1. Objects, Classes and ADTs Reference vs. Expanded Types
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COSC3311 – Software Design
Objects, Classes and ADTs
Reference vs. Expanded Types
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2. An object is a run-time instance of some class
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Objects vs. Classes
An object is a run-time instance of some class
An class is an abstract data type (ADT) with a possible partial implementation
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3. 11/8/2018 1:05 PM
Avoid “objectspeak”
The run-time structures, some of them corresponding to “objects” of the modeled system, are objects.
The software modules, each built around a type of objects, are classes.
A system does not contain any “objects” (although its execution will create objects).
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4. Why the top class is not Object
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Why the top class is not Object
ANY
CONTAINER *
BOX
COLLECTION
TRAVERSABLE
FINITE
INFINITE
UNBOUNDED
COUNTABLE
RESIZABLE
BAG
SET
HIERARCHICAL
LINEAR
TABLE
ACTIVE
INTEGER_ INTERVAL
BILINEAR
INDEXABLE
CURSOR_ STRUCTURE
DISPENSER
SEQUENCE
STRING
HASH_TABLE
STACK
QUEUE …
BOUNDED
Category error
ARRAY
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5. Abstract data type POINT
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Abstract data type POINT
x: POINT  REAL
y: POINT  REAL
: POINT  REAL
: POINT  REAL
Class POINT: Choose a representation (polar, cartesian)
In polar representation,  and  are attributes, x and y are routines. y   x
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6. Abstract data type POINT
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Abstract data type POINT
x: POINT  REAL
y: POINT  REAL
: POINT  REAL
: POINT  REAL
Axiom: y   x
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7. What is OO programming? OO = ADTs + inheritance
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8. A simple class class POINT feature
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A simple class
class POINT
feature
x, y: REAL -- Point cartesian coordinates
ro: REAL is -- Distance to origin (0, 0) do
Result := sqrt (x^2 + y^2)
end
theta: REAL is -- Angle to horizontal axis …
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9. Uniform access through feature call
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Uniform access through feature call
To access a property of a point p1, the notation is the same regardless of the representation, e.g.
p1.x
which is applicable both in cartesian representation (x is an attribute) and in polar representation (x is a function without arguments).
In the first case the feature call is a simple field access; in the second it causes a computation to be performed.
There is no difference for clients (except possibly in terms of performance).
uniform access principle
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10. Class POINT (cont’d) distance (p: POINT): REAL is -- Distance to p do
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Class POINT (cont’d)
distance (p: POINT): REAL is -- Distance to p do
Result := sqrt ((x – p.x)^2 + (y – p.y)^2)
end
move (a, b: REAL) is -- Move by a horizontally and by b vertically.
x := x + a
y := y + b
scale (factor: REAL) is -- Scale by factor.
x := factor * x
y := factor * y
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11. Use of the class in a client (1/5)
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Use of the class in a client (1/5)
class GRAPHICS
feature
p, q: POINT
-- Graphic points …
some_routine is -- Use p and q.
local u, v: REAL do
-- Creation instructions
create p
create q
end p
0.0
0.0
(POINT) q
0.0
0.0
(POINT)
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12. Use of the class in a client (2/5)
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Use of the class in a client (2/5)
class GRAPHICS
feature
p, q: POINT
-- Graphic points …
some_routine is -- Use p and q.
local u, v: REAL do
-- Creation instructions
create p
create q
p.move (4.0, -2.0)
-- Compare with Pascal, C, Ada:
-- Move (p, 4.0, -2.0)
end p
4.0
-2.0
(POINT) q
0.0
0.0
(POINT)
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13. Use of the class in a client (3/5)
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Use of the class in a client (3/5)
class GRAPHICS
feature
p, q: POINT
-- Graphic points …
some_routine is -- Use p and q.
local u, v: REAL do
-- Creation instructions
create p
create q
p.move (4.0, -2.0)
-- Compare with Pascal, C, Ada:
-- Move (p, 4.0, -2.0)
p.scale (0.5)
end p
2.0
-1.0
(POINT) q
0.0
0.0
(POINT)
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14. Use of the class in a client (4/5)
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Use of the class in a client (4/5)
class GRAPHICS
feature
p, q: POINT
-- Graphic points …
some_routine is -- Use p and q.
