Objective-C for Java Developers, Lesson 1

Objective-C for Java
Developers, Lesson 1
Raffi Khatchadourian
For Quinnipiac University
Monday, March 3, 2014
Inspired by:
Learn Objective-C for Java Developers by James Bacanek,
Apress 2009

Expected
Background
Familiarity with:
Object-Oriented Programming
concepts.
How they are manifested in Java.

Why is this topic
important?
Objective-C is the language typically
used to write native software for
Mac.

Why is this topic
important?
Mac.
No cross-
platform
interpretation

Why is this topic
important?
Mac.
No cross-
platform
interpretation
Mac UI look-
and-feel

Why is this topic
important?
Mac.
No cross-
platform
interpretation
Mac UI look-
and-feel
Non-native
languages?

Why is this topic
important?
Objective-C is the language
used to write
Mac.
Objective-C is the language used to
write native software for iOS.

Why is this topic
important?
Objective-C is the language
used to write
Mac.
Objective-C is the language used to
write native software for iOS.
Not
browser based

Overview
Objective-C basics.
How Objective-C is:
Similar to Java.
Different from Java.
Not directly about iOS development.
But! Topics can be applied to iOS
development.

Objective-C
High-level, Object-Oriented language.

Objective-C
Roots in SmallTalk.

Objective-C
Roots in SmallTalk.
Strict superset of C with a lot of “twitter” (@
symbols).

Objective-C
Roots in SmallTalk.
symbols).
Strongly-typed with dynamic features!

Objective-C
Roots in SmallTalk.
symbols).
Strongly-typed
Requires a compiler (llvm).

Objective-C
Roots in SmallTalk.
symbols).
Strongly-typed
Requires a compiler (llvm).
Language is multi-platform but not all libraries are
(Cocoa/Cocoa Touch).

Main Differences
with Java
Fast.

Main Differences
with Java
Fast.
Dynamic.

Main Differences
with Java
Fast.
Dynamic.
Syntax.

Main Differences
with Java
Fast.
Dynamic.
Syntax.
Memory management and object
ownership.

Brief History
Began as a way to add Object-Oriented
Programming to C [Bacanek09].
Driven by Brad Cox and Tim Love in 1986.
Adopted by NeXT and used in the NEXTSTEP OS.
Faced fierce competition with C++.
Apple purchased NeXT in 1996, released Mac OS
X.

Student Class Declaration
// Student.h
#import <Foundation/Foundation.h>
@interface Student : NSObject
@property (copy) NSString *firstName;
@property (copy) NSString *lastName;
@property (readonly, nonatomic) NSString *fullName;
@property int age;
@property (getter = isEnrolled) BOOL enrolled;
@end

Header Files
// Student.h
Objective-C classes are split into two different files.

Header Files
// Student.h
A header file (.h).

Header Files
// Student.h
A header file (.h).
Contains the class declaration.

Header Files
// Student.h
A header file (.h).
An implementation file (.m).

Header Files
// Student.h
A header file (.h).
Contains the class definition.

Header Files
// Student.h
A header file (.h).
Clients (other programmers) receive the .h file and the
object file but not the .m file.

Header Files
// Student.h
A header file (.h).
Clients (other programmers) receive the .h file and the
object file but not the .m file.
Information
hiding,
abstraction

#import statements
// Student.h

#import statements
// Student.h
#import is similar to Java import
statements.

#import statements
// Student.h
statements.
Must by accessible to the compiler.

#import statements
// Student.h
statements.
Processed by the pre-compiler.

#import statements
// Student.h
statements.
Processed by the pre-compiler.
Must specify framework/library header file.

Class Declarations
// Student.h
@end

Class Declarations
// Student.h
@end
@interface specifies the:

Class Declarations
// Student.h
@end
Class name.
Class name

Class Declarations
// Student.h
@end
Class name.
Information about the class (e.g., subclass, implemented
protocols).
Similar to
java.lang.Object

Class Declarations
// Student.h
@end
Class name.
protocols).
No
multiple
inheritance

Class Declarations
// Student.h
@end
Class name.
protocols).
The interface of the class (instance variables, methods,
and properties)

Class Declarations
// Student.h
@end
Class name.
protocols).
The interface of the class (instance variables, methods,
and properties)
Delimited by @end.

Class Declarations
// Student.h
@end
No packages (namespaces) in
Objective-C.
No
packages

Class Declarations
// Student.h
@end
No packages (namespaces) in
Objective-C.
Use prefixes in class names (e.g.,
FBStudent)
No
packages

Class Properties
// Student.h
@end

Class Properties
// Student.h
@end
A way for clients to indirectly access instance variables.

Class Properties
// Student.h
@end
Can be read/write and read-only.

Class Properties
// Student.h
@end
Compiler synthesizes accessor and mutator methods based on
property attributes.

