1. Inheritance and Polymorphism OOSC2 chapters 14 and parts of 15-17
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COSC3311 – Software Design
Inheritance and Polymorphism OOSC2 chapters 14 and parts of 15-17
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2. The Open-Closed Principle
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The Open-Closed Principle B A C E D F A’ G H I
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3. Open-Closed Principle
Modules should be both Open and Closed
It is open if it is available for extension (Volatile?)
It is closed if it’s API is fixed i.e. clients can rely on well-defined interface that does not change (Stable?)
(The implementation may change)
It’s a contradiction!
Resolved via inheritance!
Inheritance is not just for classification (taxonomy) but also for re-use
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4. 1/16/2019 1:45 PM
What is inheritance?
Principle: Describe a new class not from scratch but as extension or specialization of one existing class — or several in the case of MULTIPLE inheritance.
From the module viewpoint: if B inherits from A, all the services of A are potentially available in B (possibly with a different implementation).
From the type viewpoint: inheritance is the ‘‘is-a’’ relation. If B inherits from A, whenever an instance of A is required, an instance of B will be acceptable.
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5. Terminology Parent, Heir Ancestor, Descendant
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Terminology
Parent, Heir
Ancestor, Descendant
The ancestors of B are B itself
and the ancestors of its parents.
Proper ancestor, Proper descendant
Direct instance, Instance
The instances of A are the
direct instances of its descendants.
(Other terminology: subclass, superclass, base class)
feature A
feature B
Definition: heir, parent If class C has a Parent clause for B, then C is said to inherit from B; B is said to be a parent of C, and C is said to be an heir of B. Definition: ancestor, descendant Class A is an ancestor of class B if and only if A is B itself or, recursively, an ancestor of one of B's parents. Class B is a descendant of class A if and only if A is an ancestor of B, in other words if B is A or (recursively) a descendant of one of its heirs. Definition: proper ancestor, descendant The proper ancestors of a class C are its ancestors other than C itself. The proper descendants of a class B are its descendants other than B itself.
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6. Polymorphism & Dynamic Binding
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7. Design Problem
Suppose we had an array of figures and want to rotate all the figures in it.
Each figure has its own rotation method

8. The traditional architecture
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The traditional architecture
figures: ARRAY[ANY]
rotate_all_figures_in_array_by ( d : DEGREE)
require d > 0
local
f: ANY; i: INTEGER do
from i := figures.lower
until i = figures.upper
loop
f := figures[i]
rotate(f, d) -- what happens with an unknown f?
i := i + 1
end
Each routine display, move, scale would have to be changed with a new figure class
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9. Traditional rotate (f: ANY; d: DEGREE) do if ‘‘f is a CIRCLE’’ then …
...
elseif ‘‘f is a POLYGON’’ then
end
and similarly for all other routines such as display etc.!
What happens when we add a new figure type?
We must update all routines rotate, display etc.
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10. Single Choice Principle
What happens when we add a new figure type?
We must update all routines rotate, display etc.
Single Choice Principle
Whenever a software system must support a set of alternatives (e.g. variants of a figure in graphic systems), one and only one module in the system should know their exhaustive list
Later, if variants are added (e.g. by introducing a class in the figure hierarchy for a new figure), we only have to update a single module – the point of single choice!
Follows from the Open-Close Principle
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11. New solutions required!
Want to be able to add new kinds of figures without
breaking previous programs
without modifying the method that rotates all figures
Solution
polymorphism & dynamic binding
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12. FIGURE polymorphism FIGURE * CLOSED_ FIGURE POLYGON ELLIPSE + display*
barycenter* …
display*
rotate*
perimeter*
POLYGON
ELLIPSE
perimeter+ +
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13. Example hierarchy * FIGURE * * OPEN_ FIGURE CLOSED_ FIGURE + + SEGMENT
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Example hierarchy
extent*
display*
* deferred
+ effective
++ redefined
barycenter* *
rotate* …
FIGURE … * *
OPEN_ FIGURE
CLOSED_ FIGURE
perimeter*
perimeter+
perimeter+ + +
SEGMENT
POLYLINE
POLYGON
ELLIPSE
perimeter++
...
