Generics
EECS3311 A: Software Design Fall 2018
CHEN-WEI WANG
Motivating Example: A Book of Any Objects
class BOOK
names: ARRAY[STRING]
records: ARRAY[ANY]
— Create an empty book
make do … end
— Add a name-record pair to the book
add (name: STRING; record: ANY) do … end
— Return the record associated with a given name get (name: STRING): ANY do … end
end
Question: Which line has a type error?
1 2 3 4 5 6 7 8
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birthday: DATE; phone_number: STRING b: BOOK; is_wednesday: BOOLEAN create {BOOK} b.make
phone_number := “416-677-1010”
b.add (“SuYeon”, phone_number)
create {DATE} birthday.make(1975, 4, 10)
b.add (“Yuna”, birthday)
is_wednesday := b.get(“Yuna”).get_day_of_week = 4
Motivating Example: Observations (1)
● In the BOOK class:
○ In the attribute declaration
ANY is the most general type of records.
Each book instance may store any object whose static type is a
descendantclass ofANY.
records: ARRAY[ANY]
○ Accordingly, from the return type of the get feature, we only know that the returned record has the static type ANY , but not certain about its dynamic type (e.g., DATE, STRING, etc.).
∴ a record retrieved from the book, e.g., b.get(“Yuna”), may only be called upon features defined in its static type (i.e,. ANY ).
● In the tester code of the BOOK class:
○ In Line 1, the static types of variables birthday (i.e., DATE) and
phone_number (i.e., STRING) are descendant classes of ANY.
∴ Line 5 and Line 7 compile. 3 of 16
Motivating Example: Observations (2)
Due to polymorphism , in a collection, the dynamic types of
stored objects (e.g., phone number and birthday) need not be the same.
○ Features specific to the dynamic types (e.g., get_day_of_week of class Date) may be new features that are not inherited from ANY.
○ This is why Line 8 would fail to compile, and may be fixed using an explicit :
cast
check attached {DATE} b.get(“Yuna”) as yuna_bday then is_wednesday := yuna_bday.get_day_of_week = 4
end
○ But what if the dynamic type of the returned object is not a DATE?
⇒ An assertion violation at runtime! 4 of 16
check attached {DATE} b.get(“SuYeon”) as suyeon_bday then is_wednesday := suyeon_bday.get_day_of_week = 4
end
Motivating Example: Observations (2.1)
● It seems that a combination of attached check (similar to an instanceof check in Java) and type cast can work.
● Can you see any potential problem(s)? ● Hints:
○ Extensibility and Maintainability
○ What happens when you have a large number of records of
distinct dynamic types stored in the book
(e.g., DATE, STRING, PERSON, ACCOUNT, ARRAY CONTAINER, DICTIONARY, etc.)? [ all classes are descendants of ANY ]
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Motivating Example: Observations (2.2)
Imagine that the tester code (or an application) stores 100 different record objects into the book.
rec1: C1
… — declarations of rec2 to rec99
rec100: C100
create {C1} rec1.make(…) ; b.add(…, rec1)
… — additions of rec2 to rec99
create {C100} rec100.make(…) ; b.add(…, rec100)
where static types C1 to C100 are descendant classes of ANY. ○ Every time you retrieve a record from the book, you need to check
“exhaustively” on its dynamic type before calling some feature(s).
— assumption: ’f1’ specific to C1, ’f2’ specific to C2, etc.
check attached {C1} b.get(“Jim”) as c1 then c1.f1 end
… — casts for C2 to C99
check attached {C100} b.get(“Jim”) as c100 then c100.f100 end
○ Writing out this list multiple times is tedious and error-prone! 6 of 16
Motivating Example: Observations (3)
We need a solution that:
● Eliminates runtime assertion violations due to wrong casts
● Saves us from explicit attached checks and type casts
As a sketch, this is how the solution looks like:
● When the user declares a BOOK object b, they must commit to
the kind of record that b stores at runtime.
e.g., b stores either DATE objects (and its ) only
or String objects (and its only, but .
● When attempting to store a new record object rec into b, if
rec’s static type is not a descendant class of the type of book that the user previously commits to, then:
○ It is considered as a compilation error
○ Rather than triggering a runtime assertion violation
● When attempting to retrieve a record object from b, there is no
longer a need to check and cast.
