# 10 Classes and Objects

A Class in Oz is a chunk that contains:

- A collection of methods in a method table.

- A description of the attributes that each instance of the class will possess. Each attribute is a stateful cell that is accessed by the attribute-name, which is either an atom or an Oz-name.

- A description of the features that each instance of the class will possess. A feature is an immutable component (a variable) that is accessed by the feature-name, which is either an atom or an Oz-name.

- Classes are stateless Oz-values1. Contrary to languages like Smalltalk, or Java etc., they are just descriptions of how the objects of the class should behave.

## 10.1 Classes from First Principles

Figure 10.1 shows how a class is constructed from first principles as outlined above. Here we construct a `Counter` class. It has a single attribute accessed by the atom val. It has a method table, which has three methods accessed through the chunk features `browse`, `init` and `inc`. A method is a procedure that takes a message, always a record, an extra parameter representing the state of the current object, and the object itself known internally as `self`.

> `declare Counter
> local
> Attrs = [val]
> MethodTable = m(browse:MyBrowse init:Init inc:Inc)
> proc {Init M S Self}
> init(Value) = M in
> (S.val) := Value
> end
> proc {Inc M S Self}
> X inc(Value)=M
> in
> X = @(S.val) (S.val) := X+Value
> end
> proc {MyBrowse M=browse S Self}
> {Browse @(S.val)}
> end
> in
> Counter = {NewChunk c(methods:MethodTable attrs:Attrs)}
> end`

Figure 10.1: An Example of Class construction

As we can see, the method `init` assigns the attribute `val` the value `Value`, the method `inc` increments the attribute `val`, and the method `browse` browses the current value of `val`.

## 10.2 Objects from First Principles

Figure 10.2 shows a generic procedure that creates an object from a given class. This procedure creates an object state from the attributes of the class. It initializes the attributes of the object, each to a cell (with unbound initial value). We use here the iterator `Record.forAll/2` that iterates over all fields of a record. `NewObject` returns a procedure `Object` that identifies the object. Notice that the state of the object is visible only within `Object`. One may say that `Object` is a procedure that encapsulates the state2.

> `proc {NewObject Class InitialMethod ?Object}
> State O
> in
> State = {MakeRecord s Class.attrs}
> {Record.forAll State proc {$ A} {NewCell _ A} end}
> proc {O M}
> {Class.methods.{Label M} M State O}
> end
> {O InitialMethod}
> Object = O
> end`

Figure 10.2: Object Construction

We can try our program as follows

> `declare C
> {NewObject Counter init(0) C}
> {C inc(6)} {C inc(6)}
> {C browse}`

Try to execute the following statement.

> `local X in {C inc(X)} X=5 end {C browse}`

You will see that nothing happens. The reason is that the object application

> `{C inc(X)}`

suspends inside the procedure `Inc/3` that implements method `inc`. Do you know where exactly? If you on the other hand execute the following statement, things will work as expected.

> `local X in thread {C inc(X)} end X=5 end {C browse}`

## 10.3 Objects and Classes for Real

Oz supports object-oriented programming following the methodology outlined above. There is also syntactic support and optimized implementation so that object application (calling a method in objects) is as cheap as procedure calls. The class `Counter` defined earlier has the syntactic form shown in Figure 10.3:

> `class Counter
> attr val
> meth browse
> {Browse @val}
> end
> meth inc(Value)
> val := @val + Value
> end
> meth init(Value)
> val := Value
> end
> end`

Figure 10.3: Counter Class

A class X is defined by:

> `class` X`... end`

Attributes are defined using the attribute-declaration part before the method-declaration part:

> `attr` A1`...` AN

Then follows the method declarations, each has the form:

> `meth` E`` S `end`

where the expression E evaluates to a method head, which is a record whose label is the method name. An attribute A is accessed using the expression `@`A. It is assigned a value using the statement A`:=` E.

A class can be defined anonymously by:

> `X = class $ ... end`

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 in
> C = {New Counter init(0)}
> {C browse}
> {C inc(1)}
> local X in thread {C inc(X)} end X=5 end`

### 10.3.1 Static Method Calls

Given a class C and a method head m`(...)`, a method call has the following form:

> C`,` m`(...)`

A method call invokes the method defined in the class argument. A method call can only be used inside method definitions. This is because a method call takes the current object denoted by `self` as implicit argument. The method could be defined at the class `C` or inherited from a super class. Inheritance will be explained shortly.