TRIANGLE
RECTANGLE
diagonal
side1
CIRCLE
side2
perimeter++
perimeter++
SQUARE
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14. Redefinition class POLYGON inherit CLOSED_FIGURE create make feature
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Redefinition
class POLYGON inherit CLOSED_FIGURE create
make
feature
vertices: ARRAY [POINT]
vertices_count: INTEGER
perimeter: REAL
-- Perimeter length do
from ... until ... loop
Result := Result +
(vertices[i]) . distance (vertices [i + 1])
...
end end
invariant
vertices.count >= 3
vertices_count = vertices.count end
vertices[i]
vertices[i+1]
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15. Redefinition (cont’d)
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Redefinition (cont’d)
class RECTANGLE
inherit
POLYGON
redefine
perimeter
end create
make
feature diagonal, side1, side2: REAL
perimeter: REAL is
-- Perimeter length do
Result := 2 * (side1 + side2)
end
invariant vertices_count = 4 end
side2
side1
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16. Solution
figures: ARRAY[FIGURE] rotate_all_figures_in_array_by ( d : DEGREE) require d > 0 local f: FIGURE; i: INTEGER do from i := fig_array.lower until i = fig_array.upper loop -- static typing f := fig_array[i] -- dynamic binding f.rotate(d) -- polymorphism i := i + 1 end
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17. Applying the Single Choice Principle
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Applying the Single Choice Principle
f: FIGURE
c: CIRCLE
p: POLYGON
...
create c.make (...)
create p.make (...)
if ... then
f := c
else
f := p
end
f.move (...)
f.rotate (...)
f.display (...)
Dynamic binding ensures that the right routine e.g. `move’ is selected.
Do not need a long time If statement
Easy to add a new kind of figure
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18. Polymorphism Typing Binding 1/16/2019 1:45 PM
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19. Inheritance and polymorphism
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Inheritance and polymorphism
Assume:
p: POLYGON; r: RECTANGLE; t: TRIANGLE;
x: REAL 
Permitted:
x := p.perimeter
x := r.perimeter
x := r.diagonal
p := r
NOT permitted:
x := p.diagonal (even just after p := r  !) r := p p
(POLYGON) r
(RECTANGLE)
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20. 1/16/2019 1:45 PM
Dynamic binding
What is the effect of the following (assuming some_test true)?
if some_test then
p := r
else
p := t
end
x := p.perimeter
Redefinition: A class may change an inherited feature, as with RECTANGLE redefining perimeter.
Polymorphism: p may have different forms at run-time.
Dynamic binding: Effect of p.perimeter depends on run-time form of p.
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21. Polymorphism Polymorphism means the ability to take several forms
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Polymorphism
Polymorphism means the ability to take several forms
In OO, it is entities (attributes, arguments, local variables) which can become attached to objects of different types
P: POLYGON; r:RECTANGLE; t: TRIANGLE The following are both valid:
p := r
p := t
Assignments in which the type of the source (RHS) is different than the target (LHS) are called polymorphic assignments
Since both RECTANGLE and TRIANGLE inherit from POLYGON we say that the type of the source conforms to the type of the target.
In the reverse direction the assignment r := p is not valid because p does not conform to r.
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22. Type checking When should we check typing and binding?
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Type checking
When should we check typing and binding?
statically (compile time)
dynamically (runtime)
Runtime or Compile time
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23. Typing – OOSC2 chapter 17 * AIRCRAFT * * PLANE COPTER BOEING AIRBUS
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Typing – OOSC2 chapter 17
* deferred *
+ effected
AIRCRAFT
++ redefined * *
PLANE
COPTER
lower_landing_gear*
BOEING
AIRBUS
A_320
B_737
B_747
a: ARRAY[AIRCRAFT]
a[i].lower_landing_gear
target.f(arg)
lower_landing_gear+
B_747_400
lower_landing_gear++
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24. Check at compile time or runtime?
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Check at compile time or runtime?
when should we check that there is a feature lower_landing_gear (compile time or runtime)?