local u, v: REAL do
-- Creation instructions
create p
create q
p.move (4.0, -2.0)
-- Compare with Pascal, C, Ada:
-- Move (p, 4.0, -2.0)
p.scale (0.5)
u := p.distance (q)
v := p.x
p := q
end p
2.0
-1.0
(POINT) q
0.0
0.0
(POINT)
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15. Use of the class in a client (5/5)
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Use of the class in a client (5/5)
class GRAPHICS
feature
p, q: POINT
-- Graphic points …
some_routine is -- Use p and q.
local u, v: REAL do
-- Creation instructions
create p
create q
p.move (4.0, -2.0)
-- Compare with Pascal, C, Ada:
-- Move (p, 4.0, -2.0)
p.scale (0.5)
u := p.distance (q)
v := p.x
p := q
p.scale (-3.0)
end p
2.0
-1.0
(POINT) q
0.0
0.0
(POINT)
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16. The module-type merge A class is both: A module A type
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The module-type merge
A class is both:
A module
A type
Much of the conceptual power of the method comes from the fusion of these two notions.
From the module viewpoint:
Set of available services (“features”).
From the type viewpoint:
Description of set of possible run-time objects (its instances).
Connection: The services of the class, viewed as a module, are the operations applicable to the instances of the class, viewed as a type.
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17. Applying abstraction principles
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Applying abstraction principles
Privileges of a client C of a class A on query balance:
Read access if attribute is exported.
a.balance is a legal query in class C.
a.balance := 800 is an illegal assignment
read privleges (but not write) C A
a: A
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18. Creating an object With the class POINT as given: ... create my_point
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Creating an object
With the class POINT as given:
my_point: POINT
...
create my_point
Effect of such a creation instruction:
Allocate new object of the type declared for my_point.
Initialize its fields to default values (0 for numbers, false for booleans, null for characters, void for references).
Attach it to the instruction’s target, here my_point.
Without a creation clause
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19. Specific creation procedures
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Specific creation procedures
class POINT
create
make_cartesian, make_polar
feature -- Initialization
make_cartesian (a, b: REAL) is Initialize to abscissa a, ordinate b. do x := a
y := b end
make_polar(a, b: REAL) is ...
feature
... The rest as before ...
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20. If there is a creation clause
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If there is a creation clause
Creation instructions must be “creation calls”, such as
create my_point.make_polar (1, Pi / 2)
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21. If there is no creation clause
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If there is no creation clause
An absent creation clause, as in
class POINT
-- No creation clause
feature
… The rest as before …
end
is understood as one that would only list default_create, as if it had been written
create
default_create
Procedure default_create is defined in ANY as doing nothing; any class can redefine it to provide proper default initializations.
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22. Associated convention
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Associated convention
The notation
create x
is understood (if permitted) as an abbreviation for
create x.default_create
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23. To allow both forms To make both forms valid:
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To allow both forms
To make both forms valid:
create my_point.make_cartesian (0, 1)
as well as
create my_point.make_polar (1, Pi / 2)
it suffices to make default_create (redefined or not) one of the creation procedures:
class POINT
create
make_cartesian, make_polar, default_create
feature
... The rest as before ...
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24. To prohibit instantiating a class
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To prohibit instantiating a class
class NOT_CREATABLE
create
-- Nothing here!
feature
... The rest as before ...
end
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25. Forms of assignment and copy
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Forms of assignment and copy
Reference assignment (a and b of reference types): b := a
Object duplication (shallow): c := a.twin
Object duplication (deep): d := a.deep_twin
Also: shallow field-by-field copy (no new object is created): e.copy (a)
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26. Shallow and deep cloning
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Shallow and deep cloning a O1
Initial situation:
Result of:
b := a
c := a.twin
d := a.deep_twin
name
“Almaviva”
landlord
loved_one O3 O2
“Figaro”
“Susanna” b O4
“Almaviva” c
The Barber of seville
Almavia – local nobelman d
name
“Almaviva” O5
landlord
loved_one O7 O6
“Figaro”
“Susanna”
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27. Equality – Eiffel vs. Java
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Equality – Eiffel vs. Java
Eiffel
Expanded vs. Reference types (You choose)
Reference equality: x = y
Object equality: x ~ y x.is_equal(y)
Deep equality: deep_equal(x, y)
Change the default by redefining is_equal
STRING redefines is_equal to one level deeper than shallow is_equal
Java
Primitives vs. Reference types (No choice)
Reference equality: x == y
Object equality: x.equals(y) (assuming x /= Void)
No built-in deep equality
Change the default by over-riding equals
java.lang.String overrides the java.lang.Object equals() (same as ==)
Java 1.5
equals
public boolean equals(Object obj)
Indicates whether some other object is "equal to" this one. The equals method implements an equivalence relation on non-null object references:
It is reflexive: for any non-null reference value x, x.equals(x) should return true.