### 10.3.2 Classes as Modules

Static method calls have in general the same efficiency as procedure calls. This allows classes to be used as module-specification. This may be advantageous because classes can be built incrementally by inheritance. The program shown in Figure 10.4 shows a possible class acting as a module specification. The class `ListC` defines some common list-procedures as methods. `ListC` defines the methods `append/3`, `member/2`, `length/2`, and `nrev/2`. Notice that a method body is similar to any Oz statement but in addition, method calls are allowed. We also see the first example of inheritance.

> `class ListC from BaseObject`

We also show functional methods, i.e. methods that return results similar to functions. A functional method has in general the following form:

> `meth m( ... $) S E end`

Here the class `ListC` inherits from the predefined class `BaseObject` that has only one trivial method: `meth noop() skip end`.

> `class ListC from BaseObject
> meth append(Xs Ys $)
> case Xs
> of nil then Ys
> [] X|Xr then
> X|(ListC , append(Xr Ys $))
> end
> end
> meth member(X L $)
> {Member X L} % This defined in List.oz
> end
> meth length(Xs $)
> case Xs
> of nil then 0
> [] _|Xr then
> (ListC , length(Xr $)) + 1
> end
> end
> meth nrev(Xs ?Ys)
> case Xs
> of nil then Ys = nil
> [] X|Xr then Yr in
> ListC , nrev(Xr Yr)
> ListC , append(Yr [X] Ys)
> end
> end
> end`

Figure 10.4: List Class

To create a module from the module specification one needs to create an object from the class. This is done by:

> `declare ListM = {New ListC noop}`

`ListM` is an object that acts as a module, i.e. it encapsulates a group of procedures (methods). We can try this module by performing some method calls:

> `{Browse {ListM append([1 2 3] [4 5] $)}}`

> `{Browse {ListM length([1 2 3] $)}}`

> `{Browse {ListM nrev([1 2 3] $)}}`

## 10.4 Inheritance

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.

Inheritance is a way to construct new classes from existing classes. It defines what attributes, features3, and methods are available in the new class. We will restrict our discussion of inheritance to methods. Nonetheless, the same rules apply to features and attributes.

The methods available in a class C (i.e. visible) are defined through a precedence relation on the methods that appear in the class hierarchy. We call this relation the overriding relation:

- A method in a class C overrides any method, with the same label, in any super class of C.

Now a class hierarchy with the super-class relation can be seen as a directed graph with the class being defined as the root. The edges are directed towards the subclasses. There are two requirements for the inheritance to be valid. First, the inheritance relation is directed and acyclic. So the following is not allowed:

> `class A from B ... end
> class B from A ... end`

Figure 10.5: Illegal class hierarchy

Second, after striking out all overridden methods each remaining method should have a unique label and is defined only in one class in the hierarchy. Hence, class `C` in the following example is not valid because the two methods labeled `m` remain.

> `class A1 meth m(...) ... end end
> class B1 meth m(...) ... end end
> class B from B1 end
> class A from A1 end
> class C from A B end`

Figure 10.6: Illegal class hierarchy in method `m`

Also the class `C` below is invalid, since two methods `m` is available in `C`.

> `class A meth m(...) ... end end
> class B meth m(...) ... end end
> class C from A B end`

Notice that if you run a program with an invalid hierarchy, the system will not complain until an object is created that tries to access an invalid method. Only at this point of time, you are going to get a runtime exception. The reason is that classes are partially formed at compile time, and are completed by demand, using method caches, at execution time.

### 10.4.1 Multiple inheritance or Not

My opinion is the following:

- In general, to use multiple inheritance correctly, one has to understand the total inheritance hierarchy, which is sometimes worth the effort. This is important when there is a shared common ancestor.

- Oz restricts multiple inheritance in a way that most the problems with it do not occur.

- Oz enforces a programming methodology which requres one to override a method which is defined at more than one superclass, one has to define the method locally to overrides the conflict-causing methods.