If there is more than one (polymorphism), when should we determine which one to use?
a: PLANE b: B_747 a := b a.take_off ... a.lower_landing_gear
Question (1) is the typing problem
Question (2) is the binding problem
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25. Check at compile time or runtime?
a: PLANE b: B_747 a := b a.take_off ... a.lower_landing_gear
runtime is much too late to find out that you do not have landing gear
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26. Typing vs. Binding Binding should be checked at runtime (dynamically)
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Typing vs. Binding
Binding should be checked at runtime (dynamically)
So that the correct version of lower_landing_gear is invoked
Typing should be checked at compile time (statically)
Runtime is much too late to determine that you do not have lower_landing_gear
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27. Typing vs. binding What do we know about the feature to be called?
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Typing vs. binding
What do we know about the feature to be called?
Static typing:
At least one
Dynamic binding:
The right one
Example:
my_aircraft.lower_landing_gear
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28. The Typing problem Basic OO Construct: x.f (arg)
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The Typing problem
Basic OO Construct: x.f (arg)
meaning: execute on the object attached to x the operation f, using the argument arg
The model is simple – at run time, this is what our systems do – calling features on objects, and passing arguments as necessary
From the simplicity of the model follows the simplicity of the typing problem, whose statement mirrors the structure of the Basic Construct:
When, and how, do we know that:
There is a feature corresponding to f and applicable to the object?
arg is an acceptable argument for that feature?
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29. Definition – statically typed system
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Definition – statically typed system
Type violation: a runtime type violation occurs in the execution of a call x.f(arg) where x is attached to an object OBJ if either
there is no feature corresponding to f and and applicable to OBJ
There is a feature but arg is not acceptable
An OO language is statically typed if it is equipped with a set of consistency rules so that a compiler can detect that there will be no runtime type violations merely by inspecting the program text (at compile time)
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30. Consistency Rules (partial)
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Consistency Rules (partial)
x: D … x.f(arg)
Declaration Rule: Every entity (e.g. x) must be explicitly declared to be of a certain type
Call rule: If a class C contains the call x.f(arg) there must be feature f in class D exported to C
Attachment Rule: In an assignment x := y (or argument passing) the base class of y must be a descendant of the base class of x.
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31. (After Barry W. Boehm) LARGE PROJECTS SMALL PROJECTS 1/16/2019 1:45 PM
Cost of Fixing a Problem (y-axis) versus time/development phase (y-axis)
Detecting problems (like type violations) earlier (at compile time) saves development costs
SMALL PROJECTS
Requirements
Design
Code
Development
Acceptance
Operation
test
test
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32. Dynamic Language: Python
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Dynamic Language: Python
Part of the program executed correctly!
There is no compile time!
Runtime error!
Python is the interpreter language, you do not need to compile your code, and also you have no way to check for syntax errors until you run your python script. Either syntax errors or runtime errors will be throw to standard output through python exception handler by default.
Python throws the exception with traceback info indicate which part of your scripts has gone wrong, it looks like this:
Traceback (most recent call last): File "except.py", line 9, in main() File "except.py", line 5, in main print l[4] IndexError: list index out of range
The traceback indicates that there is an IndexError happening in main function.
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33. Dynamic Typing: php Type Error: Argument “b” is not a number
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Dynamic Typing: php
Type Error: Argument “b” is not a number
Type Error: There is no routine “withdra”
(spelling error)
PHP Notice is a Warning: But now we are producing incorrect results $224 (instead of $223)
PHP Fatal Error stops the program from further execution
indigo 309 % /xsys/pkg/php/bin/php bank.php PHP Notice: Object of class Account could not be converted to int in /cs/home/jonathan/tools/php/bank.php on line (This is wrong! Account “b” interpreted as withdraw “1”) PHP Fatal error: Call to undefined method Account::withdra() in /cs/home/jonathan/tools/php/bank.php on line 18
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34. Casting breaks the type system
What programming languages offer genericity that you know of? Java? C++? Other?