It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
For any non-null reference value x, x.equals(null) should return false.
The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
Parameters:
obj - the reference object with which to compare.
Returns:
true if this object is the same as the obj argument; false otherwise.
See Also:
hashCode(), Hashtable
java.lang.String overrides the java.lang.Object equals() method to return true if and only if the objects being compared contain the same sequence of characters.
String s0 = "Hello"; String s1 = new String("Hello"); // force new string object String s2 = s0; s0.equals(s1) // true (diff objects, same chars) s0.equals(s2) // true (same chars, coincidence // they are same objects)
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28. Reference vs. Expanded
Every entity must be declared to be of a certain type (based on a class)
Every type is either a reference type or an expanded type
how to treat attachments in assignments, argument passing & comparisons
In reference types
x := y: x attaches to same object as y
x = y: compares references
In expanded types
x := y: copy contents of y into x
x = y: compares value (x ~ y)
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29. A simple book – basic types
A typical instance b of a BOOK is
class BOOK feature
title: STRING
in_print: BOOLEAN
pages: INTEGER
end
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30. Basic types insufficient
class BOOK feature
title: STRING
in_print: BOOLEAN
pages: INTEGER
author: WRITER
end
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31. author: WRITER
It wastes memory space – the author is the same “Stendhall” in either case.
Does not express sharing. The author field refers to the same instance of WRITER. If you update the WRITER object (e.g. to record year_of_death), you will have to change each sub-object.
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32. Reference types
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33. Reference types [OOSC2, p224]
A reference is a run-time value that is either Void or attached.
If attached, a reference identifies a single object (and is said to be attached to that object).
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34. The dynamic model for references
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The dynamic model for references
States of a reference:
Operations on references:
create p
p := q
p := Void
if p = Void then ...
create p
p := q (where q is attached) p
ATTACHED
VOID
ATTACHED p
VOID
p := Void
p := q (where q is void)
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35. 11/8/2018 1:05 PM
Modelling Fidelity
A void reference can denote an unknown author. “Candide” was published anonymously.
Voltaire’s parady of Leibniz
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36. Aliasing
Dynamic aliasing is not just a consequence of a programmer’s dirty tricks — it is a consequence of the human ability to name things, and to give more than one name to the same thing.
However, aliasing is shocking – changing author_relativity has the side effect of changing nobel_winner_1921 – this causes many programmer errors!
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37. Aliasing problems
Blurb from the International Workshop on Aliasing, Confinement and Ownership in Object-Oriented programming (IWACO) July 22, 2003:
“The power of objects lies in the flexibility of their interconnection structure. But this flexibility comes at a cost.
Because an object can be modified via any alias, object-oriented programs are hard to understand, maintain, and analyse.
Aliasing makes objects depend on their environment in unpredictable ways, breaking the encapsulation necessary for reliable software components, making it difficult to reason about and optimize programs, obscuring the flow of information between objects, and introducing security problems.”
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38. Why aliasing? Modelling Fidelity (see earlier slide)
We need references (particularly sharing) just to do certain kinds of data structures.
Consider a LINKED_CIRCULAR_LISTS — with a node first linked to one of the nodes (say n) in the circle.
Then first and node (n-1) both share a reference to n.
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39. Expanded vs. References
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40. Expanded [OOSC2, 8.7 and 8.8] class BOOK feature title: STRING
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Expanded [OOSC2, 8.7 and 8.8]
class BOOK feature
title: STRING
in_print: BOOLEAN
pages: INTEGER
author: expanded WRITER
end
Conceptually, a reference to a WRITER is not the same thing as the WRITER themselves.
