.equals 与其他布尔函数

Question

In a small segment of my program I'm trying to validate if someone enter M,m,F,f and for the most part I got it, but something seems wrong when I put the statements together. For example,

   String _gender = "M";
   if(!"m".equals(_gender.toLowerCase())||!"f".equals(_gender.toLowerCase())){
      System.out.println("try again");
   }else 
      System.out.println("success");

It tells me to try again, but when I put it as:

  String _gender = "M";
   if(!"m".equals(_gender.toLowerCase())){
      System.out.println("try again");
   }else 
      System.out.println("success");

It gives me a success. I don't know if its something with how I'm using the or, but thats what I've guessed. I also think it may be something to do with the fact that the check is checking both or something. Idk I've been at this for a while and its been confusing me.

Answer 1

There is logical mistake. It should go into "try again" when both equal() results are false.

String _gender = "M";
if(!"m".equals(_gender.toLowerCase()) && !"f".equals(_gender.toLowerCase())){
  System.out.println("try again");
}else 
  System.out.println("success");

Answer 2

Let's evaluate your code:

String _gender = "M";
if(!"m".equals(_gender.toLowerCase())||!"f".equals(_gender.toLowerCase()))

if(!true || !false) => if(false || true) => true

Now you understand the reason why you got the output, try again .

Now, let's fix it:

Do it as

String _gender = "M";
if (!("m".equals(_gender.toLowerCase()) || "f".equals(_gender.toLowerCase()))) 

which evaluates to if (!(true || "f".equals(_gender.toLowerCase()))) => (!(true)) => false

which will give you the intended output, success