ObjectOriented Programming Objects Classes and Inheritance VRH 7

Object-Oriented Programming Objects, Classes and Inheritance (VRH 7. 1 -4) Polymorphism (VRH 6. 4. 3) Carlos Varela RPI Adapted with permission from: Seif Haridi KTH Peter Van Roy UCL C. Varela; Adapted from S. Haridi and P. Van Roy 1

Overview • What is object-oriented programming? – Objects – Classes – Interfaces • • Inheritance Polymorphism Static and dynamic binding Multiple Inheritance Method dispatching, multi-methods Reflection Active Objects Relationship to Other Programming Models C. Varela; Adapted from S. Haridi and P. Van Roy 2

System building • Abstraction is the best tool to build complex systems • Complex systems are built by layers of abstractions • Each layer has two parts: – Specification, and – Implementation • Any layer uses the specification of the lower layer to implement its functionality C. Varela; Adapted from S. Haridi and P. Van Roy 3

Properties needed to support the principle of abstraction • Encapsulation – Hide internals from the interface • Compositionality – Combine parts to make new parts • Instantiation/invocation – Create new instances of parts C. Varela; Adapted from S. Haridi and P. Van Roy 4

Component-based programming • Supports – Encapsulation – Compositionality – Instantiation C. Varela; Adapted from S. Haridi and P. Van Roy 5

Object-oriented programming • Supports – Encapsulation – Compositionality – Instantiation • Plus – Inheritance C. Varela; Adapted from S. Haridi and P. Van Roy 6

Inheritance • Programs can be built in hierarchical structure from ADT’s that depend on other ADT’s (Components) • Object-oriented programming (inheritance) is based on the idea that ADTs have so much in common • For example, sequences (stacks, lists, queues) • Object oriented programming enables building ADTs incrementally, through inheritance • An ADT can be defined to inherit from another abstract data type, substantially sharing functionality with that abstract data type • Only the difference between an abstract datatype and its ancestor has to be specified C. Varela; Adapted from S. Haridi and P. Van Roy 7

What is object-oriented programming? • OOP (Object-oriented programming) = encapsulated state + inheritance • Object – An entity with unique identity that encapsulates state – State can be accessed in a controlled way from outside – The access is provided by means of methods (procedures that can directly access the internal state) • Class – A specification of objects in an incremental way – Incrementality is achieved inheriting from other classes by specifying how its objects (instances) differ from the objects of the inherited classes C. Varela; Adapted from S. Haridi and P. Van Roy 8

Instances (objects) Interface (what methods are available) State (attributes) Procedures (methods) C. Varela; Adapted from S. Haridi and P. Van Roy 9

Classes (simplified syntax) A class is a statement class Class. Variable attr Attr. Name 1 : Attr. Name. N meth Pattern 1 Statement end : meth Pattern. N Statement end C. Varela; Adapted from S. Haridi and P. Van Roy 10

Classes (simplified syntax) A class can also be a value that can be in an expression position class $ attr Attr. Name 1 : Attr. Namen meth Pattern Statement end : meth Pattern Statement end C. Varela; Adapted from S. Haridi and P. Van Roy 11

Classes in Oz The class Counter has the syntactic form class Counter attr val meth display {Browse @val} end meth inc(Value) val : = @val + Value end meth init(Value) val : = Value end C. Varela; Adapted from S. Haridi and P. Van Roy 12

Attributes of Classes The class Counter has the syntactic form class Counter attr val meth display {Browse @val} end meth inc(Value) val : = @val + Value end meth init(Value) val : = Value end val is an attribute: a modifiable cell that is accessed by the atom val C. Varela; Adapted from S. Haridi and P. Van Roy 13

Attributes of classes The class Counter has the syntactic form class Counter attr val meth display {Browse @val} end meth inc(Value) val : = @val + Value end meth init(Value) val : = Value end the attribute val is accessed by the operator @val C. Varela; Adapted from S. Haridi and P. Van Roy 14

Attributes of classes The class Counter has the syntactic form class Counter attr val meth display {Browse @val} end meth inc(Value) val : = @val + Value end meth init(Value) val : = Value end the attribute val is assigned by the operator : = as val : =. . . C. Varela; Adapted from S. Haridi and P. Van Roy 15

