Objective-c for Java Developers

Mohammad [email protected]

SiliconExpert technologies

OBJECTIVE-C FOR JAVA DEVELOPERS

Key Points

• What is Objective-c?

• Syntaxo Type systemo Classeso Message Passingo Protocolso Dynamic Typingo Categories

• Memory Management• Objective-c 2.0 features

What is Objective-c?

• Compiled, Object-Oriented and more dynamic than Java.

• Objective-C was created in the early 1980s.

• Type system: static, dynamic, weak (Java: static, strong)

• Objective-c = C + Smalltalko Superset of C Programming

language. (so, it is better to have a background knowledge with C first)

o Adds Smalltalk-style messaging to the C.

What is Objective-c? (cont.)

• Since Objective-c is a superset of C and Java is a c-like language, so the syntax of most of the languages are the same (for,while,if,switch,brackets)

• The Primary language for Cocoa API. (used in Mac OS X)

• Major Implementations: GCC and Clang• Has a very rich Class APIs with two large

APIs (Foundation and AppKit)

What is Objective-c? (cont.)

• Objective-c uses different runtime model than java.o Java uses, VM and Class Loader.

.java => .class (bytecode) => ClassLoader load .class

o Objective-c uses traditional C Linker model. .m => .o (object file) => linker produces

executable file. (also supports dynamic class loading using NSBundle class)

Syntax: Type System

• Objective-c is a statically typed language (except for id type) with the variable types declared at compile-time.int x; float y; short s = 3;

• have the same types as in C (machine depended, java have platform independent types:bool, int, long int, short int, long long int, float, double and long double.

• Has 3 levels of variables, local, instance and global.

• using global variables without caution may cause linking errors.

Syntax: Type System (cont.)• Follow C in which a local variable may be

used without initialization (compile error in Java)

• instance variables are initialized to nil (Objective-c null keyword)

• static variables are not as in Java.

• instance variables may use @public, @protected (default) or @private access modifiers.

• Has no namespace concepts (Java has), and all classes are public.

• String literals starts with @ ( @"BAV99")

Syntax: Classes

• Objective-c is like java in that it is a class-based Object-Oriented Programming language.

• Class declared in two files; interface (header) file (.h) and implementation file (.m).

• In java, we declare the class in one .class file.

• A common convention is to name the header file after the name of the class. (in java it should)

Syntax: Classes (cont.)

Class definition in java:// Component.javapublic class Component{ int comId; String comNumber; public Component(){/* implementation goes here*/} public void updateCompNumber(String newComNumber) {/* implementation goes here*/} public void save(){/* implementation goes here*/} public static Component addComponent (Component firstComp, Component secondComp){/*

implementation goes here*/}}

Syntax: Classes (cont.)Class definition in Objective-c:// Component.h@interface Component : NSObject{ int comId; NSString* comNumber;}-(id) init;-(void) updateCompNumber:(NSString*) newComNumber;-(void) save;+(Component*) addComponent:(Component*) firstComp

to:(Component*) secondComp;@end

Syntax: Classes (cont.)// Component.m#import "Component.h"@implementation Component-(id) init{ /* implementation goes here*/ return self; }-(void) updateCompNumber:(NSString*) newComNumber{ /* implementation goes here*/}-(void) save { /* implementation goes here*/ }+(Component*) addComponent:(Component*) firstComp

to:(Component*) secondComp{ /* implementation goes here*/ }@end

Syntax: Classes (cont.)

• In java each class is implicitly a child of java.lang.Object, in Objective-c you have to tell yourself what is the parent class.

• NSObject is the parent of all classes in the class library

• use NSObject if no other parent is needed.

• Objective-c Has the concept of struct (inherited from c)

Syntax: Classes (cont.)

• Object instantiation in Objective-c is a two steps process. in java it done in one step:

//Java

Component c = new Component();

// Objective-c

Component* c = [[Component alloc] init];

• No special operator for instantiation (new operator)

• Both create Objects on Heap (vs stack objects)

• Both follow the same pattern, first allocate memory for the new object then call the initialization method (init methods in Objective-c; Constructors in java)

• Both have GC (but iOS runtime doesn't provide GC).

Syntax: Classes (cont.)

• init implementation:

- (id)init { self = [super init]; if (self) { // perform initialization of object here } return self;}

Syntax: Message Passing

• Objective-c Uses smalltalk style messaging.

• In Objective-C one does not simply call a method; one sends a message.

• In simula-style messaging languages, The method name is in most cases bound to a section of code in the target class by the compiler.

• In Smalltalk (and Objective-C), the target of a message is resolved at runtime, with the receiving object itself interpreting the message.

Syntax: Message Passing (cont.)

• The object to which the message receiver is not guaranteed to respond to a message, and if it does not, it simply raises an exception.

• A method is identified by a selector or SEL and resolved to a C method pointer implementing it (an IMP).[obj method:argument];Transformed at runtime to:method(obj, argument);

Syntax: Message Passing (cont.)