- There is another problem with multiple inheritance when sibling super-classes share (directly or indirectly) a common ancestor-class that is stateful (i.e. has attributes). One may get replicated operations on the same attribute. This typically happens when executing an initialization method in a class, one has to initialize its super classes. The only remedy here is to understand carefully the inheritance hierarchy to avoid such replication. Alternatively, you should only inherit from multiple classes that do not share stateful common ancestor. This problem is known as the implementation-sharing problem.

## 10.5 Features

Objects may have features similar to records. Features are stateless components that are specified in the class declaration:

> `class` C `from ...
> feat` A1`...` AN`
> ...
> end`

As in a record, a feature of an object has an associated field. The field is a logic variable that can be bound to any Oz value (including cells, objects, classes etc.). Features of objects are accessed using the infix '`.`' operator. The following shows an example using features:

> `class ApartmentC from BaseObject
> meth init skip end
> end
> class AptC from ApartmentC
> feat
> streetName: york
> streetNumber:100
> wallColor:white
> floorSurface:wood
> end`

### 10.5.1 Feature initialization

The example shows how features could be initialized at the time the class is defined. In this case, all instances of the class `AptC` will have the features of the class, with their corresponding values. Therefore, the following program will display `york` twice.

> `declare Apt1 Apt2
> Apt1 = {New AptC init}
> Apt2 = {New AptC init}
> {Browse Apt1.streetName}
> {Browse Apt2.streetName}`

We may leave a feature uninitialized as in:

> `class MyAptC1 from ApartmentC
> feat streetName
> end`

In this case whenever an instance is created, the field of the feature is assigned a new fresh variable. Therefore, the following program will bind the feature `streetName` of object `Apt3` to the atom `kungsgatan`, and the corresponding feature of `Apt4` to the atom `sturegatan`.

> `declare Apt3 Apt4
> Apt3 = {New MyAptC1 init}
> Apt4 = {New MyAptC1 init}
> Apt3.streetName = kungsgatan
> Apt4.streetName = sturegatan`

One more form of initialization is available. A feature may be initialized in the class declaration to a variable or an Oz-value that has a variable. In the following, the feature is initialized to a tuple with an anonymous variable. In this case, all instances of the class will share the same variable. Consider the following program.

> `class MyAptC1 from ApartmentC
> feat streetName:f(_)
> end
> local Apt1 Apt2 in
> Apt1 = {New MyAptC1 init}
> Apt2 = {New MyAptC1 init}
> {Browse Apt1.streetName}
> {Browse Apt2.streetName}
> Apt1.streetName = f(york)`

If entered incrementally, will show that the statement

> `Apt1.streetName = f(york)`

binds the corresponding feature of `Apt2` to the same value as that of `Apt1`.

What has been said of features also holds for attributes.

## 10.6 Parameterized Classes

There are many ways to get your classes more generic, which later may be specialized for specific purposes. The common way to do this in object-oriented programming is to define first an abstract class in which some methods are left unspecified. Later these methods are defined in the subclasses. Suppose you have defined a generic class for sorting where the comparison operator `less` is needed. This operator depends on what kinds of data are being sorted. Different realizations are needed for integer, rational, or complex numbers, etc. In this case, by subclassing we can specialize the abstract class to a concrete class.

In Oz, we have also another natural method for creating generic classes. Since classes are first-class values, we can instead define a function that takes some type argument(s) and return a class that is specialized for the type(s). In Figure 10.7, the function `SortClass` is defined that takes a class as its single argument and returns a sorting class specialized for the argument.

> `fun {SortClass Type}
> class $ from BaseObject
> meth qsort(Xs Ys)
> case Xs
> of nil then Ys = nil
> [] P|Xr then S L in
> {self partition(Xr P S L)}
> ListC, append({self qsort(S $)} P|{self qsort(L $)} Ys)
> end
> end
> meth partition(Xs P Ss Ls)
> case Xs
> of nil then Ss = nil Ls = nil
> [] X|Xr then Sr Lr in
> case Type,less(X P $) then
> Ss = X|Sr Lr = Ls
> else
> Ss = Sr Ls = X|Lr
> end
> {self partition(Xr P Sr Lr)}
> end
> end
> end
> end`

Figure 10.7: Parameterized Classes

We can now define two classes for integers and rationals:

> `class Int
> meth less(X Y $)
> X<Y
> end
> end
> class Rat from Object
> meth less(X Y $)
> '/'(P Q) = X
> '/'(R S) = Y
> in
> P*S < Q*R
> end
> end`

Thereafter, we can execute the following statements:

> `{Browse {{New {SortClass Int} noop} qsort([1 2 5 3 4] $)}}
> {Browse {{New {SortClass Rat} noop}
> qsort(['/'(23 3) '/'(34 11) '/'(47 17)] $)}}`

## 10.7 Self Application

The program in Figure 10.7 shows in the method `qsort` an object application using the keyword `self` (see below).

> `meth qsort(Xs Ys)
> case Xs
> ...
> {self partition(Xr P S L)}
> ...
> end`

We use here the phrase object-application instead of the commonly known phrase message sending because message sending is misleading in a concurrent language like Oz. When we use `self` instead of a specific object as in

> `{self partition(Xr P S L)}`

We mean that we dynamically pick the method `partition` that is defined (available) in the current object. Thereafter we apply the object (as a procedure) to the message. This is a form of dynamic binding common in all object-oriented languages.

## 10.8 Attributes

We have touched before on the notion of attributes. Attributes are the carriers of state in objects. Attributes are declared similar to features, but using the keyword `attr` instead. When an object is created each attribute is assigned a new cell as its value. These cells are initialized very much the same way as features. The difference lies in the fact that attributes are cells that can be assigned, reassigned and accessed at will. However, attributes are private to their objects. The only way to manipulate an attribute from outside an object is to force the class designer to write a method that manipulates the attribute. In the Figure 10.8 we define the class `Point`. Note that the attributes `x` and `y` are initialized to zero before the initial message is applied. The method `move` uses `self`-application internally.

> `class Point from BaseObject
> attr x:0 y:0
> meth init(X Y)
> x := X
> y := Y % attribute update
> end
> meth location(L)
> L = l(x:@x y:@y) % attribute access
> end
> meth moveHorizontal(X)
> x := X
> end
> meth moveVertical(Y)
> y := Y
> end
> meth move(X Y)
> {self moveHorizontal(X)}
> {self moveVertical(Y)}
> end
> meth display
> % Switch the browser to virtual string mode
> {Browse "point at ("#@x#" , "#@y#")\n"}
> end
> end`

Figure 10.8: The class Point

Try to create an instance of `Point` and apply some few messages:

> `declare P
> P = {New Point init(2 0)}
> {P display}
> {P move(3 2)}`

## 10.9 Private and Protected Methods

Methods may be labeled by variables instead of literals. These methods are private to the class in which they are defined, as in:

> `class C from ...
> meth A(X) ... end
> meth a(...) {self A(5)} ... end
> ....
> end`

The method `A` is visible only within the class `C`. In fact the notation above is just an abbreviation of the following expanded definition:

> `local A = {NewName} in
> class C from ...
> meth !A(X) ... end
> meth a(...) {self A(5)} ... end
> ...
> end
> end`

A is bound to a new name in the lexical scope of the class definition.

Some object-oriented languages have also the notion of protected methods. A method is protected if it is accessible only in the class it is defined or in descendant classes, i.e. subclasses and subsubclasses etc. In Oz there is no direct way to define a method to be protected. However there is a programming technique that gives the same effect. We know that attributes are only visible inside a class or to descendants of a class by inheritance. We may make a method protected by first making it private and second by storing it in an attribute. Consider the following example:

> `class C from ...
> attr pa:A
> meth A(X) ... end
> meth a(...) {self A(5)} ... end
> ...
> end`

Now, we create a subclass `C1` of `C` and access method `A` as follows:

> `class C1 from C
> meth b(...) L=@pa in {self L(5)} ... end
> ...
> end`

Method `b` accesses method `A` through the attribute `pa`.

Let us continue our simple example in Figure 10.8 by defining a specialization of the class that in addition of being a point, it stores a history of the previous movement. This is shown in Figure 10.9.

