1>初始化:
stringWithContentsOfFile:
stringWithContentsOfURL:
stringByAppendingString:
initWithString:
stringWithString：
stringWithFormat：
writeToFile:
stringWithContentsOfFile:
2>比较：isEqualToString，compare，NSComparisonResult
BOOL result = [astring01 isEqualToString:astring02]
compare方法(comparer返回的三种值：NSOrderedSame，NSOrderedAscending，NSOrderedDescending)
NSString *astring01 = @"This is a String!"
caseInsensitiveCompare 不区分大小写
[astring01 compare:astring02 options:NSCaseInsensitiveSearch | NSNumericSearch]
//NSCaseInsensitiveSearch:不区分大小写比较， NSLiteralSearch:进行完全比较，区分大小写， NSNumericSearch:比较字符串的字符个数，而不是字符值。
NSComparisonResult result=[self.birthDate compare:another.birthDate];
if(result==NSOrderdSame) return 0;
if(result==NSOrderdAscending) return -1;
else return 1；
3>截取：
rangeOfString:
substringToIndex:3
substringFromIndex:
substringWithRange:
hasPrefix:
hassuffix: