How can I use a while to continuously ask for input from a user and exit the program when “quit” is typed without using system.exit()?

问题

I am currently taking an AP Computer Science class in my school and I ran into a little trouble with one of my projects! The project requires me to create a calculator that can evaluate an expression and then solve it. I have got most of that down, but I ran into a little trouble because my teacher asked me to use a while loop to continuously ask for input and display the answer, and I am stuck on that. To end the program the user has to type in "quit" and I can't use system.exit() or any cheating thing like that, the program has to just run out of code. Does anyone have any tips?

import java.util.*;

public class Calculator {
   public static void main(String[] args) {
      System.out.println("Welcome to the AP Computer Science calculator!!");
      System.out.println();
      System.out.println("Please use the following format in your expressions: (double)(space)(+,-,*,/...)(space)(double)");
      System.out.println("or: (symbol)(space)(double)");
      System.out.println();
      next();
   }

   public static void next() {
      Scanner kb = new Scanner(System.in);
      System.out.print("Enter an expression, or quit to exit: ");
      String expression = kb.nextLine();
      next3(expression);
   }

   public static void next3(String expression) {   
      while (!expression.equals("quit")) {
         next2(expression);
         next();
      }  
   }

   public static void next2(String expression) {
      if (OperatorFor2OperandExpressions(expression).equals("+")) {
         System.out.println(FirstOperandFor2OperandExpressions(expression) + " " + OperatorFor2OperandExpressions(expression) + " " + SecondOperandFor2OperandExpressions(expression) + " = " + (FirstOperandFor2OperandExpressions(expression) + SecondOperandFor2OperandExpressions(expression)));          
      }
      else if (OperatorFor2OperandExpressions(expression).equals("*")) {
         System.out.println(FirstOperandFor2OperandExpressions(expression) + " " + OperatorFor2OperandExpressions(expression) + " "  + SecondOperandFor2OperandExpressions(expression) + " = " + (FirstOperandFor2OperandExpressions(expression) * SecondOperandFor2OperandExpressions(expression)));
      }
      else if (OperatorFor2OperandExpressions(expression).equals("-")) {
         System.out.println(FirstOperandFor2OperandExpressions(expression) + " " + OperatorFor2OperandExpressions(expression) + " " + SecondOperandFor2OperandExpressions(expression) + " = " + (FirstOperandFor2OperandExpressions(expression) - SecondOperandFor2OperandExpressions(expression)));       
      }
      else if (OperatorFor2OperandExpressions(expression).equals("/")) {
         System.out.println(FirstOperandFor2OperandExpressions(expression) + " " + OperatorFor2OperandExpressions(expression) + " "  + SecondOperandFor2OperandExpressions(expression) + " = " + (FirstOperandFor2OperandExpressions(expression) / SecondOperandFor2OperandExpressions(expression)));
      }
      else if (OperatorFor2OperandExpressions(expression).equals("^")) {
         System.out.println(FirstOperandFor2OperandExpressions(expression) + " " + OperatorFor2OperandExpressions(expression) + " " + SecondOperandFor2OperandExpressions(expression) + " = " + Math.pow(FirstOperandFor2OperandExpressions(expression),SecondOperandFor2OperandExpressions(expression)));
      }
      else if (OperatorFor1OperandExpressions(expression).equals("|")) {
         System.out.println(OperatorFor1OperandExpressions(expression) + " " + OperandFor1OperatorExpressions(expression) + " = " + Math.abs(OperandFor1OperatorExpressions(expression)));
      }
      else if (OperatorFor1OperandExpressions(expression).equals("v")) {
         System.out.println(OperatorFor1OperandExpressions(expression) + " " + OperandFor1OperatorExpressions(expression) + " = " + Math.sqrt(OperandFor1OperatorExpressions(expression)));
      }
      else if (OperatorFor1OperandExpressions(expression).equals("~")) {
         double x = 0.0;
         System.out.println(OperatorFor1OperandExpressions(expression) + " " + OperandFor1OperatorExpressions(expression) + " = " + (Math.round(OperandFor1OperatorExpressions(expression))+ x));
      }
      else if (OperatorFor1OperandExpressions(expression).equals("s")) {
         System.out.println(OperatorFor1OperandExpressions(expression) + " " + OperandFor1OperatorExpressions(expression) + " = " + Math.sin(OperandFor1OperatorExpressions(expression)));
      }
      else if (OperatorFor1OperandExpressions(expression).equals("c")) {
         System.out.println(OperatorFor1OperandExpressions(expression) + " " + OperandFor1OperatorExpressions(expression) + " = " + Math.cos(OperandFor1OperatorExpressions(expression)));
      }
      else if (OperatorFor1OperandExpressions(expression).equals("t")) {
         System.out.println(OperatorFor1OperandExpressions(expression) + " " + OperandFor1OperatorExpressions(expression) + " = " + Math.tan(OperandFor1OperatorExpressions(expression))); 
      } 
   }

      public static double FirstOperandFor2OperandExpressions(String expression) {
         String[] tokens = expression.split(" ");
         String OperandOrOperator = tokens[0];
         double y = Double.parseDouble(OperandOrOperator);
         return y;         
   }
      public static double SecondOperandFor2OperandExpressions(String expression) {
         String[] tokens = expression.split(" ");
         String OperandOrOperator = tokens[2];
         double y = Double.parseDouble(OperandOrOperator);
         return y;
   }
       public static String OperatorFor2OperandExpressions(String expression) {
         String[] tokens = expression.split(" ");
         String OperandOrOperator = tokens[1];
         return OperandOrOperator;
   }
      public static String OperatorFor1OperandExpressions(String expression) {
         String[] tokens = expression.split(" ");
         String OperandOrOperator = tokens[0];
         return OperandOrOperator; 
   }
      public static double OperandFor1OperatorExpressions(String expression) {
         String[] tokens = expression.split(" ");
         String OperandOrOperator = tokens[1];
         double y = Double.parseDouble(OperandOrOperator);
         return y;  
   }
      public static boolean QuitFunction(String expression) {
         if (expression.equalsIgnoreCase("quit")) {
            System.out.println("Goodbye!");
            return false;

         }
         else {
            return true;
         }
      }           
}

回答1:

Take a look at this code. I think this might help you in the right direction. It's similar to what you have already written except it eliminates the need for method calls in your while loop.

Scanner input = new Scanner(System.in);

while (!input.hasNext("quit")) {

    String expression = input.nextLine(); // gets the next line from the Scanner
    next2(expression); // process the input

}
// once the value "quit" has been entered, the while loop terminates
System.out.println("Goodbye");

Writing it this way drastically cleans up your code and prevents a new declaration of Scanner kb = new Scanner(System.in); each time an input is processed.

来源：https://stackoverflow.com/questions/26836258/how-can-i-use-a-while-to-continuously-ask-for-input-from-a-user-and-exit-the-pro