may lead to deep subclassing hierarchies. Beware of that when designing [[API]].

== Do Not Expose Deep Hierarchies ==

=== [[OOP]] Models the World ===

Classic [[OOP|object-oriented programming languages]] are said to be inspired by nature and its

Classic [[OOP|object-oriented programming languages]] are said to be inspired by nature and its

That is not a failure of the usage, but a failure of the [[GUI]] toolkit [[API]] designer!

That is not a failure of the usage, but a failure of the [[GUI]] toolkit [[API]] designer!

== Avoid Deep Hierarchies in Your [[API]]! ==

== Do Not Expose Deep Hierarchies! ==

Rather than exposing [[DeepHierarchy]] separate [[APIvsSPI]] and stay as flat as possible! If you have to expose subtyping, remember [[wikipedia:Liskov_substitution_principle|Liskov's substitution principle]]: if something is a subtype, it has to inherently

Rather than exposing [[DeepHierarchy]] separate [[APIvsSPI]] and stay as flat as possible! If you have to expose subtyping, remember [[wikipedia:Liskov_substitution_principle|Liskov's substitution principle]]: if something is a subtype, it has to inherently

== Avoid Deep Hierarchies in Your [[API]]! ==

== Avoid Deep Hierarchies in Your [[API]]! ==

Rather than exposing [[DeepHierarchy]] separate [[APIvsSPI]] and stay as flat as possible! If you have to expose subtyping, remember [[wikipedia:Liskov_substitution_principle|Liskov's substitution principle]]: if something is a subtype, it has inherently

Rather than exposing [[DeepHierarchy]] separate [[APIvsSPI]] and stay as flat as possible! If you have to expose subtyping, remember [[wikipedia:Liskov_substitution_principle|Liskov's substitution principle]]: if something is a subtype, it has to inherently

be ready to be used in place of the original super class or interface.

be ready to be used in place of the original super class or interface.

[[Category:APIDesignPatterns]]

[[Category:APIDesignPatterns]]

[[Category:APIDesignPatterns:Anti]]

[[Category:APIDesignPatterns:Anti]]

== Avoid Deep Hierarchies in Your [[API]]! ==

== Avoid Deep Hierarchies in Your [[API]]! ==

Rather than exposing [[DeepHierarchy]] separate [[APIvsSPI]] and stay as flat as possible! If you have to expose subclasses, remember [[wikipedia:Liskov_substitution_principle|Liskov's substitution principle]]: if something is a subclass or a subinterface, it has inherently

Rather than exposing [[DeepHierarchy]] separate [[APIvsSPI]] and stay as flat as possible! If you have to expose subtyping, remember [[wikipedia:Liskov_substitution_principle|Liskov's substitution principle]]: if something is a subtype, it has inherently

be ready to be used in place of the original superclass or interface.

be ready to be used in place of the original super class or interface.

[[Category:APIDesignPatterns]]

[[Category:APIDesignPatterns]]

[[Category:APIDesignPatterns:Anti]]

[[Category:APIDesignPatterns:Anti]]

== Avoid Deep Hierarchies in Your [[API]]! ==

== Avoid Deep Hierarchies in Your [[API]]! ==

The advice here is: instead of big, rooted families, try to define the real interface of the application and let users

Rather than exposing [[DeepHierarchy]] separate [[APIvsSPI]] and stay as flat as possible! If you have to expose subclasses, remember [[wikipedia:Liskov_substitution_principle|Liskov's substitution principle]]: if something is a subclass or a subinterface, it has inherently

implement it. Always remember the ''Liskov's principle'': if something is a subclass or a subinterface, it can inherently

be ready to be used in place of the original superclass or interface.

be used in place of the original superclass or interface.

 

[[Category:APIDesignPatterns]]

[[Category:APIDesignPatterns]]

[[Category:APIDesignPatterns:Anti]]

[[Category:APIDesignPatterns:Anti]]

maintainable.

maintainable.

=== Liskov's Substitution Principle ===

=== Substitution Principle ===

So, simply exposing a deep hierarchy of classes is unlikely to improve an API’s usability.

Simply exposing a deep hierarchy of classes is unlikely to improve an [[API]]’s usability.

When writing an API, the term “subclass” shouldn’t be used for someone who can “participate

When writing an [[API]], the term ''subclass'' shouldn’t be used for someone who can ''participate

in a switch,” but for someone who ''behaves exactly like me'', plus adds some additional behavior.

in a switch'', but for someone who ''behaves exactly like me'', plus adds some additional behavior.

True, many would agree that a '''Human''' is a subclass (a specialization) of a '''Mammal'''. But how

True, many would agree that a '''Human''' is a subclass (a specialization) of a '''Mammal'''. But how

much can you use this fact in an API design? Especially [[GUI]] toolkits got this all wrong.

much can you use this fact in an API design?

 

Especially [[GUI]] toolkits got this all wrong.

They like to claim that a {{JDK|javax.swing|JFrame}} is a specialization of {{JDK|javax.swing|JComponent}},

They like to claim that a {{JDK|javax.swing|JFrame}} is a specialization of {{JDK|javax.swing|JComponent}},

but try to use {{JDK|javax.swing|JFrame}} where you can use {{JDK|javax.swing|JComponent}}! It is going to fail.

but try to use {{JDK|javax.swing|JFrame}} where you can use {{JDK|javax.swing|JComponent}}! It is going to fail.

That is not failure of the usage, but failure of the [[GUI]] toolkit [[API]] designer!

That is not a failure of the usage, but a failure of the [[GUI]] toolkit [[API]] designer!

 

== Avoid Deep Hierarchies in Your [[API]]! ==

== Avoid Deep Hierarchies in Your [[API]]! ==

why computer courses are likely to stick with the '''Mammal''' example.

why computer courses are likely to stick with the '''Mammal''' example.

=== Switch with No Limits! ===

=== Switch with no Limits! ===

However, I’ll use the switch

However, I’ll use the switch