Methods of classes The class Counter has the syntactic form class Counter attr val meth display {Browse @val} end meth inc(Value) val : = @val + Value end meth init(Value) val : = Value end methods are statements method head is a record (tuple) pattern C. Varela; Adapted from S. Haridi and P. Van Roy 16

Classes in Oz The class Counter has the syntactic form class Counter attr val meth display {Browse @val} end meth inc(Value) val : = @val + Value end meth init(Value) val : = Value end C. Varela; Adapted from S. Haridi and P. Van Roy 17

Example • An object is created from a class using the procedure New/3, whose first argument is the class, the second is the initial method, and the result is the object (such as in the functor and procedure implementation approaches) • New/3 is a generic procedure for creating objects from classes. declare C = {New Counter init(0)} {C display} {C inc(1)} {C display} C. Varela; Adapted from S. Haridi and P. Van Roy 18

Example • The following shows how an object is created from a class using the procedure New/3, whose first argument is the class, the second is the initial method, and the result is the object. • New/3 is a generic procedure for creating objects from classes. declare C = {New Counter init(0)} {C display} Object interface is as a {C inc(1)} procedure of one argument, {C display} which expects a record representing a method C. Varela; Adapted from S. Haridi and P. Van Roy 19

Summary • A class X is defined by: – class X. . . end • Attributes are defined using the attribute-declaration part before the method-declaration part: – attr A 1. . . AN • Then follow the method declarations, each has the form: – meth E S end • The expression E evaluates to a method head, which is a record whose label is the method name. C. Varela; Adapted from S. Haridi and P. Van Roy 20

Summary • An attribute A is accessed using @A. • An attribute is assigned a value using A : = E • A class can be defined as a value: • X = class $. . . end C. Varela; Adapted from S. Haridi and P. Van Roy 21

Attribute Initialization • Stateful (may be updated by : =) • Initialized at object creation time, all instances have the initial balance = 0 • class Account attr balance: 0 meth … end In general the initial value of an attribute could be any legal value (including classes and objects) C. Varela; Adapted from S. Haridi and P. Van Roy 22

Attribute Initialization • Initialization by instance class Account attr balance meth init(X) balance : = X end … end • C 1 = {New Account init(100)} • C 1 = {New Account init(50)} C. Varela; Adapted from S. Haridi and P. Van Roy 23

Attribute Initialization • Initialization by brand declare L=linux class Red. Hat attr ostype: L meth get(X) X = @ostype end class Su. SE attr ostype: L meth get(X) X = @ostype end class Debian attr ostype: L meth get(X) X = @ostype end C. Varela; Adapted from S. Haridi and P. Van Roy 24

Example class Queue attr front back count meth init Q in front : = Q back : = Q count : = 0 end meth put(X) Q in @back = X|Q back : = Q count : = @count + 1 end. . . end C. Varela; Adapted from S. Haridi and P. Van Roy 25

Example class Queue attr front back count meth init Q in front : = Q back : = Q count : = 0 end meth put(X) Q in @back = X|Q back : = Q count : = @count + 1 end. . . end front Q 0 back put(a) front a | Q 1 back C. Varela; Adapted from S. Haridi and P. Van Roy 26

Example class Queue attr front back count. . . meth get(? X) Q in X|Q = @front : = Q count : = @count - 1 end meth count(X) X = @count end. . . end front a | Q 1 back X C. Varela; Adapted from S. Haridi and P. Van Roy 27

Classes as incremental ADTs • Object-oriented programming allows us to define a class by extending existing classes • Three things have to be introduced – How to express inheritance, and what does it mean? – How to access particular methods in the new class and in preexisting classes – Visibility – what part of the program can see the attributes and methods of a class C. Varela; Adapted from S. Haridi and P. Van Roy 28

Inheritance • Inheritance should be used as a way to specialize a class while retaining the relationship between methods • In this way it is just an extension of an ADT • The other view is inheritance is just a (lazy) way to construct new abstract data types! • No relationships are preserved general class specialized class C. Varela; Adapted from S. Haridi and P. Van Roy 29

Inheritance class Account attr balance: 0 meth transfer(Amount) balance : = @balance+Amount end meth get. Bal(B) B = @balance end A={New Account transfer(100)} C. Varela; Adapted from S. Haridi and P. Van Roy 30

