NSDictionary

//官网原文:The

NSDictionary

classdeclarestheprogrammaticinterfacetoobjectsthatmanageimmutableassociationsofkeysandvalues.Usethisclassoritssubclass

NSMutableDictionary

whenyouneedaconvenientandefficientwaytoretrievedataassociatedwithanarbitrarykey.

NSDictionary

createsstaticdictionaries,and

NSMutableDictionary

createsdynamicdictionaries.(Forconvenience,thetermdictionaryreferstoanyinstanceofoneoftheseclasseswithoutspecifyingitsexactclassmembership.)Akey-valuepairwithinadictionaryiscalledanentry.Eachentryconsistsofoneobjectthatrepresentsthekeyandasecondobjectthatisthatkey’svalue.Withinadictionary,thekeysareunique.Thatis,notwokeysinasingledictionaryareequal(asdeterminedby

isEqual:

).Ingeneral,akeycanbeanyobject(providedthatitconformstothe

NSCopying

protocol—seebelow),butnotethatwhenusingkey-valuecodingthekeymustbeastring(seeKey-ValueCodingFundamentals).Neitherakeynoravaluecanbe

nil

;ifyouneedtorepresentanullvalueinadictionary,youshoulduse

NSNull

.翻译:使用NSDictionary用产生的对象是用来管理不变的键值对,使用它或者它的子类NSMutableDictionary可以很方便的使用key来查找到它所对应的value,使用NSDictionary创建静态的(一经创建内容不可变)的字典,而使用NSMutableDictionary创建动态的字典(内容可更改);一个键值对在字典中被称为一个表项,每一个表项都有一个键和一个与该键对应的值,在字典中,所有的键都是唯一的,也就是说在一个字典中不存在两个相同的键,通常,一个键可以是任何OC中得对象,但是注意在使用key-value进行编码时,键必须是一个字符串,在字典中,键和值不能为nil,若需要将value置为nil,应写成NSNUll(mark:因为在对每个字典进行初始化时都是以nil作为结束);