C++ has the template: Set < int > s ;
Java 5 added genericity
What is the effect of genericity on
compile time
size of the generated code
execution time
execution space
Warning: generics cheap in Eiffel – expensive in C++

35. Non-generic Java Stack
public class Stack {
private int count;
private int capacity;
private int capacityIncrement;
private Object[] itemArray; // instead of ARRAY[G]
Stack() { //constructor limited to class name
count= 0;
capacity= 10;
capacityIncrement= 5;
itemArray= new Object[capacity]; }
public boolean empty() {
return (count == 0);

36. Java Stack (cont.) public void push(Object x) {
if(count == capacity) {
capacity += capacityIncrement;
Object[] tempArray= new Object[capacity];
for(int i=0; i < count; i++)
tempArray[i]= itemArray[i];
itemArray= tempArray; }
itemArray[count++]= x;
public Object pop() {
if(count == 0)
return null;
else return itemArray[--count];

37. Java Stack (cont.) public Object peek() { if(count == 0) return null;
else return itemArray[count-1]; }
public class Point {
public int x;
public int y;

38. Java Stack runtime error
class testStack {
public static void main(String[] args) {
Stack s = new Stack();
Point upperRight = new Point();
upperRight.x= 1280; // breaks information hiding
upperRight.y= 1024; // cannot guarantee any contract
s.push("red");
s.push("green"); // push some String
s.push("blue"); // objects on the stack
s.push(upperRight); // push a POINT object on the stack
while(!s.empty()) { // cast all items from OBJECT
String color = (String) s.pop(); // to STRING in order to print
System.out.println(color);
}}} // causes a run-time error
// ClassCastException: Point at testStack.main(testStack.java:18)
// With genericity, it is a compile time error

39. Why Twitter Switched to Scala (2009)
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Why Twitter Switched to Scala (2009)
Twitter began its life as a Ruby on Rails application, and still uses Ruby on Rails to deliver most user-facing web pages. In this interview, three developers from Twitter—Steve Jenson, system engineer; Alex Payne, API lead; and Robey Pointer, member of the service team—sit down with Bill Venners to discuss Twitter's real-world use of Scala.
Bill Venners: I’m curious, and the Ruby folks will want it spelled out: Can you elaborate on what you felt the Ruby language lacked in the area of reliable, high performance code?
Steve Jenson: ... Another thing we really like about Scala is static typing that’s not painful ...
Reliable, high performance code
Bill Venners: I’m curious, and the Ruby folks will want it spelled out: Can you elaborate on what you felt the Ruby language lacked in the area of reliable, high performance code?
Steve Jenson:One of the things that I’ve found throughout my career is the need to have long-lived processes. And Ruby, like many scripting languages, has trouble being an environment for long lived processes. But the JVM is very good at that, because it’s been optimized for that over the last ten years. So Scala provides a basis for writing long-lived servers, and that’s primarily what we use it for at Twitter right now. Another thing we really like about Scala is static typing that’s not painful. Sometimes it would be really nice in Ruby to say things like, here’s an optional type annotation. This is the type we really expect to see here. And we find that really useful in Scala, to be able to specify the type information.
Robey Pointer: Also, Ruby doesn’t really have good thread support yet. It’s getting better, but when we were writing these servers, green threads were the only thing available. Green threads don't use the actual operating system’s kernel threads. They sort of emulate threads by periodically stopping what they are doing and checking whether another “thread” wants to run. So Ruby is emulating threads within a single core or a processor. We wanted to run on multi-core servers that don’t have an infinite amount of memory. And if you don’t have good threading support, you really need multiple processes. And because Ruby’s garbage collector is not quite as good as Java’s, each process uses up a lot of memory. We can’t really run very many Ruby daemon processes on a single machine without consuming large amounts of memory. Whereas with running things on the JVM we can run many threads in the same heap, and let that one process take all the machine’s memory for its playground.
Alex Payne: I’d definitely want to hammer home what Steve said about typing. As our system has grown, a lot of the logic in our Ruby system sort of replicates a type system, either in our unit tests or as validations on models. I think it may just be a property of large systems in dynamic languages, that eventually you end up rewriting your own type system, and you sort of do it badly. You’re checking for null values all over the place. There’s lots of calls to Ruby’s kind_of? method, which asks, “Is this a kind of User object? Because that’s what we’re expecting. If we don’t get that, this is going to explode.” It is a shame to have to write all that when there is a solution that has existed in the world of programming languages for decades now.
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40. Why Twitter Switched to Scala (2009)
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Why Twitter Switched to Scala (2009)
Alex Payne: I’d definitely want to hammer home what Steve said about typing. As our system has grown, a lot of the logic in our Ruby system sort of replicates a type system, either in our unit tests or as validations on models. I think it may just be a property of large systems in dynamic languages, that eventually you end up rewriting your own type system, and you sort of do it badly.
