The WebScript Language

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Creating Instances of Classes

The section "Variables" told you how to declare variables, which represent objects. Before you use a variable, you must first create the object, or instantiate the class that defines the object. In WebScript, there are two different ways to create objects. The first approach, which applies only to the most commonly used classes, is to initialize with constant objects as described in the section "Assigning Values to Variables". The second approach, which applies to all classes, is to use creation methods.

All classes provide creation methods that you can use to create an instance of that class. Depending on the class and the particular creation method, the instances of the class you create are either mutable (modifiable) or immutable (constant). Usually, the instances of a class are always mutable or always immutable. (You must read the specifications in the Foundation Framework Reference to find out if a particular class is immutable or mutable.) However, some classes, including NSString, NSArray, and NSDictionary provide both mutable and immutable forms, and you can choose which one to create. It's best to create immutable objects wherever possible. Use mutable objects only if you need to change its value after you initialize it.

Here are some examples of using creation methods to create mutable and immutable NSString, NSArray, and NSDictionary objects:

// Create a mutable string

string = [NSMutableString stringWithFormat:@"The string 
is %@", aString];



// Create an immutable string

string = [NSString stringWithFormat:@"The string is %@", 
aString];



// Create a mutable array

array = [NSMutableArray array];

anotherArray = [NSMutableArray 
arrayWithObjects:@"Marsha", @"Greg", 

	@"Cindy", nil];



// Create an immutable array

array = [NSArray arrayWithObjects:@"Bobby", @"Jan", 
@"Peter", nil];



// Create a mutable dictionary

dictionary = [NSMutableDictionary dictionary];

// Create an immutable dictionary

id stooges = [NSDictionary

		dictionaryWithObjects:@("Mo", "Larry", "Curley")

		forKeys:@("Stooge1", "Stooge2", "Stooge3")];

The following examples show how you can create and work with NSCalendarDates, which are always immutable:

// Using the creation method date, create an 
NSCalendarDate instance 

// 'now' that contains the current date and time

now = [NSCalendarDate date];



// Return a string representation of 'now' using a format 
string

dateString = [now descriptionWithCalendarFormat:@"%B %d, 
%Y"];



// Using the creation method dateWithString:, create an 
NSCalendarDate

// instance 'newDate' from 'dateString'

newDate = [NSCalendarDate dateWithString:dateString 

	calendarFormat:@"%B %d, %Y"];



// Return a new date in which newDate's day field is 
decremented

date = [newDate addYear:0 month:0 day:-1 hour:0 minute:0 
second:0];

For a detailed discussion of these classes and a more complete listing of methods, see the chapter "WebScript Programmer's Quick Reference to Foundation Classes".

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