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Objective-C provides the NSNull class as an object placeholder for nil values. This provides an object that can be stored in collections, archived (serialized), or otherwise used to represent nothing where nil is unacceptable. The method +[NSNull nil] returns the singleton instance of NSNull created by the Objective-C runtime. The single NSNull object is immutable and immortal. Listing 8-4 demonstrates a simple technique for writing a method that accepts an object, an instance of NSNull, or nil treating the last two equally.
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Listin g 8-4. Method That Accepts an Object, nil, or NSNull
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- (void)doSomethingWithObject:(id)object { if (object==[NSNull null]) object = nil; }
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Strings are the odd duck in both Java and Objective-C. They are so fundamental to programming that both languages include special syntax for declaring string literals. Yet beyond declaring string literals, the languages provide little direct support for strings, expecting the programmer to manipulate them as objects. The exception to this is Java s string concatenation operator (+), which Objective-C does not include. This section covers string literals, string comparison, string manipulation, converting strings to and from scalar values, and complex formatting. At many levels, strings in Objective-C follow the familiar patterns they do in Java. All Objective-C strings are objects of class NSString. NSString characters are represented internally using Unicode. NSString objects are immutable. Literal Objective-C string objects are written using the @"string" directive. Table 8-4 lists common string operations and their Objective-C counterparts. Table 8-4. Common String Classes and Methods
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"string" java.lang.String java.lang.StringBuffer java.lang.StringBuilder Object.toString() new String(byte[],String)
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@"string" NSString NSMutableString (although not thread safe) NSMutableString -[NSObject description] +[NSString stringWithCString:(const char*)cString encoding:(NSStringEncoding)enc]
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String.length() String.charAt(int) String.equals(String) String.compareTo(String) String.compareToIgnoreCase(String) String.concat(String) String.substring(int) String.substring(int,int) String.toLowerCase() String.toUpperCase() String.trim()
-[NSString length] -[NSString characterAtIndex:(int)index] -[NSString isEqualToString:(NSString*)string] -[NSString compare:(NSString*)string] -[NSString caseInsensitiveCompare: (NSString*)string] -[NSString stringByAppendingString:(NSString*)string] -[NSString substringFromIndex:(int)index] -[NSString substringWithRange:(NSRange)range] -[NSString lowercaseString] -[NSString uppercaseString] -[NSString stringByTrimmingCharactersInSet: (NSCharacterSet*)set] +[NSString stringWithFormat:(NSString*)format, ]
String.format(String,Object )
You will find many other analogous methods in String and NSString. NSString tends to prefer more generic methods that handle a wide variety of cases, where the Java classes implement many simplistic methods. A good example is java.lang.String.trim(). The Objective-C equivalent is -stringByTrimmingCharactersInSet:, which takes an NSCharacterSet object as a parameter. To accomplish exactly what String.trim() does, use [string stringByTrimmingCharactersInSet: [NSCharacterSet characterSetWithRange:NSMakeRange(0,0x20+1)]]. While more verbose, it has the advantage of being able to trim any arbitrary set of characters from a string. If you do this a lot, use a Category to add your own -trim method to NSString, as explained in 5.
Note Modern Objective-C development tools coalesce identical literal strings when your application is built. No
matter how many times the string literal @"Welcome" appears in your application, every occurrence will refer to a single instance at runtime.
There are, however, several key differences between String and NSString that you need to be aware of: There are subclasses of NSString. A string object might be mutable.
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There are two kinds of strings: Objective-C strings and C strings. The additional operator (+) will not concatenate two strings. Objective-C string literals do follow the C language s compile-time string concatenation syntax. The literal string expressions @"Hello" @", World!" and @"Hello, World!" are identical. NSString follows a consistent pattern in the Cocoa class framework; when a class has immutable and mutable variants, the mutable class is the subclass of the immutable class. This has three important consequences. First, the class for mutable string objects is the NSMutableString subclass. Use NSMutableString where you would use java.lang.StringBuilder. The second consequence is that any NSString pointer or parameter could potentially contain a reference to a mutable subclass. In Java, the java.lang.String class is final and cannot be subclassed, guaranteeing that all String objects are immutable. To dynamically create a string requires first constructing a StringBuffer object, then converting that into a String object using toString(). In Objective-C, you can use NSMutableString to assemble a string then simply return it or use it as a regular NSString object. If you do not modify it thereafter, it s indistinguishable from an immutable string. In the situation where you must guarantee that a string object is immutable, construct a copy of the suspicious string object using [NSString stringWithString:possiblyMutableString]. Also, see 12 for information about copying objects. The last consequence is more of a benefit. Since NSMutableString is a subclass of NSString, it inherits all of the methods of NSString. This includes methods added to NSString by categories.
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