IOS新手问个字符串判断为空的问题

Question

NSLog(@"echo>>>>>>%@",echo);
 NSLog(@"echo>>>>>>%d",echo==nil);
 NSLog(@"echo>>>>>>%d",[echo length]);
    
016-06-14 15:13:29.739 OA_App[4990:1558049] echo>>>>>><null>

2016-06-14 15:13:29.740 OA_App[4990:1558049] echo>>>>>>0 //这里不应该是1 true吗？

2016-06-14 15:13:29.740 OA_App[4990:1558049] -[NSNull length]: unrecognized selector sent to instance 0x34c15850

Answer 1

NSNull is not equal to nil

Answer 2

It is not nil, it is an instance of NSNull. If you want to judge, it should be enough to judge whether it is NSNull class.

Answer 3

Directly determine the length of the string, if it is equal to 0, it is empty