Classes

Classes enable you to create objects based on templates. You can use JSL to create a definition of a class. A class definition is a template, which contains definitions for member variables, methods, and functions. After you have defined a class in JSL, you can instantiate an object that has all of the characteristics defined in the class definition.

Use the JSL function Define Class() to create a JSL class definition. Use the JSL function New Object() to instantiate an instance of a JSL class.

The JSL function Define Class() has a number of arguments that define the contents of the class:

Define Class( "class name",

	<Base Class( "base class name", <"base class name", ...> ),>

	<Show( All( Boolean ) ) | Show( <Members( Boolean ),> <Methods( Boolean ),> <Functions( Boolean )> ),>

	<Assignment Statements>

);

Where:

•

"class name" is the name of the class being defined.

•

"base class name" is one or more class names that make the set of base classes that are used as the foundation of the class being created.

•

Show defines the value that appears when a class object is written out using the Show(), Print(), or Write() functions. Options include:

–

All( Boolean ) shows all members, methods, and functions defined in a class object instance. The default value for All is true. If you specify the All option, that setting overrides any other arguments in the Show option.

–

Members( Boolean ) shows all member variables contained in a class object instance.

–

Methods( Boolean ) shows all methods contained in a class object instance.

–

Functions( Boolean ) shows all functions contained in a class object instance.

Note: You can specify either the All option or any number of the Members, Methods, and Functions options, but you cannot specify both options at the same time.

Note: If a class contains a _show_ method, that method overrides the Show option.

•

Assignment Statements are zero or more of the following assignment statements, separated by semicolons:

–

Member = JSL Expression

–

Method Name = Method( {<argument, ...>}, body )

–

Function Name = Function( {<argument, ...}, {<local variable, ...>}, body )

Note: The Assignment Statements can alternatively be specified as a list of JSL expressions.

There are three specialized methods that you can use in a class definition:

_init_

Initializes the object at the time it is instantiated. This is considered the constructor for the class. You can have only one constructor per class.

_show_

Defines the output of the Show(), Print(), and Write() JSL functions when an instance of the class is used as the argument.

_to string_

Defines the output of the Char() JSL function when an instance of the class is used as the argument.

Notes:

•

All methods, member variables, and functions are public.

•

Functions defined within a class cannot access method variables or methods within the class.

Example of Defining a Class

The following example defines a simple class for a pet, containing the following information: name, breed, and color.

In this example, you define the constructor to require all information to create a new Pet object. Also, you define a set of accessor methods to obtain each of the attributes of a Pet object.

Define Class( "Pet",

	name = ""; breed = ""; color = "";

	_init_ = Method( { name, breed, color },

		this:name = name;

		this:color = color;

		this:breed = breed;

);

	_show_ = Method( {},

		Return( name || " is a " || color || " " || breed || "." )

);

	Get Pet Name = Method( {}, Return( name ) )

);

Instantiating a Class

To instantiate an instance of a JSL class object, use New Object() JSL function with any of the following functions and arguments:

New Object( "Class Name"( constructor arguments ) );

New Object( Class Name( constructor arguments ) );

New Object( Class Reference( constructor arguments ) );

New Object( "Class Name", { constructor arguments } );

New Object( Class Name, { constructor arguments } );

New Object( Class Reference, { constructor arguments } );

Examples of Instantiating and Using a Class

The following example shows how to instantiate a class to create and access JSL class objects.

class = "Pet";

xa = New Object( Pet( "Oliver", "Schnauzer", "Black" ) );

xb = New Object( class, { "Flynn", "Cockapoo", "Brown" } );

xc = New Object( class( "Mandi", "King Charles Cavalier", "Ruby" ) );

xd = New Object( "Pet"( "Charlie", "Asian", "White" ) );

xd1 = New Object( xd( "Fred", "Siamese", "Silver" ) );

xd2 = New Object( xd, { "Sam", "German Shepard", "Black and White" } );

Show( xa );

Show( xb );

Show( xc );

Show( xd );

Show( xd1 );

Show( xd2 );

xd2 = xd;                 // creates a second reference to xd

xdclone = xd << Clone;    // copies xd to a new instance, named xdclone

xd2:name = "Jerry";

Show( xd:Get Pet Name(), xd2:Get Pet Name(), xdclone:Get Pet Name() );

This example creates multiple instances of the "Pet" class. The output of the Show() JSL function is determined by the _show_ method that is defined in the "Pet" class definition. This example also illustrates the difference between creating a second JSL reference to a class instance and cloning a class instance. When using the Clone message, you are creating a new class instance that is independent of the original. However, the new instance starts out as a copy of the original.