NSMutableDictionary 

        - (void)setObject:(id) for :(NSstring *)key         當id 為 nil，會出現 crash。         但是，當用setValue 則沒有事情。目前 Dic 用setValue 代替 setObject 構建MutableDictionary，沒有出現什麼大問題   例子：   NSMutableDictionary  *dic1 = [NSMutableDictionary dictionaryWithCapacity:1];       [dic1 setObject:@"1" forKey:@"1"];       [dic1 setObject:@"2" forKey:@"2"];       [dic1 setObject:@"" forKey:@"3"];       [dic1 setObject:@"4" forKey:@"4"];            //[dic setObject:nil forKey:@"41"]; wrong            [dic1 setValue:nil forKey:@"5"];            [dic1 setValue:@"6" forKey:@"6"];            [dic1 setValue:[NSDate date] forKey:@"date"];       [dic1 setValue:[NSString string] forKey:@"nil string"];            [dic1 setValue:[NSDictionary dictionaryWithObject:[[UIView alloc] init] forKey:@"View"] forKey:@"復雜非String"];           2013-03-15 14:36:42.089 GuessMovies[5924:c07] {       1 = 1;       2 = 2;       3 = "";       4 = 4;       6 = 6;       date = "2013-03-15 06:35:56 +0000";       "nil string" = "";       "/U590d/U6742/U975eString" =     {           View = "<UIView: 0xa421a10; frame = (0 0; 0 0); layer = <CALayer: 0xa24be80>>";       };   }