Class Properties
// Student.h
@end
“Provide
clients with a copy of
firstName”

Class Properties
// Student.h
@end
“Provide
firstName”
Could
also use strong or
weak

Class Properties
// Student.h
@end
You can create your own in the implementation file.
“Provide
firstName”
Could
also use strong or
weak

Class Properties
// Student.h
@end
You can create your own in the implementation file.
NSString * is a reference to a string object (String firstName).
“Provide
firstName”
Could
also use strong or
weak

Objective-C Memory
Management
No garbage collector.

Objective-C Memory
Management
No garbage collector. Why?

Objective-C Memory
Management
Automated Reference Counting.

Objective-C Memory
Management
Automated Reference Counting.
Each object is associated with a
count of (other) objects that refer to
them.

Objective-C Memory
Management
Two types of references:
Weak:
“I need to refer to this object but I don’t
own it.”
Strong:
“I am an owner of this object.”

Objective-C Memory
Management
Two types of references:
Weak:
“I need to refer to this object but I don’t
own it.”
Strong:
“I am an owner of this object.”
Objects are deallocated when no strong references
to them remain.

Objective-C Memory
Management
An object with only weak references
may be deallocated.

Objective-C Memory
Management
An object
may be deallocated.
The reference is set to nil (null).

Objective-C Memory
Management
An object
may be deallocated.
The reference is set to nil (null).
What
happens when a
method is called on
a null reference
in Java?

ARC = Garbage
Collection?
Not exactly.

ARC = Garbage
Collection?
Not exactly.
Need to:

ARC = Garbage
Collection?
Not exactly.
Need to:
Specify ownership.

ARC = Garbage
Collection?
Not exactly.
Need to:
Specify ownership.
Beware of “retain cycles.”

“Retain Cycles”
o1 o2
strong

“Retain Cycles”
o1 o2
strong
strong

“Retain Cycles”
o1 o2
strong
strong
Objects are deallocated when no strong
references to them remain.

“Retain Cycles”
o1 o2
strong
strong
Leak!
Objects are deallocated when no strong
references to them remain.

“Retain Cycles”
o1
o2
strong
strong
o3

“Retain Cycles”
o1
o2
strong
strong
o3
strong

Derived Properties
// Student.h
@end

Derived Properties
// Student.h
@end
readonly so that we will derive this
property from other properties.

Derived Properties
// Student.h
@end
readonly
atomic is similar to synchronized in
Java.

Derived Properties
// Student.h
@end
atomic by
default
readonly
atomic is similar to synchronized in
Java.

Integer Property
// Student.h
@property int age;
@end

Boolean Property
// Student.h
@property int age;
@end

Boolean Property
// Student.h
@property int age;
@end
Changes
the name of the
accessor

Student Class Definition
// Student.m
#import "Student.h"
@implementation Student
- (NSString *)fullName {
return [NSString stringWithFormat:@"%@ %@",
[self firstName],
[self lastName]];
}
@end

// Student.m
Implementation files have an
extension of .m

// Student.m
#import "Student.h"
#import the header file.

// Student.m
#import "Student.h"
@end
Class definition body

// Student.m
#import "Student.h"
}
@end

// Student.m
#import "Student.h"
}
@end
Method definition for derived property.

// Student.m
#import "Student.h"
}
@end
Read-only, no mutator method.

// Student.m
#import "Student.h"
}
@end
Instance method

// Student.m
#import "Student.h"
}
@end
Instance method
Method name

// Student.m
#import "Student.h"
}
@end
Instance method
Return
type
Method name

// Student.m
#import "Student.h"
[self firstName],
[self lastName]];
}
@end
Method invocation between []

// Student.m
#import "Student.h"
[self firstName],
[self lastName]];
}
@end
Format string is similar to C and Java OutputStream.format()

// Student.m
#import "Student.h"
[self firstName],
[self lastName]];
}
@end
@ means object reference

// Student.m
#import "Student.h"
[self firstName],
[self lastName]];
}
@end
Class
method

// Student.m
#import "Student.h"
[self firstName],
[self lastName]];
}
@end
Argument list

// Student.m
#import "Student.h"
[self firstName],
[self lastName]];
}
@end
Method
name

// Student.m
#import "Student.h"
[self firstName],
[self lastName]];
}
@end
Like Java
this

// Student.m
#import "Student.h"
[self firstName],
[self lastName]];
}
@end
Property

Conclusion
Both Objective-C and Java are high-level, compiled,
Object-Oriented languages.
Objective-C is not cross-platform but can be faster
than Java.
Java is very structured, while Objective-C can be
either structured or dynamic.
Objective-C is a strict superset of C.
Objective-C uses automatic reference counting
(ARC) instead of garbage collection.

Objective-C for Java Developers, Lesson 1

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