You’re checking for null values all over the place. There’s lots of calls to Ruby’s kind_of? method, which asks, “Is this a kind of User object? Because that’s what we’re expecting. If we don’t get that, this is going to explode.” It is a shame to have to write all that when there is a solution that has existed in the world of programming languages for decades now.
Reliable, high performance code
Bill Venners: I’m curious, and the Ruby folks will want it spelled out: Can you elaborate on what you felt the Ruby language lacked in the area of reliable, high performance code?
Steve Jenson:One of the things that I’ve found throughout my career is the need to have long-lived processes. And Ruby, like many scripting languages, has trouble being an environment for long lived processes. But the JVM is very good at that, because it’s been optimized for that over the last ten years. So Scala provides a basis for writing long-lived servers, and that’s primarily what we use it for at Twitter right now. Another thing we really like about Scala is static typing that’s not painful. Sometimes it would be really nice in Ruby to say things like, here’s an optional type annotation. This is the type we really expect to see here. And we find that really useful in Scala, to be able to specify the type information.
Robey Pointer: Also, Ruby doesn’t really have good thread support yet. It’s getting better, but when we were writing these servers, green threads were the only thing available. Green threads don't use the actual operating system’s kernel threads. They sort of emulate threads by periodically stopping what they are doing and checking whether another “thread” wants to run. So Ruby is emulating threads within a single core or a processor. We wanted to run on multi-core servers that don’t have an infinite amount of memory. And if you don’t have good threading support, you really need multiple processes. And because Ruby’s garbage collector is not quite as good as Java’s, each process uses up a lot of memory. We can’t really run very many Ruby daemon processes on a single machine without consuming large amounts of memory. Whereas with running things on the JVM we can run many threads in the same heap, and let that one process take all the machine’s memory for its playground.
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41. Using Scala/Joda library
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Using Scala/Joda library
scala> import org.joda.time._                       import org.joda.time._ scala> def time = new DateTime(2010, 5, 7, 0, 0, 0, 0) time: org.joda.time.DateTime scala> time res0: org.joda.time.DateTime = T00:00: :00 scala> def time = new DateTime(2010, 5, 7, 0, 0, 0)   time: org.joda.time.DateTime scala> time                                         java.lang.IllegalArgumentException: No instant converter found for type: scala.Tuple6 ....
(July 2010)
There is this library joda time which I'd always heard nice things about so I set out to use it today.  Let me show you something. scala> import org.joda.time._                       import org.joda.time._ scala> def time = new DateTime(2010, 5, 7, 0, 0, 0, 0) time: org.joda.time.DateTime scala> time res0: org.joda.time.DateTime = T00:00: :00 scala> def time = new DateTime(2010, 5, 7, 0, 0, 0)   time: org.joda.time.DateTime scala> time                                         java.lang.IllegalArgumentException: No instant converter found for type: scala.Tuple6         at org.joda.time.convert.ConverterManager.getInstantConverter(ConverterManager.java:165)         at org.joda.time.base.BaseDateTime.<init>(BaseDateTime.java:169) This is a DEBACLE! The joda time constructors include one which takes Object.  That means you lose 100% of your type safety unless you luck into a conflict it will notice, because scala thinks "oh, you must have meant to call the Object constructor with a 6-tuple, let me help you." No scala buddy, I didn't mean that. I knew this risk existed but now having experienced it in this way I consider it past indecent.  I think we must do something.
====
Oh, wait, I didn't look closely enough at the example.  I assume you specified the wrong number of arguments for the DateTime constructor in the second definition of time, so scalac happily tried to find a way to make it the right number of arguments.  So when one would expect a compile error due to the wrong number of arguments, one receives runtime error because scalac tuplized your arguments into a single object.
