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Good Objective-C follows the same practice for all of the reasons Java does and a few more. In Objective-C, the instance variable int value would be accessible via the methods -(int)value and -(void)setValue:(int)newValue. Objetive-C encourages the use of accessor methods so much, the definition and construction of accessor methods has been built into the language in the form of properties. How to declare properties is described immediately after the section on the isa variable.

Note Objective-C uses the names value and setValue for the methods that get and set the property value,

rather than getValue and setValue which are more common in Java. Technologies that use introspection to identify properties via accessor methods will often accept the getValue form as an alternative, but the value form for getters is preferred.

Every object in Objective-C inherits an isa instance variable; isa literally defines the class of the object. At runtime, there is a single instance of a Class object for every class. The Class object defines the behavior for all instances of that class. Every instance of that class refers to its defining Class object via its isa variable. You should not directly access an object s isa variable. If you want to get an object s Class, the -(Class)class method will return it. You may also find -(NSString*)className useful: it returns the name of the object s class as a string. Several technologies alter an object s isa variable (a technique known as isa swizzling ) to dynamicaly alter that object s behavior. Never change the value of an object s isa variable yourself, until you have a clear and deep understanding of Objective-C s runtime architecture.

Objective-C 2.0 adds the @property and @synthesize keywords for defining object properties and implementing their matching accessor methods. A property is a value that is fetched using an accessor method and set using a mutator method. A property s value is typically stored in an instance variable, but that s not a requirement. The @property directive declares a property of the class. A @property directive typically appears in the @interface directive. It does not implement the accessor methods or create any instance variables. It is simply a promise that the class implements that property. Listing 3-9 shows the Person class, in both Java and Objective-C. The class defines five properties: tag, firstName, lastName, fullName, and adult.

Java public class Person { int tag; String firstName; String secondName; boolean adult;

synchronized int getTag( ) { return (tag); } synchronized void setTag( int tag ) { this.tag = tag; } public synchronized String getFirstName() { return firstName; } public synchronized void setFirstName( String firstName ) { this.firstName = firstName; } public synchronized String getLastName() { return secondName; } public synchronized void setLastName( String lastName ) { secondName = lastName; } public synchronized boolean isAdult() { return adult; } public synchronized void setAdult( boolean adult ) { this.adult = adult; } public String getFullName( ) { return (firstName+" "+secondName); } }

Objective-C @interface Person : NSObject { int tag; NSString *firstName; NSString *secondName; BOOL adult; } @property int tag; @property (copy) NSString *firstName; @property (copy) NSString *lastName; @property (getter=isAdult) BOOL adult; @property (readonly,nonatomic) NSString *fullName; @end @implementation Person @synthesize @synthesize @synthesize @synthesize tag; firstName; lastName = secondName; adult;

- (NSString *)fullName { return ([NSString stringWithFormat:@"%@ %@",firstName,secondName]); } @end The @synthesize directive appears in the @implementation of the class. A @synthesize directive tells the compiler to generate the accessor and mutator methods needed to implement the property. In the absense of a @synthesize directive, it is up to the programmer to implement the methods that sastify the contract declared in the @property directive. Failing to either include a @synthesize directive or implement the expected accessor methods is an error.

Note The special @dynamic directive suppresses the compiler s expectation for the required accessor methods.

In some cases (i.e., class extensions, categories, subclassing, dynamically-loaded frameworks), it s possible that the class will implement methods at runtime that the compiler is not aware of. The @dynamic directive tells the compiler not to worry about it; the programmer promises that the expected methods will exist when the program executes. Use of @dynamic is rare.

The first two properties are straightforward. The @property int tag declares that the class Person implements getter and setter methods that satisfy the contract of an integer property named tag. The @synthesize tag directive instructs the compiler to generate the methods -(int)tag and -(void)setTag:(int)newTag for you. The tag property will be stored in the instance variables of the same name.