Classes, objects and inheritance.

Classes, objects and encapsulation.

In the 9618 pseudocode a class is written between CLASS and ENDCLASS. The constructor is always a procedure called NEW, and the keywords PUBLIC and PRIVATE set who may use each attribute and method.

CLASS Account
    PRIVATE AccountID : STRING
    PRIVATE Balance : REAL
    PUBLIC PROCEDURE NEW(GivenID : STRING)
        AccountID ← GivenID
        Balance ← 0
    ENDPROCEDURE
    PUBLIC FUNCTION GetBalance() RETURNS REAL
        RETURN Balance
    ENDFUNCTION
    PUBLIC PROCEDURE Deposit(Amount : REAL)
        IF Amount > 0 THEN
            Balance ← Balance + Amount
        ENDIF
    ENDPROCEDURE
    PUBLIC FUNCTION Withdraw(Amount : REAL) RETURNS BOOLEAN
        IF Amount > 0 AND Amount <= Balance THEN
            Balance ← Balance - Amount
            RETURN TRUE
        ELSE
            RETURN FALSE
        ENDIF
    ENDFUNCTION
ENDCLASS

DECLARE MyAccount : Account
MyAccount ← NEW Account("A101")
MyAccount.Deposit(50)
OUTPUT MyAccount.GetBalance()

Inheritance and polymorphism.

A subclass is declared with INHERITS. It gets every attribute and method of the superclass, adds its own, and can override a method by redefining it with the same name. Its constructor calls the superclass constructor with SUPER.NEW.

CLASS Shape
    PRIVATE Name : STRING
    PUBLIC PROCEDURE NEW(GivenName : STRING)
        Name ← GivenName
    ENDPROCEDURE
    PUBLIC FUNCTION GetName() RETURNS STRING
        RETURN Name
    ENDFUNCTION
    PUBLIC FUNCTION Area() RETURNS REAL
        RETURN 0
    ENDFUNCTION
ENDCLASS

CLASS Rectangle INHERITS Shape
    PRIVATE Width : REAL
    PRIVATE Height : REAL
    PUBLIC PROCEDURE NEW(GivenName : STRING, GivenWidth : REAL, GivenHeight : REAL)
        SUPER.NEW(GivenName)
        Width ← GivenWidth
        Height ← GivenHeight
    ENDPROCEDURE
    PUBLIC FUNCTION Area() RETURNS REAL
        RETURN Width * Height
    ENDFUNCTION
ENDCLASS

CLASS Circle INHERITS Shape
    PRIVATE Radius : REAL
    PUBLIC PROCEDURE NEW(GivenName : STRING, GivenRadius : REAL)
        SUPER.NEW(GivenName)
        Radius ← GivenRadius
    ENDPROCEDURE
    PUBLIC FUNCTION Area() RETURNS REAL
        RETURN 3.142 * Radius * Radius
    ENDFUNCTION
ENDCLASS

DECLARE Shapes : ARRAY[1:3] OF Shape
DECLARE Index : INTEGER
Shapes[1] ← NEW Rectangle("R1", 4, 2.5)
Shapes[2] ← NEW Circle("C1", 2)
Shapes[3] ← NEW Rectangle("R2", 3, 3)
FOR Index ← 1 TO 3
    OUTPUT Shapes[Index].GetName(), " ", Shapes[Index].Area()
NEXT Index

• Inheritance: Rectangle and Circle never define GetName, but they inherit it from Shape. A subclass is a more specialised version of its superclass ("a Circle is a Shape").
• SUPER.NEW(GivenName): Name is PRIVATE to Shape, so the subclass constructor sets it by calling the superclass constructor.
• Polymorphism: the same call Shapes[Index].Area() runs a different method depending on the class of the object: Width * Height for a Rectangle, 3.142 * Radius * Radius for a Circle. Each subclass overrides Shape's Area.
• Because every Rectangle and Circle is a Shape, they can all be stored in one ARRAY OF Shape and processed by the same loop.

Containment and class design.

Not every relationship is inheritance. If an object has other objects as parts, use containment (aggregation): the container holds them as attributes, often in an array.

CLASS Drawing
    PRIVATE Shapes : ARRAY[1:20] OF Shape
    PRIVATE NumShapes : INTEGER
    PUBLIC PROCEDURE NEW()
        NumShapes ← 0
    ENDPROCEDURE
    PUBLIC FUNCTION AddShape(NewShape : Shape) RETURNS BOOLEAN
        IF NumShapes < 20 THEN
            NumShapes ← NumShapes + 1
            Shapes[NumShapes] ← NewShape
            RETURN TRUE
        ELSE
            RETURN FALSE
        ENDIF
    ENDFUNCTION
    PUBLIC FUNCTION TotalArea() RETURNS REAL
        DECLARE Total : REAL
        DECLARE Index : INTEGER
        Total ← 0
        FOR Index ← 1 TO NumShapes
            Total ← Total + Shapes[Index].Area()
        NEXT Index
        RETURN Total
    ENDFUNCTION
ENDCLASS