Inheritance II Conservative extension The class Verbose. Account has the methods: transfer, get. Bal, and verbose. Transfer class Verbose. Account from Account meth verbose. Transfer(Amount). . . end C. Varela; Adapted from S. Haridi and P. Van Roy 31

Inheritance II Non-Conservative extension class Account. With. Fee from Verbose. Account attr fee: 5 meth transfer(Amount). . . end The class Account. With. Fee has the methods: transfer, get. Bal, and verbose. Transfer The method transfer has been redefined (overridden) with another definition C. Varela; Adapted from S. Haridi and P. Van Roy 32

Inheritance II Non-Conservative extension class Account. With. Fee from Verbose. Account attr fee: 5 meth transfer(Amount). . . end Account Verbose. Account. With. Fee C. Varela; Adapted from S. Haridi and P. Van Roy 33

Polymorphismn The ability for operations to take objects (instances) of different types. For example, the transfer method can be invoked in account object instances of three different classes. Account Verbose. Account. With. Fee The most specific behavior should be executed. C. Varela; Adapted from S. Haridi and P. Van Roy 34

Static and dynamic binding Dynamic binding • Inside an object O we want to invoke a method M • This is written as {self M}, and chooses the method visible in the current object (M of D) O an instance of D class C meth M class D a subclass of C meth M C. Varela; Adapted from S. Haridi and P. Van Roy 35

Static and dynamic binding Static binding • Inside an object O we want to invoke a method M in a specific (super) class • This is written as C, M and chooses the method visible in the super class C (M of C) O an instance of D class C meth M class D a subclass of C meth M C. Varela; Adapted from S. Haridi and P. Van Roy 36

Static method calls • Given a class and a method head m(…), a static method-call has the following form: C, m(…) • Invokes the method defined in the class argument. • A static method call can only be used inside class definitions. • The method call takes the current object denoted by self as implicit argument. • The method m could be defined in the class C, or inherited from a super class. C. Varela; Adapted from S. Haridi and P. Van Roy 37

Inheritance class Account attr balance: 0 meth transfer(Amount) balance : = @balance+Amount end meth get. Bal(B) B = @balance end A={New Account transfer(100)} C. Varela; Adapted from S. Haridi and P. Van Roy 38

Inheritance II Conservative extension The class Verbose. Account has the methods: transfer, get. Bal, and verbose. Transfer class Verbose. Account from Account meth verbose. Transfer(Amount) {self transfer(Amount)} {Browse @balance} end C. Varela; Adapted from S. Haridi and P. Van Roy 39

Inheritance II Non-Conservative extension class Account. With. Fee from Verbose. Account attr fee: 5 meth transfer(Amount) Verbose. Account, transfer(Amount - @fee) end The class Account. With. Fee has the methods: transfer, get. Bal, and verbose. Transfer The method transfer has been redefined (overridden) with another definition C. Varela; Adapted from S. Haridi and P. Van Roy 40

Inheritance II Non-Conservative extension class Account. With. Fee from Verbose. Account attr fee: 5 meth transfer(Amount) Verbose. Account, transfer(Amount - @fee) end Non-Conservative inheritance is dangerous because it might change the relationship between methods and the invariants the programmer depends on Account get. Balance(B); transfer(S); get. Balance(B 1) => B 1 = B+S C. Varela; Adapted from S. Haridi and P. Van Roy 41

Inheritance II Non-Conservative extension class Account. With. Fee from Verbose. Account attr fee: 5 meth transfer(Amount) Verbose. Account, transfer(Amount - @fee) end Non-Conservative inheritance is dangerous because it might change the relationship between methods and the invariants the programmer depends on Account. With. Fee get. Balance(B); transfer(S); get. Balance(B 1) => B 1 = B+S-@fee C. Varela; Adapted from S. Haridi and P. Van Roy 42

Inheritance III • • • Classes may inherit from one or several classes appearing after the keyword: from. A class B is a superclass of a class A if: – B appears in the from declaration of A, or – B is a superclass of a class appearing in the from declaration of A. The methods (attributes and features) available in a class C (i. e. visible) are defined through a precedence relation on the methods that appear in the class hierarchy based on the overriding relation: – A method in a class C overrides any method, with the same label, in any super class of C. Varela; Adapted from S. Haridi and P. Van Roy 43

Super. Class relation • Super. Class relation is directed and acyclic. C C. Varela; Adapted from S. Haridi and P. Van Roy 44