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42. Genericity + Inheritance: Polymorphic data structures
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Genericity + Inheritance: Polymorphic data structures
class STACK [G]
feature
...
item: G is ...
put (x: G) is ...
end
fs: STACK [FIGURE]
r: RECTANGLE
s: SQUARE
t: TRIANGLE
p: POLYGON
fs.put (p); fs.put (t); fs.put (s); fs.put (r) fs.item.display fs
(RECTANGLE)
(SQUARE)
(TRIANGLE)
(POLYGON)
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43. Genericity vs. Inheritance
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Genericity vs. Inheritance
Abstraction
SET_OF_BOOKS
Type parameterization
Type parameterization
LIST_OF_PEOPLE
LIST_OF_BOOKS
LIST_OF_
JOURNALS
LIKED_LIST_OF_BOOKS
Specialization
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44. Inheritance & Contracts
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45. Invariant accumulation
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Invariant accumulation
Every class inherits all the invariant clauses of its parents.
These clauses are conceptually “and”-ed.
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46. Accumulating invariants
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Accumulating invariants
Every routine will fail with an invariant contract violation
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47. Inheritance and assertions
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Inheritance and assertions
What is the relationship between postcondition C in POLYGON and D in RECTANGLE?
In the descendant RECTANGLE
Require less
Ensure more
i.e.
Why?
So that RECTANGLE should not be able to change the meaning of perimeter to area
Substitutivity
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48. 1/16/2019 1:45 PM
Substitutivity
Require less / Ensure more constrains an instance of RECTANGLE to act like an instance of POLYGON.
Hence the instance of RECTANGLE can always be substituted for an instance of POLYGON without changing the meaning of function routine perimeter
Without contracts you could not truly ensure substitutivity
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49. Precondition Subcontracting
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Precondition Subcontracting
Correct call:
if a1. then
a1.r (...)
else ...
end
r is C A
require
a1: A  …
ensure
a1.r (…)  B
r is
require
   so that if a1. holds then so does a1. 
Require Less
Ensure more
Substitutability
ensure 
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50. Assertion redeclaration rule
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Assertion redeclaration rule
Redefined version may not have require or ensure.
May have nothing (assertions kept by default), or
require else new_pre
ensure then new_post
Resulting assertions are:
original_precondition or new_pre
original_postcondition and new_post
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51. Why Contracts? Specifications (vs. implementations)
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Why Contracts?
Specifications (vs. implementations)
Documents the contract between client and supplier
contract view (information hiding)
Validating the implementation
Check that the implementation satisfies the specification
Catches many errors
Verification via formal methods
Defines an exception (contract violation)
Subcontracting (ensures substitutivity)
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52. The meaning of inheritance
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The meaning of inheritance
The type perspective:
Automatic adaptation of operations to the dynamic form of their targets (extendibility): Representation independence.
Factoring out reusable operations: Commonality.
The module perspective (reusability):
Open-closed modules
Modularity
The Single Choice Principle
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53. Hoare specification of code
{P} prog {Q}
Execution of the code prog
started in a state satisfying P
is guaranteed to terminate in a state satisfying Q
{a:ARRAY} sort {i:INT | 1ia.count  a[i]  a[i+1]}
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54. The dangers of static binding
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The dangers of static binding
create a.make (…)
For every creation procedure cp: {precp} docp {postcp and INV}
For every exported routine r: {INV and prer} dor {INV and postr}
The worst possible erroneous run-time situation
in object-oriented software development:
Producing an object that does not satisfy
the invariant of its class. S1
a.f (…) S2
a.g (…) S3
a.f (…) S4
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55. The dangers of static binding
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The dangers of static binding
{INVA} dorA {INVA}
{INVB} dorB {INVB}
Consider a call of the form a1.r where a1 is polymorphic
rA preserves INVA and rB preserves INVB
But rA has no reason to preserves INVB
So a1.r (under static binding) executes rA on objects of type B, thus resulting in an inconsistent object (one that does not satisfy INVB) rA A
rB++ B
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56. Using original version of redefined feature
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Using original version of redefined feature
class BUTTON inherit
WINDOW redefine
display
end
feature
display is do Precursor display_border display_label end
display_label is do
...
display_border is
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57. A concrete example w: WINDOW b: BUTTON create b w := b w.display
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A concrete example
w: WINDOW
b: BUTTON
create b
w := b
w.display
display
WINDOW
display++
BUTTON
Static binding would execute {WINDOW}.display on the button rather than {BUTTON}.display
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58. Use of Precursor Not necessarily the first feature statement.
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Use of Precursor
Not necessarily the first feature statement.