• If messages are sent to nil (the null object pointer), they will be silently ignored or raise a generic exception, depending on compiler options. (default behaviour is to ignore)

• Multi-parameter method take the form:

-(type) metho:(type1) param1 dName:(type2)param2

-(void) add:(int) x to:(int)y So "add:to:" is the SEL name and called as:

[MyObject add:10 to:30]

Syntax: Protocols

• The same concept as Interface in Java.

• Types of Protocols:o Informal Protocolso Formal Protocols

• Formal Protocols is the same as

interfaces in java, example:@protocol Locking

-(void)lock; -(void)unlock;

@end@interface SomeClass : SomeSuperClass <Locking>@end

Syntax: Protocols (cont.)

• From Objective-c 2.0, Formal Protocols can contains optional (@optional) methods.

• Informal Protocols used extensively in Cocoa API.

• Common usage is to implement callbacks.

Syntax: Protocols (cont.)

Example on informat Protocols:

• Suppose we have a library that used to download a file from a URL:@interface DownloadHelper

-(byte[]) download:(NSURL*) url target:(id)target;@end

• This library documents that, the target object should supply a function with the following signature to be called when download complete:-(void)downloadComplete;

• The Library come with a Category on NSObject with a default implementation for this method.

Syntax: Dynamic Typing

• Objective-c is a statically typed language with some dynamic typing support.

• An object can be sent a message that is not specified in its interface.

• Dynamic typing on variables level is achieved using the id type.id anObject = [SomeClass someMethod];

[anObject doSomeMethod]; // no compile-time check if this method belongs to this type

• Example:- (void)setMyValue:(id)foo;- (void)setMyValue:(id<aProtocol>)foo;- (void)setMyValue:(NSNumber *)foo;- (void)setMyValue:(NSNumber<aProtocol> *)foo;

Syntax: Dynamic Typing (cont.)

• Dynamic Typing is used to compensate the missing of Generics in Objective-c. (Java has Generics)Java (No Generics): ArrayList arr = new ArrayList();arr.add(new Employee());Employee e = (Employee) arr.get(0); // should do cast which breaks concept of static typingObjective-c:NSMutableArray arr = [NSMutableArray array];Employee* e1 = [[[Employee alloc]init]autorelease];[arr addObject:e1];Employee* e2 = [arr objectAtIndex:0]; // no cast needed

• Containers in Objective-c uses id type for dynamic typing.

Syntax: Categories• Similar to partial classes in C# but more

powerful.

• Add functionality to existing classes without editing the source file.

• extend classes at runtime (Java only supports compile-time inheritance)

• Used extensively by Objective-c.

• Example: extending the String class to allow get nanPartNumber from comPartNum // NSString+NanPartNum.h#import <Foundation/NSString.h>@interface NSString (NanPartNum)-(NSString*) nanPartNum;@end

Syntax: Categories (cont.)// NSString+NanPartNum.m

@implementation NSString (NanPartNum)

-(NSString*) nanPartNum{

return [self removeNonAlpha];

}

// other methods like removeNonAlpha

@end

// main.m

#import <Foundation/NSString>

#import "NSString+NanPartNum.h"

int main(void){

NSString* comPartNumber = @"BAV99-70";

NSString nanPartNum = [comPartNumber nanPartNum]; // method resolved at runtime

NSLog(@"%@", nanPartNum); /* prints BAV9970*/ }

Syntax: Categories (cont.)

• A category has full access to all of the instance variables within the class, including private variables.

• Overrides methods in the class with the same signature (can used to fix bugs in existing classes by rewriting its methods)

• Used in Informal Protocols.

• Cannot add variables in categories, just methods.

• Other languages uses Prototype-oriented solutions to add functionality at runtime (e.g.: Javascript)

Memory Management• Objective-c 2.0 has a Garbage Collector

but it is not available in iOS (when writing mobile apps)

• Manual memory management in Objective-c is much easier from C and C++.

• Memory Management Rules:o If you create an object using the manual

alloc style, you need to release the object later.

o You should not manually release an autoreleased object.NSString* string2 = [[NSString alloc] init];[string2 release];

Memory Management (cont.)

o You should provide a dealloc method for each object you create.- (void) dealloc{

[instanceVar1 release];

[super dealloc];

}

o Objective-C memory management system is called reference counting

o Golden Rule: Simply, you alloc an object, maybe retain it at some point, then send one release for each alloc/retain you sent. So if you used alloc once and then retain once, you need to release twice.

Memory Management (cont.)

o You create an object as: instance variable function local variable

o For Instance variables, make sure it is deallocated in the dealloc method (shown before) and when setting it just autorelease the old value and retain the new one.-(void) setPartNumber:(NSString*) partNum{

[self->partNum autorelease];

self->partNum = [partNum retain];

}

alloc (+1)retain (+1)

release (-1)

release (-1)

1 2 1 0

Memory Management (cont.)

o For local variables, whenever you create an object using alloc, release it using release or autorelease, that's all.

o Always remember, you are not the owner of objects you donot create!

Objective-c 2.0 features

• Properties

• Fast Enumerations

Questions?

Resources

• http://en.wikipedia.org/wiki/Objective-C

• http://www.gnustep.org/

• http://en.wikipedia.org/wiki/Java_(programming_language)

• http://stackoverflow.com/questions/2010058

• http://cocoadevcentral.com/d/learn_objectivec/