∵ Static types of all records in b are guaranteed to be the same. 7 of 16
descendants
descendants )
not a mix
Parameters
● In mathematics:
○ The same function is applied with different argument values.
e.g.,2 + 3,1 + 1,10 + 101,etc.
○ We generalize these instance applications into a definition.
e.g., + ∶ (Z × Z) → Z is a function that takes two integer parameters and returns an integer.
● In object-oriented programming:
○ We want to call a feature, with different argument values, to
achieve a similar goal.
e.g., acc.deposit(100), acc.deposit(23), etc.
○ We generalize these possible feature calls into a definition.
e.g., In class ACCOUNT, a feature deposit(amount: REAL)
takes a real-valued parameter .
● When you design a mathematical function or a class feature,
always consider the list of parameters , each of which
representing a set of possible argument values. 8 of 16
Generics: Design of a Generic Book
class BOOK[ ]
names: ARRAY[STRING]
records: ARRAY[ G ]
— Create an empty book
make do … end
/* Add a name-record pair to the book */
add (name: STRING; record: G ) do . . . end
/* Return the record associated with a given name */ get (name: STRING): G do . . . end
end
G
Question: Which line has a type error?
1 birthday: DATE; phone_number: STRING 2 ; is_wednesday: BOOLEAN 3
4 phone_number = “416-67-1010”
5 b.add (“SuYeon”, phone_number)
6 create {DATE} birthday.make (1975, 4, 10)
7 b.add (“Yuna”, birthday)
8 is_wednesday := b.get(“Yuna”).get_day_of_week == 4
b: BOOK[DATE]
create BOOK[DATE] b.make
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Generics: Observations
● In class BOOK:
○ At the class level, we :
parameterize the type of records
class BOOK[G]
○ Every occurrence of ANY is replaced by E.
● As far as a client of BOOK is concerned, they must instantiate G.
⇒ This particular instance of book must consistently store items of
that instantiating type.
● As soon as E instantiated to some known type (e.g., DATE,
STRING), every occurrence of E will be replaced by that type. ● For example, in the tester code of BOOK:
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○ In Line 2, we commit that the book b will store DATE objects only.
○ Line 5 fails to compile. [ ∵ STRING not descendant of DATE ]
○ Line 7 still compiles. [ ∵ DATE is descendant of itself ]
○ Line 8 does not need any attached check and type cast, and
does not cause any runtime assertion violation.
ì All attempts to store non-DATE objects are caught at compile time.
Bad Example of using Generics
Has the following client made an appropriate choice?
book: BOOK[ANY]
NO!!!!!!!!!!!!!!!!!!!!!!!
○ It allows all kinds of objects to be stored.
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∵ All classes are descendants of ANY .
○ We can expect very little from an object retrieved from this book.
∵ The static type of book’s items are ANY , root of the class hierarchy, has the minimum amount of features available for use. ∵ Exhaustive list of casts are unavoidable.
[ bad for extensibility and maintainability ]
Instantiating Generic Parameters
● Say the supplier provides a generic DICTIONARY class:
class DICTIONARY[V, K] — V type of values; K type of keys add_entry (v: V; k: K) do … end
remove_entry (k: K) do … end
end
● Clients use DICTIONARY with different degrees of instantiations:
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.
class DATABASE_TABLE[K, V] imp: DICTIONARY[V, K]
end
e.g., Declaring
instantiates
e.g., Declaring
instantiates
DATABSE_TABLE[INTEGER, STRING]
DICTIONARY[STRING, INTEGER]
.
class STUDENT_BOOK[V]
imp: DICTIONARY[V, STRING]
end
STUDENT_BOOK[ARRAY[COURSE]]
DICTIONARY[ARRAY[COURSE], STRING]
Generics vs. Inheritance (1)
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Generics vs. Inheritance (2)
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Beyond this lecture . . .
● Study the “Generic Parameters and the Iterator Pattern” Tutorial Videos.
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Index (1)
Motivating Example: A Book of Any Objects Motivating Example: Observations (1) Motivating Example: Observations (2) Motivating Example: Observations (2.1) Motivating Example: Observations (2.2) Motivating Example: Observations (3) Parameters
Generics: Design of a Generic Book Generics: Observations
Bad Example of using Generics Instantiating Generic Parameters Generics vs. Inheritance (1) Generics vs. Inheritance (2)
Beyond this lecture . . . 16 of 16