Java做while循环不断重复，我不知道为什么

Question

So I've attached a debugger, and tried different inputs and I can't seem to figure out why this won't get past the loop. When ran I enter "l" or "L", then entry gets set to that, then input is set to the capitalized version and then it repeats.

public static char displayMenu(){
    char input;
    sc.nextLine();//clear junk
    do {
        System.out.println();
        System.out.println("\t\t Enter L to (L)oad ");
        String entry = sc.nextLine();
        input = entry.toUpperCase().charAt(0);
    } while (input != 'L' || input!='M' || input != 'P' || input != 'Q');

Answer 1

input will only have one value. That value cannot be both L and M . You need to change the termination condition.

Answer 2

Your boolean || is incorrect. If a value is L it is then not M , P or Q so your loop will continue to iterate. I think you wanted something like,

while (input != 'L' && input != 'M' && input != 'P' && input != 'Q');

or

while (!(input == 'L' || input == 'M' || input == 'P' || input == 'Q'));

consider when input is L , clearly L is not M and so your initial while condition would continue to iterate.

Answer 3

您已经使用了逻辑或条件，即使您输入'L'，它也只需要运行一个真实的语句，这时您的一个语句为假，而其他语句变为真，因此它会重复。

Answer 4

Like

public static void main(String[] args) {
    char input;
    Scanner sc = new Scanner(System.in);
    input = sc.nextLine().charAt(0);//clear junk
    do {
        System.out.println();
        System.out.println("\t\t Enter L to (L)oad ");
        String entry = sc.nextLine();
        input = entry.toUpperCase().charAt(0);
    } while (input != 'L' && input!='M' && input != 'P' && input != 'Q');
}

Answer 5

public static char displayMenu(){
    char input;
    sc.nextLine();//clear junk
    do {
        System.out.println();
        System.out.println("\t\t Enter L to (L)oad ");
        String entry = sc.nextLine();
        input = entry.toUpperCase().charAt(0);
    } while ((input != 'L') && (input!='M') && (input != 'P') && (input != 'Q'));

try this

Answer 6

De Morgan's laws tell us that

(input != 'L' || input!='M' || input != 'P' || input != 'Q')

is the same as

! (input == 'L' && input=='M' && input == 'P' && input == 'Q')

which must always be true because

(input == 'L' && input=='M' && input == 'P' && input == 'Q')

must always be false because

input can only be equal to one thing at a time.

Use De Morgan's laws to avoid extra NOT's when you can. Computers don't care but humans don't process NOT's very well.