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CHAPTER 3
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This chapter describes the core Objective-C language and how its syntax differs from Java. Functional differences and more esoteric language features are covered in later chapters. Objective-C enhances C by adding an additional layer of syntax. It does not redefine C, or limit its capabilities, in any meaningful way. Objective-C syntax is easily recognized. If Objective-C had a logo, it would probably be the at sign (@); all Objective-C directives, including string constants, begin with @ (as in @interface, @selector(), @"string"). Other notable traits are the use of square brackets ([ ]) to invoke methods and very descriptive method names. But if you ignore these peculiarities, you won t find any significant discord between the two. Both are object-oriented languages that let you define classes, declare instance variables and methods, instantiate instances of those classes, inherit from subclasses, override methods, invoke methods, pass parameters, and return values. Good Objective-C programming embraces the same design patterns and practices that you are familiar with in Java.
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Objective-C classes are defined in an @interface directive. Its implementation is defined in an @implementation directive. This differs from Java s single class definition, which defines both the class s interface and its implementation, as shown in Listing 3-1.
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Listin g 3-1. Objective-C Class Definition
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Java import com.apress.java.SuperClass; public class NewClass extends SuperClass { int instanceVariable; Object method( ) { return (null); }
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Object method(Object param ) { return (null); }
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CHAPTER 3 WELCOME TO OBJECTIVE-C
} Objective-C #import "SuperClass.h" @interface NewClass : SuperClass { int instanceVariable; } -method; -methodWithParameter:param; @end @implementation NewClass -method { return (nil); } -methodWithParameter:param { return (nil); } @end Listing 3-1 shows the definition of the NewClass class in Java and an equivalent class in Objective-C. The @interface part of a class definition is typically in a header (.h) file for inclusion by other modules, while the @implementation portion is in a source (.m) file. See the #include and #import section of the previous chapter if you need a refresher on C source file organization. The @interface portion of a class declaration has two parts. The first part contains the instance variable declarations surrounded by braces. This portion is similar to a C struct declaration. Following the variable declaration are the class s method prototypes. The hyphen prefix indicates that the method is an instance method. A plus prefix denotes a class method, which is similar to a static method in Java. The @implementation directive contains the actual code for the methods that were described in the @interface section. It is an error to declare a method and not implement it, although the opposite is permitted. One of Java s more elegant design features is that a class file defines both a class s interface and its implementation. In Objective-C, it s the programmer s responsibility to keep the interface and the implementation in agreement. Both the @interface and @implementation sections are terminated with an @end directive. Class inheritance works the same way it does in Java. The class NewClass inherits all of the instance variables and methods of the class SuperClass. Declaring a method in NewClass with the same name as a method inherited from SuperClass overrides the inherited method.
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CHAPTER 3 WELCOME TO OBJECTIVE-C
Caution Always declare the superclass in the @interface declaration, even when the superclass is NSObject.
NSObject is the logical root class in the Cocoa framework, functionally equivalent to Java s Object class. It is not, however, the root class of Objective-C. Objective-C s actual root class, Object, is so primitive that a direct subclass is essentially useless. It is unlikely that you will ever want to create a direct subclass of Object.
Object Pointers
A variable containing a pointer to an Objective-C object is an object pointer (reference) or object identifier. Both are equivalent to a Java object reference. Listing 3-2 shows the two ways of declaring an object pointer. An object pointer can be declared as a pointer to a specific class or as a generic object identifier (id). Both contain a pointer to the memory address of an object. The difference is in how they are treated by the compiler.
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