IV. Objects in MODSIM III
 

Objective

• Understanding objects and their relationship
• Learn tools for manipulating  methods and fields.

   4.1 Object:

• Dynamically allocated data structures coupled with routines (called methods).
• Data structures are abstract.
• The state and status of objects are called fields. Fields can only be modified by its own method.
• Object type is specified by its fields and methods.
• Object type is dynamic (i.e. behaves like arrays).

Example:

                                        Note: object includes methods (functions) and
                                        fields (data).

                                        In this case field of object is hidden, it is the
                                        value returned by method.
 
 

   4.2 Capabilities:
 

• Encapsulation of data and code: Fields are tied with  procedures (called methods) which define object's behaviors.

• Inheritance: New types of objects can be defined based on the existing type. Each descendant in the hierarchy can add its own fields and method definitions to those of its ancestors.

• Message passing: An object's methods are invoked by sending a message to the object asking it to perform a specific method.

• Polymorphism: Allowing differing object types in a hierarchy to share the same method name but provide their own implementation of that method. This results in a generic invocation producing different behaviors appropriate to the object being referenced.

• Hierarchical types: A descendant is type compatible with any of its ancestors.

Object type needs to be defined before an object can be introduced.
 
Example:
 
TYPE
  VehicleObject = OBJECT   { <=  object type declaration }
     course : [ 0 .. 359 ];
     speed : INTEGER;
     position : PositType;
    ASK METHOD GoTo(IN destination : PositType);
END OBJECT;

OBJECT VehicleObject;     { <= object declaration }
     ASK METHOD GoTo(IN destination : PositType);
    BEGIN METHOD
     ...
     executable code goes here
     ...
    END METHOD;
END OBJECT;

Methods ate declared along with objects and object types.
 

• Use of commands: ASK METHOD, TELL METHOD or 
  WAITFOR METHOD.

Examples:
 
    OBJECT VehicleObject;
ASK METHOD GoTo(IN destination : PositType);
BEGIN
implementation code ...
END METHOD;
    END OBJECT;
 

TELL METHOD Stop;

BEGIN
speedKm := 0;
END METHOD;
 

• Use of PROCEDURE.

Example:
 
MyGroupObj = OBJECT
      ASK METHOD First : Aobj;
      ASK METHOD Next (IN obj : Aobj) : Aobj;
      ASK METHOD Prev (IN obj : Aobj) : Aobj;
      ASK METHOD Last : Aobj;
END OBJECT;
PROCEDURE iterate (IN grp: MyGroupObj);
      VAR
      a : Aobj;
      BEGIN
      FOREACH a IN grp
      {Use 'a' }
      END FOREACH;
 END PROCEDURE;
 
• Use of Definition and Implementation modules:

• Object type declaration is placed in a DEFINITION MODULE.
• The corresponding object declaration is placed in an IMPLEMENTATION MODULE.

• Method Invocation:
 
   Syntex:

ASK object TO method ( parameter list)
or
value := ASK object TO method ( parameter list);
 

Example:
 
TELL Aircraft TO FlyTo(OHare);
location := ASK Ship position(   );
 
• Built-in Reference Constant SELF: used to refer to the object itself, within one of its methods.

Examples:

TELL SELF TO FlyTo(OHare);

location := ASK SELF position(   );

TELL Tiger20 TO ReportDistance(SELF);

Syntex:  ASK object field

Examples:
 

range := ASK Car fuelLeft * ASK Car MPG;
 

range := ASK SELF fuelLeft * ASK SELF MPG;
 

IF ASK Tiger20 position <> HomeBase
   TELL Tiger20 TO ProceedTo(HomeBase);
    OUTPUT("Not at home base, but returning");
ELSE
   OUTPUT("Already at home base");
END IF;
 

ASK (ASK object group) numberIn;

            Example:

DEFINITION MODULE MovingMod;                           {<= Module header}

TYPE                                                                                     {<= User defined type}
   fuelType = (Unleaded, Diesel, AvGas, JetFuel);

locationType = RECORD                                                   {<= define record}

latitude,
longtitude : REAL
END RECORD;

EngineObj = OBJECT                                                         {<= define objects}

power : INTEGER;
fuel : fuelType;
kmPerLtr : REAL;
TELL METHOD ConsumeFuel;
END OBJECT;

VehicleObj = OBJECT( EngineObj )

position : locationType;
course : [ 0..359 ];
speedKm : INTEGER;
fuelLevel : REAL;
TELL METHOD GoTo(IN destination: locationType);
TELL METHOD Stop;
END OBJECT;

AircraftObj = OBJECT( VehicleObj )

altitude : INTEGER;
END OBJECT;

HelicopterObj = OBJECT( AircraftObj )                              { <= define inherited object}

inHover : BOOLEAN;
TELL METHOD Hover(IN posit : locationType;
                                          IN alt : INTEGER);
END OBJECT;

END MODULE.

TELL METHOD Stop;                             {<= old definition of "stop" method}
BEGIN
speedKm := 0;
END METHOD;

AircraftObj = OBJECT( VehicleObj )
altitude : INTEGER;
OVERRIDE                                               {<= use a new definition for an object}

TELL METHOD Stop;
END OBJECT;
 

OBJECT AircraftObj                        {<= new defintion given here for that object}
  TELL METHOD Stop;
  BEGIN

{... make sure we're on the ground first!}
altitude := 0;
speedKm := 0;
   END METHOD;
END OBJECT;
 

• The PRIVATE section lists all of the fields and methods which are part of the object type declaration, but which cannot be accessed from outside of the object.

Example:
 
TYPE
AircraftObj = OBJECT( VehicleObj )
     altitude : INTEGER;
     TELL METHOD ClimbTo(IN height: REAL);
     TELL METHOD Circle;
     ASK METHOD FindTarget(IN enemy: VehicleObj);
     PRIVATE
       liftCoefficient : REAL;
       ASK METHOD CalcLiftCoeff;
       WAITFOR METHOD DeployLandingGear;
      OVERRIDE
                     TELL METHOD Stop;
END OBJECT;
 
 

Summary
 

• Objects contain methods and fields (functions+data)

• Objects have multiple capabilities.

• Objects are introduced by two steps: type + object definition.

• Various means for introduction of methods and reference of fields.