Foundation_NSStringAndNSMutableString

常用方法：

－length

－stringByAppendingString

－isEqualToString

－uppercaseString

－lowercaseString

+stringWithString

    NSString * string = @"This is string";    NSLog(@"%@",[string substringFromIndex:1]);    NSLog(@"%@",[string substringToIndex:2]);    NSLog(@"%@",[string substringWithRange:NSMakeRange(3, 7)]);            NSMutableString * muString = [NSMutableString stringWithString:@"This is mutableString "];    [muString appendString:@"world "];    NSLog(@"1--%@",muString);    [muString insertString:@"hello " atIndex:1];    [muString deleteCharactersInRange:NSMakeRange(0, 1)];    [muString replaceCharactersInRange:NSMakeRange(2, 4) withString:@"haha "];    NSLog(@"2--%@",muString);