May have arguments.
class B inherit
A redefine
my_feature
end
feature
my_feature (args: SOME_TYPE) is do Something here
Precursor (args)
-- Something else here
my_feature A
my_feature++ B
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59. Assignment Attempt (OOSC2 16.5)
Suppose we want to find out the longest diagonal (rectangles only) in a list of figures.
Suppose we were retrieving the list from a network or from a stored file where we could not guarantee the type of the object.
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60. Forcing a type: the problem
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Forcing a type: the problem
list: LIST [FIGURE]
x: RECTANGLE
list.store ("FILE_NAME")
Two years later:
list := retrieved ("FILE_NAME") x := list.item -- [1]
print (x.diagonal) -- [2]
But
If x is declared of type RECTANGLE, [1] will not compile.
If x is declared of type FIGURE, [2] will not compile.
[1] is invalid because POLYGONS on the stack
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61. What not to do! if “list.item is of type RECTANGLE” then … elseif
“list.item is of type CIRCLE”
etc.
We could so the above as in ANY we have:
conforms_to (other: ANY): BOOLEAN is
-- Is type of current object identical to type of `other'?
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62. Assignment attempt (cont’d)
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Assignment attempt (cont’d)
x ?= y
with
x: A
If y is attached to an object whose type conforms to A, perform normal reference assignment.
Otherwise, make x void.
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63. Calculating largest diagonal in list – Solution
list: LIST[FIGURE]
maximum_diagonal (file_name: STRING): REAL
-- Max value of diagonals of rectangles in list retrieved from storage or network
-- -1 if none
local r: RECTANGLE do
Result := -1
list ?= retrieved(“file_name”)
if list /= Void then
-- file contains an object structure that conforms to LIST[FIGURE]
from list.start;
until list.after
loop
r ?= list.item
if r /= Void then
-- list.item conforms to RECTANGLE
Result := Result.max(r.diagonal)
end
list.forth
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64. Feature Call Rule is Preserved
r ?= list.item
if r /= Void then
Result := Result.max(r.diagonal)
end
r.diagonal is guarded
statically, by a compiler check that diagonal is a feature of class RECTANGLE
dynamically by a guarantee that r /= Void
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65. Multiple inheritance A class may have two or more parents.
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Multiple inheritance
A class may have two or more parents.
What not to use as an elementary example: TEACHING_ASSISTANT inherits from TEACHER and STUDENT.
TEACHER
STUDENT
TEACHING_ASSISTANT
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66. The teaching assistant example
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The teaching assistant example
This is in fact a case of repeated inheritance: id
UNIVERSITY_PERSON ?? ??
TEACHER
STUDENT
Chapter 15: Page 543.
Introduces ambiguities.
But “id” is ok  sharing
TEACHING_ASSISTANT
????
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67. Sharing and replication
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Sharing and replication
Features such as age and birthday, not renamed along any of the inheritance paths, will be shared.
Features such as tax_id, violation_count inherited under different names, will be replicated.
address
tax_id
TAXPAYER
age
pay_taxes
US_ TAXPAYER
SWISS_ TAXPAYER
address us_address
address swiss_address
tax_id us_tax_id
tax_id swiss_tax_id
SWISS_US_ TAXPAYER
pay_taxes pay_us_taxes
pay_taxes pay_swiss_taxes

68. Common examples of multiple inheritance
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Common examples of multiple inheritance
Combining separate abstractions:
Restaurant, train car
Calculator, watch
Plane, asset
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69. Multiple inheritance: Combining abstractions
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Multiple inheritance: Combining abstractions * *
COMPARABLE
NUMERIC
INTEGER
REAL
STRING
COMPLEX
DOUBLE
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70. Multiple inheritance: Nested windows
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Multiple inheritance: Nested windows
‘‘Graphical’’ features: height, width, change_height, change_width, xpos, ypos, move...
