Java Calculator (Not GUI)

[trixsareforkids]

I need to use StringTokenizer, which I have, but something is wrong. I keep getting errors and I'm having some trouble finding out where. I know every time the program is executed, the while statement is not implemented. Here is my code so far:

import java.util.*; 
public class Calculator
{ 
    private StringTokenizer tokenizer; 
    private String token; 
    
       
    public Calculator(String line) 
    { 
        tokenizer = new StringTokenizer(line); 
        token = tokenizer.nextToken(); 
    } 

    public double Evaluate() 
    { 
        return Expression(); 
    } 
    
    private double Primary() 
    { 
        double result; 
      
        if(token.equals(""))
        { 
            token = tokenizer.nextToken(); 
            result = Expression(); 
        } 
        else 
        { 
            result = Double.valueOf(token).doubleValue(); 
        } 
      
        token = tokenizer.nextToken(); 
        return result; 
    } 
    
    private double Term() 
    { 
        double nextValue; 
        double result; 
      
        result = Primary(); 
      
        while(token.equals("*")) 
        { 
            token = tokenizer.nextToken(); 
            nextValue = Primary(); 
            result *= nextValue; 
        } 
        
        while(token.equals("/")) 
        { 
            token = tokenizer.nextToken(); 
            nextValue = Primary(); 
            result /= nextValue; 
        }   
        return result;
    } 
    
    private double Expression() 
    { 
        double nextValue; 
        double result; 
     
        result = Term(); 
     
        while(token.equals("+")) 
        {           
            token = tokenizer.nextToken(); 
            nextValue = Term(); 
            result += nextValue; 
        } 
        
        while(token.equals("-")) 
        { 
            token = tokenizer.nextToken(); 
            nextValue = Term(); 
            result -= nextValue; 
        } 
      
        return result; 
     } 
    
     public static void main (String[] args)
     { 
             Scanner input = new Scanner(System.in);            
             String line; 
      
             String choice = "y";
             while(choice.equalsIgnoreCase("y")) 
             { 
                 System.out.print("Enter an expression: "); 
                 line = input.nextLine(); 
                 
                 if(line.length() == 0) 
                 { 
                     System.out.println("Invalid input."); 
                     System.out.println("Try again? (Y/N): "); 
                     choice = input.nextLine(); 
                     if(choice.equalsIgnoreCase("n"))
                     {
                         System.out.println("Bye.");
                         System.exit(0);
                     }
                 }   
                 
             Calculator expn = new Calculator(line); 
             System.out.println("Result is " + expn.Evaluate()); 
         } 
       } 
   } 

This is a sample output it should be:

Enter an expression: 4.45 + 1.0
Result is 5.45
Try again? y
Enter an expression: 2.3 – 10.77
Result is -8.47
Try again? y
Enter an expression: 5.3
Input error
Try again? y
Enter an expression: 5.4 * 2.7.7
Input error
Try again? y
Enter an expression: 6.0 + * 3.0
Input error
Try again? n
Bye.


This is what I get:

Enter an expression: 4.45 + 1.0 =
Result is 5.45
Enter an expression: 2.3 - 10.77 =
Result is -8.469999999999999
Enter an expression:
..............it repeats until manually terminating the program.


Any help would be appreciated. Thanks in advance.

[EINSTEIN_007]

You do not have enough libraries imported and are not making use of the try-catch-finally paradigm for throwing exception. You should study the following program:

import java.io.FileReader;
import java.io.BufferedReader;
import java.io.IOException;
import java.util.Scanner;
import java.util.Locale;

public class ScanSum {
public static void main(String[] args) throws IOException {
Scanner s = null;
double sum = 0;
try {
s = new Scanner(
new BufferedReader(new FileReader("usnumbers.txt")));
s.useLocale(Locale.US);


while (s.hasNext()) {
if (s.hasNextDouble()) {
sum += s.nextDouble();
} else {
s.next();
}
}
} finally {
s.close();
}

System.out.println(sum);
}
}