对用户输入的字符串使用 do while 循环

Question

Am currently working on a question for my Java class, I'm doing do-while loops and I'm having trouble when my while condition has to do with a user inputted String. The code compiles but no matter what I enter it fails the while condition and does the loop again. This even occurred when I hardcoded the value of unit.

I tried looking up the solution online but every example I can find uses a user inputted int value instead of a String

public class unit {
   public static void main (String[] args) {
      String prompt = "Please enter your preferred unit of mass (kg, lb, g or oz): ";
      System.out.println(preferredUnit(prompt));
   }
   
   public static String preferredUnit(String prompt) {
      Scanner sc = new Scanner(System.in);
      String unit = "";
            
      do{
         if (!unit.equals("")) {
            System.out.println("Sorry but " + unit + " is not a valid unit type");
         }
         System.out.println(prompt);
         unit = sc.nextLine();
      
      }  while(!unit.equals("kg") || !unit.equals("lb") || !unit.equals("g") || !unit.equals("oz") );
      
      return "Unit of mass: " + unit; 
   }
} 

Answer 1

!=A || !=B

is not what you're looking for. If I enter A, it's not B, so it fails. If I enter B, it's not A, so it fails. Instead you're looking for !=A && !=B

Eg

while(!unit.equals("kg") || !unit.equals("lb") || !unit.equals("g") || !unit.equals("oz"))

Should be

while(!unit.equals("kg") && !unit.equals("lb") && !unit.equals("g") && !unit.equals("oz"))

Answer 2

Two possible Fixed of your problem.

One Way:

 public static void main (String[] args) {
      String prompt = "Please enter your preferred unit of mass (kg, lb, g or oz): ";
      System.out.println(preferredUnit(prompt));
   }
   
   public static String preferredUnit(String prompt) {
      Scanner sc = new Scanner(System.in);
      String unit = "";
            
      do{
         if (!unit.equals("")) {
            System.out.println("Sorry but " + unit + " is not a valid unit type");
         }
         System.out.println(prompt);
         unit = sc.nextLine();
      
      }  while(!unit.equals("kg") && !unit.equals("lb") && !unit.equals("g") && !unit.equals("oz") );
      
      return "Unit of mass: " + unit; 
   }

Second way:

public static void main (String[] args) {
      String prompt = "Please enter your preferred unit of mass (kg, lb, g or oz): ";
      System.out.println(preferredUnit(prompt));
   }
   
   public static String preferredUnit(String prompt) {
      Scanner sc = new Scanner(System.in);
      String unit = "";
            
      do{
         if (!unit.equals("")) {
            System.out.println("Sorry but " + unit + " is not a valid unit type");
         }
         System.out.println(prompt);
         unit = sc.nextLine();
      
      }  while(!(unit.equals("kg") || unit.equals("lb") || unit.equals("g") || unit.equals("oz")) );
      
      return "Unit of mass: " + unit; 
   }

Answer 3

The do-while loop says: do something while a condition is true . Your condition is:

   !unit.equals("kg") || !unit.equals("lb") || !unit.equals("g") || !unit.equals("oz")

and it must be false to exit from the do-while loop.
If you enter kg , the first condition ( .unit.equals("kg") ) is false and it is what you want, but unfortunately, because the next condition ( .unit.equals("lb") ) is true and it is in OR ( || ), you remain in the do-while loop whatever is your input.
You need to change the condition using AND ( && ):

   !unit.equals("kg") && !unit.equals("lb") && !unit.equals("g") && !unit.equals("oz")