Conclusion: WRITER should be expanded
Alternate format:
expanded class WRITER
pseudonym: STRING
name: STRING …
end
Stendhal ( ) - Pseudonym of Marie-Henri Beyle  One of the most original French writers of the first half of the 19th century, who played a major role in the development of the modern novel. Stendhal is best known for his masterpieces LE ROUGE ET LE NOIR (1830) and LA CHARTREUSE DE PARME (1839), sharp and passionate chronicles of the intellectual and moral climate of France after Napoleon's defeat. Stendhal also wrote travel books, literature and art reviews, and biographies about such composers as W.A. Mozart and Joseph Haydn. Stendhal's subjects are often melodramatic, but they form a fascinating frame for his psychologically deep stories of selfishness and different paths towards self-discovery. "A novel is a mirror that strolls along a highway. Now it reflects the blue of the skies, now the mud puddles underfoot." (from Le Rouge et le Noir)
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41. Reference POINT vs. Expanded POINT
Reference: sharing
Aliasing
Expanded: no sharing
Sub-object a
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42. BON diagram class WORKSTATION feature k: expanded KEYBOARD
c: expanded CPU
m: expanded MONITOR
n: NETWORK
end
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43. BON: Associations vs. Aggregations
Reference: association
Expanded: aggregation
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44. extra slides
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45. A related mechanism: Persistence
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A related mechanism: Persistence
a.store (file)
....
b ?= retrieved (file)
Storage is automatic.
These features come from the library class STORABLE. The above is only an approximate form (see typing discussion).
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46. Java Strings are Immutable
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Java Strings are Immutable
public class EqualityTest2 {
public static void main(String[] args) {
char c1, c2; String s1,s2,s3;
c1 = 'c'; c2 = 'c';
s1 = "hello"; s2 = "hello"; //same string literal
s3 = s1;
String s4 = new String("hello");
check("t1", c1 == c2); //PASS
check("t2", new Character(c1).equals(new Character(c2))); //PASS
check("t4",s1 == s2); //PASS
check("t5",s1.equals(s2)); //PASS
check("t7",s1 == s3); //PASS
check("t8",s4 == "hello"); //FAIL
//above are different objects
check("t9",s4.equals("hello")); } //PASS
public static void check(String s, boolean b) {
String st = (b)?"passed":"failed";
System.out.println(s+": "+st); }}
String objects are read-only or immutable ie the contents of a String object never change
String str = "Hello"; str = "Goodbye";
in the above example, the second assignment of "Goodbye" to String, what actually happens is that a new string "Goodbye" is created and the object reference of the new string is stored in the variable str
operations that seem to modify a String object actually create new read-only String objects; leaving the original object unchanged
the StringBuffer class provides mutable or flexible string handling
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47. Java: String vs. StringBuffer
Java provides the StringBuffer and String classes, and the String class is used to manipulate character strings that cannot be changed. Simply stated, objects of type String are read only and immutable. The StringBuffer class is used to represent characters that can be modified.
The significant performance difference between these two classes is that StringBuffer is faster than String when performing simple concatenations.
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48. Eiffel Strings are Mutable
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Eiffel Strings are Mutable
test_string_equality: BOOLEAN is
local
s1, s2, s3: STRING do
check s1 = Void end
s1 := "hello"
s2 := "hello“
check s1 /= s2 end
s3 := s1
check s1 = s3 end
create s2.make_from_string ("hello")
check s1.is_equal (s2) end
check s1 = s3 and s3 /= s2 end
Result := true
end
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49. CHARACTER is expanded test_character_equality: BOOLEAN is local
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CHARACTER is expanded
test_character_equality: BOOLEAN is
local
c1,c2, c3: CHARACTER
c4, c5, c6: CHARACTER_REF do
check c1 = '%U' and c2 = '%U' end
c1 := 'c'
c2 := 'c'
c3 := c1
Result := c1 = c2 and equal(c1,c2) and c1 = c3
check Result end
c4 := 'c'
c5 := 'c'
c6 := c4
Result := c4 /= c5 and equal(c4,c5) and c4 = c6
end
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50. 11/8/2018 1:05 PM
Uniform type system
A reference to an integer is different than the integer itself
expanded class INTEGER inherit
INTEGER_REF
...
end
In INTEGER, all features of INTEGER_REF become expanded.
Same for all basic types: BOOLEAN, REAL, CHARACTER, DOUBLE etc.
Uniform type system for primitives or any other type
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51. Reference types implies Aliasing
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Reference types implies Aliasing
Bad
io.read_line -- “Jesse”
s1 := io.last_string
io.read_line -- “Helen”
s2 := io.last_string
check s1 = s2 and s1 = “Helen” end
Good
io.read_line
s1 := io.last_string.twin
s2 := io.last_string.twin
io is from ANY
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