Super. Class relation • Super. Class relation is directed and acyclic. • After striking out all overridden methods each remaining method should have a unique label and be defined only in one class in the hierarchy. C C. Varela; Adapted from S. Haridi and P. Van Roy 45

Inheritance relation m m C A (valid hierarchy) (invalid hierarchy) C. Varela; Adapted from S. Haridi and P. Van Roy 46

Multiple Inheritance Example class Account attr balance: 0 meth transfer(Amount) balance @balance+Amount end meth get. Bal(? B) B = @balance end class Customer attr name meth init(N) name N end class Customer. Account from Customer Account end A={New Customer. Account init} C. Varela; Adapted from S. Haridi and P. Van Roy 47

Illegal inheritance class Account attr balance meth init(Amount) There are two init methods balance Amount visible for Customer. Account end This is illegal meth transfer(Amount) balance @balance+Amount end meth get. Bal(B) B = @balance end class Customer attr name meth init(N) name N end class Customer. Account from Customer Account end C. Varela; Adapted from S. Haridi and P. Van Roy 48

Legal inheritance class Account attr balance meth init(Amount) balance Amount end meth transfer(Amount) balance @balance+Amount end meth get. Bal(B) B = @balance end class Customer attr name meth init(N) name N end class Customer. Account from Customer Account meth init(N A) Customer, init(N) Account, init(A) end Customer. Account has attributes balance and name methods init, transfer and get. Balance This overriding is not harmful: it does not change relationships in super classes C. Varela; Adapted from S. Haridi and P. Van Roy 49

Controlling visibility • Visibility is the control given to the user to limit access to members of a class (attributes, methods and properties) • Each member is defined with a scope (part of program text that the member can be accessed by name) • Programming languages use words like public, private and protected to define visibility • Unfortunately different languages use these keywords to define different scopes C. Varela; Adapted from S. Haridi and P. Van Roy 50

Public and private scopes in ADTs • A private member is one which is only visible in the object instance (it is used for implementing the ADT) • The object instance can see all the private members in its class and its super classes • A public member is visible anywhere in the program • It is part of the interface of the ADT • In Oz (and Smalltalk) attributes are private and methods are public (the default rule) • In Java and C++ private has another meaning C. Varela; Adapted from S. Haridi and P. Van Roy 51

The meaning of Private C Class Hierarchy Sub. C Instances I 1 I 2 I 3 I 4 C. Varela; Adapted from S. Haridi and P. Van Roy 52

The meaning of Private C Class Hierarchy Sub. C Instances I 1 I 2 I 3 According to Smalltalk and Oz All private members in this region are visible to I 3 I 4 C. Varela; Adapted from S. Haridi and P. Van Roy 53

The meaning of Private C Class Hierarchy Sub. C According to C++ and Java All private members in this region are visible to I 3 Sub. C Instances I 1 I 2 I 3 I 4 C. Varela; Adapted from S. Haridi and P. Van Roy 54

Public and private scopes in ADTs • In Oz (and Smalltalk) attributes are private and methods are public • It is possible in Oz to make a method private within a class C meth A(. . . ). . . end • Using a variable identifier as a method head will make the method local to the class • The variable is automatically bound to a unique name C. Varela; Adapted from S. Haridi and P. Van Roy 55

Public and private scopes in ADTs • In Oz (and Smalltalk) attributes are private and methods are public • It is possible in Oz to make a method private within a class • Using a variable identifier as a method head will make the method local to the class • The variable is automatically bound to a unique name • ! is an escape character, !A means escape the class scope class C meth A(. . . ). . . end local A = {New. Name} in class C meth !A(. . . ). . . end C. Varela; Adapted from S. Haridi and P. Van Roy 56

Exercises • • • Do Java and C++ object abstractions completely encapsulate internal state? If so, how? If not, why? Do Java and C++ enable static access to methods defined in classes arbitrarily high in the inheritance hierarchy? If so, how? If not, why? Do Java and C++ allow multiple inheritance? If so, how? If not, why? Exercises VRH 7. 9. 1 -4 (pg 567) Exercise VRH 7. 9. 7 (pg 568) Read VRH Sections 7. 5, 7. 7 and Multijava paper. C. Varela; Adapted from S. Haridi and P. Van Roy 57