CSE 505 Lecture Notes:

Types in Object-Oriented Languages

November 19, 1997


Some choices: In Smalltalk-80, all type checking is done dynamically.

Object-oriented languages with static type checking: Emerald, Simula, Trellis/Owl, Eiffel

Cecil: optional type declarations and static checking


Types in an object-oriented language:
A collection of operation signatures, where each signature consists of the operation name, the types of the arguments, and the type of the result(s)
We will say that an object is of an abstract type if it has the properties defined by that type. Abstract types can allow us to do static type checking in an object-oriented language, while preserving the dynamic binding of names to operations. For example, we can statically check that some object will understand a message m, even though we don't know exactly which method will be invoked.

Another definition sometimes used in type inference systems: a type is a set of classes (see e.g. Palsberg and Schwartzbach)


Contravariance vs. covariance

The contravariant rule for subtyping is used in Emerald, Cecil, Trellis/Owl, etc. (It is also the rule used in Cardelli's paper "A Semantics of Multiple Inheritance".)

Covariant rule is used in Eiffel.

Contravariant rule:

S is a subtype of T if:

  1. S provides all the operations that T does (and maybe some more)
  2. For each operation in T, the corresponding operation in S has the same number of arguments and results
  3. The types of the results of S's operations are subtypes of the types of the corresponding results of T's operations
  4. The types of the arguments of T's operations are subtypes of the types of the corresponding arguments of S's operations (note the reversal of T and S here)
N.B. If S=T, then S is a subtype of T.

Covariant rule:

same as above, except for 4:

  1. The types of the arguments of S's operations are subtypes of the types of the corresponding arguments of T's operations


Example

Type Color Type GrayScaleColor (a subtype of Color) Type Number Type Integer, Float (both subtypes of Number) Type Point x(): Number y(): Number Type ColoredPoint x(): Number y(): Number mycolor() : Color Type GrayScalePoint x(): Number y(): Number mycolor() : GrayScaleColor ColoredPoint is a subtype of Point -- anywhere a Point is needed, we can use a ColoredPoint.

Also GrayScalePoint is a subtype of ColoredPoint, and GrayScalePoint is a subtype of Point.

p : ColoredPoint; c : Color p := GrayScalePoint.new; c := p.mycolor; q , r : Point; a : Number; q := ColoredPoint.new; r := GrayScalePoint.new; a := q.x + r.x; now add a message with an argument: Type ColoredPoint x(): Number y(): Number mycolor() : Color setDotSize(c : Integer); Type GrayScalePoint x(): Number y(): Number mycolor() : GrayScaleColor setDotSize(c : Number); The contravariant rule says that GrayScalePoint is still a subtype of ColoredPoint. c, d : ColoredPoint; c := ColoredPoint.new; d := GrayScalePoint.new; c.setDotSize(3); d.setDotSize(4); However, the following statements would give compile-time errors: c.setDotSize(3.5); d.setDotSize(3.5);

Now consider:

Type ColoredPoint x(): Number y(): Number mycolor() : Color setcolor(c : Color); Type GrayScalePoint x(): Number y(): Number mycolor() : GrayScaleColor setcolor(c : GrayScaleColor); Under the contravariant rule there is no longer a subtype relation between ColoredPoint and GrayScalePoint. Example: c : ColoredPoint; c := GrayScalePoint.new; /* not permitted */ c.setcolor(yellow); /* an error would occur if we tried this */

However, under the covariant rule GrayScalePoint would still be a subtype of ColoredPoint, and the above program would type check correctly.


Multiple Inheritance

A type can be a subtype of several other types -- 'multiple inheritance' for abstract types introduces no additional complications (unlike multiple implementation inheritance).


Parameterized types

What is the type of array? We want to be able to get reasonable type information for the element access and set messages.

Statically-typed object-oriented languages usually have some notion of parameterized types to handle this.

a : Array[Integer] n : Integer; ... a[1] := 4; n := a[1]; For a type Array, we can have something like: Type Array[T] at(n : Integer) : T put(n : Integer, x : T) Question:What is the type relation between Array[Number] and Array[Integer]?

Answer: under the contravariant rule, none.


Design Decision: should abstract type and concrete implementation inheritance be separate or combined? (This question only makes sense if there are type declarations.)

Reasons for separating them:

Reasons against:

Example of multiple implementations (two concrete classes with one abstract type):

abstract type Stack implementation classes ListStack, ArrayStack both would conform to the abstract type Stack Example of inheriting implementation but not abstract type:

Stack inherits from Deque (just masks off extra operations like front)