Java：使用if条件检查char变量的内容

Question

I have a char variable that is supposed to contain either a Y,y,n or N character, I want to test if it does not contain it, then display an error message and exit the program.

This is the code I am using;

    if (userDecision != 'Y' || userDecision != 'y' || userDecision != 'n' || userDecision != 'N')
        {
            System.out.println("Error: invalid input entered for the interstate question");
            System.exit(0);
        }

Irregardless of what is in the variable it always returns true and executes the command to exit the program, what am I doing wrong?

Answer 1

|| means logical or. You want && instead.

if (userDecision != 'Y' && userDecision != 'y' ...

---

a || b a || b returns true if either a or b is true. Suppose the userDecision is 'Y' . Then

• userDecision != 'Y' is false
• userDecision != 'y' is true
• userDecision != 'N' is true
• userDecision != 'n' is true

So together the condition is true and the if branch is executed.

OTOH, a && b returns true if both a and b are true, which is what you really need.

Answer 2

Read the first part of the condition aloud: Is the choice different from Y or y ? The problem is that any character is different either from Y or y . You've picked the wrong logical operator – if you want to be sure that user picked something else than those characters in the condition, you have to pick && , logical and: Is the character different from Y and also different from y and etc.

Answer 3

Change yoiur ORs to ANDs

Or you could use

(!(userDecision == 'Y' || userDecision == 'y' || userDecision == 'n' || userDecision == 'N'))

This

!(A OR B) 

is equivelant to

!A AND !B

Have a look at Boolean algebra

De Morgans theorem

NOT (P OR Q) = (NOT P) AND (NOT Q) 
NOT (P AND Q) = (NOT P) OR (NOT Q) 

DeMorgan's Theorem

Answer 4

Your condition is "if this is not a or not b" this means it will always be true even if it is a or b. What you want to test for is "if this is not (a or b)" so:

if (! (userDecision == 'Y' || userDecision == 'y' || userDecision == 'n' || userDecision == 'N')) {
    System.out.println("Error: invalid input entered for the interstate question");
    System.exit(0);
}

If your code contains conditions like this that become long as many alternative chars must be tested for, you can use the switch construct, which makes this case easier to follow:

switch (userDecision) {
case 'y': /*fallthrough*/
case 'Y':
    // accepted
    break;
case 'n': /*fallthrough*/
case 'N':
    // rejected
    break;

default:
    System.out.println("Error: invalid input entered for the interstate question");
    System.exit(0);
}

Answer 5

You need to use && instead of || . You are asking whether "none" of those characters match, not simply whether any one of the four fail to match. (A value cannot simultaneously be Y, y, N, and n.)