‘‘Hierarchical’’ features: superwindow, subwindows, change_subwindow, add_subwindow...
class WINDOW
inherit
RECTANGLE
TREE [WINDOW]
feature ... end
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71. Multiple inheritance: Composite figures
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Multiple inheritance: Composite figures
Simple figures
A composite figure
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72. Defining the notion of composite figure
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Defining the notion of composite figure *
LIST [FIGURE]
FIGURE
display
count
hide
put
rotate
remove
move … …
COMPOSITE_FIGURE
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73. Defining the notion of composite figure through multiple inheritance
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Defining the notion of composite figure through multiple inheritance *
LIST [FIGURE]
FIGURE
OPEN_ FIGURE
CLOSED_ FIGURE
perimeter*
SEGMENT
POLYLINE
COMPOSITE_FIGURE
perimeter+
perimeter+
POLYGON
ELLIPSE …
diagonal …
perimeter++
TRIANGLE
RECTANGLE
CIRCLE
perimeter++
SQUARE
perimeter++
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74. A composite figure as a list
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A composite figure as a list
item
start
forth
after
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75. Composite figures class COMPOSITE_FIGURE inherit
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Composite figures
class COMPOSITE_FIGURE inherit
FIGURE redefine display, move, rotate, ... end
LIST [FIGURE]
feature
display is Display each constituent figure in turn. do from start
until after loop item.display forth end end
... Similarly for move, rotate etc. ...
end
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76. 1/16/2019 1:45 PM
Complex figures
Note: a simpler form of procedures display, move etc. can be obtained through the use of iterators.
Exercise: Use agents for that purpose.
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77. Name clashes under multiple inheritance
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Name clashes under multiple inheritance A B
foo
foo C
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78. Resolving name clashes
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Resolving name clashes A B
foo
foo
rename foo as fog
rename foo as zoo C
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79. Resolving name clashes (cont’d)
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Resolving name clashes (cont’d)
class C
inherit A
rename
foo as fog
end B
foo as zoo
feature
...
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80. Results of renaming a1: A b1: B c1: C ... c1.fog c1.zoo a1.foo b1.foo
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Results of renaming
a1: A
b1: B
c1: C
...
c1.fog
c1.zoo
a1.foo
b1.foo
Invalid:
a1.fog, a1.zoo, b1.zoo, b1.fog, c1.foo
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81. Another application of renaming
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Another application of renaming
Provide locally better adapted terminology.
class TREE feature
child: TREE[WINDOW]
end
class WINDOW inherit
TREE
feature
rename
child as subwindow
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82. The marriage of convenience
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The marriage of convenience
class ARRAYED_STACK [G]
inherit
STACK [G]
ARRAY [G]
feature ...
end
class LINKED_STACK [G]
LINKED_LIST [G]
feature ... end
Arrayed stack is a marriage of convenience (OOSC2 page 530) which unites the noble family of STACK with the rich family of ARRAY.
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83. The need for deferred classes
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The need for deferred classes
In the scheme seen earlier:
f: FIGURE; c: CIRCLE; p: POLYGON
...
create c.make (...); create p.make (...)
if ... then
f := c
else
f := p
end
f.move (...); f.rotate (...); f.display (...); ...
How do we ensure that a call such as f.move (...) is valid even though there is no way to implement a general-ee-purpose feature move for class FIGURE?
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84. Deferred classes deferred class FIGURE feature
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Deferred classes
deferred class FIGURE
feature
move (v: VECTOR) is deferred end
rotate (a: ANGLE; p: POINT) is deferred end
... display, hide, ...
end
Not permitted:
create f ...
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85. Example hierarchy * FIGURE * * OPEN_ FIGURE CLOSED_ FIGURE SEGMENT
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Example hierarchy
extent*
display*
barycenter* *
rotate …
FIGURE … * *
OPEN_ FIGURE
CLOSED_ FIGURE
perimeter*
perimeter+
perimeter+
SEGMENT
POLYLINE
POLYGON
ELLIPSE
diagonal
perimeter++
...
...
TRIANGLE
RECTANGLE
CIRCLE
perimeter++
perimeter++
SQUARE
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86. Deferred classes and features
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Deferred classes and features
A feature is either deferred or effective.
To effect a inherited feature (deferred in the parent) is to make it effective. No need for redefine clause.
Like a feature, a class is either deferred or effective.
A class is deferred if it has at least one deferred feature (possibly coming from an ancestor) that it does not effect. It is effective otherwise.
A deferred class may not be instantiated.
BUT: A deferred class may have assertions (in particular, a deferred routine may have a precondition and a postcondition, and the class may have a class invariant).
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