Using a do while loop with a user inputted String

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; 
   }
}

It always return true. Because when you enter anything it will check !unit.equals("kg") return true. so It's OR operator. One of them is true mean While run again — Uzair
– Uzair, Commented May 4, 2021 at 4:27

Liftoff · Accepted Answer · 2021-05-04 04:25:00Z

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

E.g.

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"))

answered May 4, 2021 at 4:25

Liftoff

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Comments

Uzair · Accepted Answer · 2021-05-04 04:35:00Z

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; 
   }

Sixro · Accepted Answer · 2021-05-04 04:37:22Z

1

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")

answered May 4, 2021 at 4:37

Sixro

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