Java程序辅导

  
Dev Seminar
Clean Object oriented programming
  
Caveat
● Everything said in this lecture is true
– Or so I'm claiming – at least for java/c++ style 
object
– But some languages (python, rust, golang) 
don't have that object model and therefore 
work differently.
– Many Principles still hold
  
Abstraction
● The whole point of Object Oriented 
Programming/Design:
– Abstraction through encapsulation
– Encapsulation: hiding object state through private 
instance variables
– Abstraction: hiding the details from users – it just 
works
– Font rendering example in pgs 126-129 of code 
complete
– Example from 94 in clean code
● Note the jab at java.
  
ADT Advantages
● Classes are Abstract Data Types (ADTs)
● What are the advantages of these?
  
ADT Advantages
● Classes are Abstract Data Types (ADTs)
● What are the advantages of these?
– As a hint, why do we say global variables are 
bad?
  
ADT Advantages
● Classes are Abstract Data Types (ADTs)
● What are the advantages of these?
– As a hint, why do we say global variables are 
bad?
– Every time we access one in a new place, have 
potential to muck up every other place the 
variable is used.
  
ADT Advantages
● Classes are Abstract Data Types (ADTs)
● What are the advantages of these?
– Now we can use whatever internal representation 
is efficient/convenient without the client code 
needing to know
– We can change the implementation without 
anyone knowing/caring
– We can make the code more readable/self 
documenting
  
We see this in language design
● Long long ago, in a world quite different than our 
own….
– C was a new and shiny language
– How did it manage text?
  
See ADT advantage in language 
design
● Long long ago, in a world quite different than our 
own….
– C was a new and shiny language
– How did it manage text?
● As an array of characters with a special end 
character
● This lived on to early versions of C++
  
See ADT advantage in language 
design
● Long long ago, in a world quite different than our 
own….
– C was a new and shiny language
– How did it manage text?
● As an array of characters with a special end 
character
● This lived on to early versions of C++
– How about any modern and semi modern langage?
● Java,C#,python3,Go,Rust,Kotlin,Swift?
  
See ADT advantage in language 
design
● Long long ago, in a world quite different than our 
own….
– C was a new and shiny language
– How did it manage text?
● As an array of characters with a special end 
character
● This lived on to early versions of C++
– How about any modern and semi modern langage?
● Java,C#,python3,Go,Rust,Kotlin,Swift?
– Text strings are an ADT
  
Aside
● Python2 died Jan 1 2020
● What is the biggest difference between python2 
and python3?
  
Aside
● Python2 dies Jan 1 2020
● What is the biggest difference between python2 
and python3?
– Text handling. Python2 really can’t handle 
Unicode/UTF-8 characters sanely. If you want 
more than just English need UTF-8
  
Advantage of ADTs II
● Easy separation of multiple objects of same 
class
● See page 132 in clean code for example of 
id/handle approach.
  
Accessors and Mutators
● We use accessors and mutators all the time
– Why?
  
Accessors and Mutators
● We use accessors and mutators all the time
– And why are we letting clients access our 
private data?
– Accessors maybe
– Mutators?!?!?!
– Why do we use accessors even? Is there a 
better way?
– (I Know some languages love them)
– “Richard” gives us another reason not to 
abuse mutators
– http://mcfunley.com/from-the-annals-of-dubiou
s-achievement
  
Data Structures vs Objects
● Book:
– Objects hide their data and expose operations
– Data structures expose their data and have no 
operations.
● (think an array or a linked list)
– Now we can try to make object oriented data 
structures 
● but there is a reason for c++ struct
– How about a Java 'class' that is really a data 
structure?
– Python? 
  
Law of Demeter
● Law of Demeter
– Ian Holland 1987 Northeastern Univ
– Don't talk to strangers only talk to friends (in the c++ 
sense of the word)
– AKA: principle of least knowledge
– common version:
● A module should not know about the innards of the 
objects it uses
– Podcast guest (changelog version) notes this ‘law’ is 
more like ‘rule of thumb of Demeter’
  
Law Of Demeter
● Law of Demeter Specifics:
– A class C with a method M, M can only call:
● Methods in class C
● Methods in an object created by M
● Methods from objects passed as parameters to M
● Methods from an instance variable of C
● Do not call methods on objects returned by any of the 
above
– Buys us automatically reduced coupling
– By which I mean?
●  
  
Train Wrecks
● Often referred to as optional part or optional 
corollary of Law of Demeter: avoid train wrecks
● Any one know what train wrecks are?
  
Train Wrecks
● Often referred to as optional part or optional 
corollary of Law of Demeter: avoid train wrecks
● Any one know what train wrecks are?
● this.configuration.getLocation().toString().toUpperC
ase().equals(otherString)
● Wow!1?
– Perfectly syntactically correct
– I’ve done a lesser version myself.
● Law of Demeter violation?
  
Train Wrecks
● Often referred to as optional part or optional 
corollary of Law of Demeter: avoid train wrecks
● Any one know what train wrecks are?
● this.configuration.getLocation().toString().toUpperC
ase().equals(otherString)
● Wow!1?
– Perfectly syntactically correct
– I’ve done a lesser version myself.
● Law of Demeter violation?
– No – but how long did it take you to tell?
  
Train Wrecks
● Main problem with train wrecks is that it is hard to 
tell if there is a law of Demeter violation.
● How about this:
– self.priceLabel.text = self.media.ad.price.value;
– Is this a Law of Demeter violation?
– And this?
  
Train Wrecks
● Main problem with train wrecks is that it is hard to 
tell if there is a law of Demeter violation.
● How about this:
– self.priceLabel.text = self.media.ad.price.value;
– Is this a Law of Demeter violation?
– No – if you can access the data members directly it 
is a data structure – not a class
– So no Law of Demeter Violation.
– Difficulty in languages like java and frameworks like 
java beans that demand all data structures use 
private instance variables and accessors.
  
Data Transfer Objects
● Data Transfer Objects (DTO)
– Pure data structures
– Public instance variables, no methods
– structs from C++
– Named tuples from python
●
  
Next Step
● The Java Bean:
– 'quasi-encapsulation'
– Private instance variables
– Accessors and mutators for all
– Robert Martin (Uncle Bob) refers to this as:
– “to make some OO purists feel better but usually 
provides no other benefit”
– Were everywhere 10-ish years ago (in heyday of 
java)
● Seem to be less widely used these days
● Lots of legacy code to support.
  
Active Records
● Refers to a 'special type of DTO
● DTO with 'navigation methods' like save and find
● Designed by Martin Fowler
● Don't add other methods
– Like business rules.
● These days almost synonymous with ruby on rails
– Which wasn’t a thing when Clean Code and code 
complete were written.
  
Composition vs Inheritance
● Did Dr. Liang or Dr. Kim cover this?
– Lots of different approaches in recent languages
– Composition
● Has-a relationship, instance variable of another 
object
– Inheritance
● Is-a relationship
● Lives in many OO languages
● Less so go/rust/objective-c
  
Now
● Assignment reminder next slide
● After that, lets take some time and let the 
groups get together to see what they need to 
do next on this project.
  
Assignment
●
●  https://changelog.com/podcast/339
● Or
● https://www.se-radio.net/2019/02/se-radio-
episode-357-adam-barr-on-code-quality/
● The software engineering radio episode is 
hosted by a professor, the changelog version 
is hosted by a former industry practitioner. 
Both of them interview the same guest about 
the same topic. He is more polished in the 
changelog episode