> `class HistoryPoint from Point
> attr
> history: nil
> displayHistory: DisplayHistory
> meth init(X Y)
> Point,init(X Y) % call your super
> history := [l(X Y)]
> end
> meth move(X Y)
> Point,move(X Y)
> history := l(X Y)|@history
> end
> meth display
> Point,display
> {self DisplayHistory}
> end
> meth DisplayHistory % made protected method
> {Browse "with location history: "}
> {Browse @history}
> end
> end`

Figure 10.9: The class History Point

There are a number of remarks on the class definition `HistoryPoint`. First observe the typical pattern of method refinement. The method `move` specializes that of class `Point`. It first calls the super method, and then does what is specific to being a `HistoryPoint` class. Second, `DisplayHistory` method is made private to the class. Moreover it is made available for subclasses, i.e. protected, by storing it in the attribute `displayHistory`. You can now try the class by the following statements:

> `declare P
> P = {New HistoryPoint init(2 0)}
> {P display}
> {P move(3 2)}`

## 10.10 Default Argument Values

A method head may have default argument values. Consider the following example.

> `meth m(X Y d1:Z<=0 d2:W<=0) ... end`

A call of the method `m` may leave the arguments of features `d1` and `d2` unspecified. In this case these arguments will assume the value zero.

We continue our `Point` example by specializing `Point` in a different direction. We define the class `BoundedPoint` as a point that moves in a constrained rectangular area. Any attempt to move such a point outside the area will be ignored. The class is shown in Figure 10.10. Notice that the method `init` has two default arguments that give a default area if not specified in the initialization of a new instance of `BoundedPoint`.

> `class BoundedPoint from Point
> attr
> xbounds: 0#0
> ybounds: 0#0
> boundConstraint: BoundConstraint
> meth init(X Y xbounds:XB <= 0#10 ybounds:YB <= 0#10)
> Point,init(X Y) % call your super
> xbounds := XB
> ybounds := YB
> end
> meth move(X Y)
> if {self BoundConstraint(X Y $)} then
> Point,move(X Y)
> end
> end
> meth BoundConstraint(X Y $)
> (X >= @xbounds.1 andthen
> X =< @xbounds.2 andthen
> Y >= @ybounds.1 andthen
> Y =< @ybounds.2 )
> end
> meth display
> Point,display
> {self DisplayBounds}
> end
> meth DisplayBounds
> X0#X1 = @xbounds
> Y0#Y1 = @ybounds
> S = "xbounds=("#X0#","#X1#"),ybounds=("
> #Y0#","#Y1#")"
> in
> {Browse S}
> end
> end`

Figure 10.10: The class BoundedPoint

We conclude this section by finishing our example in a way that shows the multiple inheritance problem. We would like now a specialization of both `HistoryPoint` and `BoundedPoint` as a bounded-history point. A point that keeps track of the history and moves in a constrained area. We do this by defining the class `BHPoint` that inherits from the two previously defined classes. Since they both share the class `Point`, which contains stateful attributes, we encounter the implementation-sharing problem. We, any way, anticipated this problem and therefore created two protected methods stored in `boundConstraint` and `displayHistory` to avoid repeating the same actions. In any case, we have to refine the methods `init`, `move` and `display` since they occur in the two sibling classes. The solution is shown in Figure 10.11. Notice how we use the protected methods. We did not care avoiding the repetition of initializing the attributes `x` and `y` since it does not make any harm. Try the following example:

> `declare P
> P = {New BHPoint init(2 0)}
> {P display}
> {P move(1 2)}`

This pretty much covers most of the object system. What is left is how to deal with concurrent threads sharing a common space of objects.

> `class BHPoint from HistoryPoint BoundedPoint
> meth init(X Y xbounds:XB <= 0#10 ybounds:YB <= 0#10)
> % repeats init
> HistoryPoint,init(X Y)
> BoundedPoint,init(X Y xbounds:XB ybounds:YB)
> end
> meth move(X Y)
> L = @boundConstraint in
> if {self L(X Y $)} then
> HistoryPoint,move(X Y)
> end
> end
> meth display
> BoundedPoint,display
> {self @displayHistory}
> end
> end`

Figure 10.11: The class BHPoint

1. In fact, classes may have some invisible state. In the current implementation, a class usually has method cache, which is stateful
2. This is a simplification; an object in Oz is a chunk that has the above procedure in one of its fields; other fields contain the object features
3. To be defined shortly