Why is the custom exception I created causing a non-static variable error?

Question

Can somebody please explain what is going on here? Why is the "non-static variable this cannot be referenced from a static context." appearing in this code? I have tried changing the parseBinary to non-static. I have tried changing the BinaryFormatException class to static... (not allowed). My understanding of this error is very weak. I know I can usually swap some static and public words around and get it to work right. No such luck with this. This is a homework question... but the work does not revolve around the error. It has to do with creating a custom exception class that is thrown when a binary string is incorrectly formed. So while my question will help me answer the problem, it will not give me the answer.

public class binaryToDecimal {

    public static void main(String[] args) {
        try {
            System.out.println(parseBinary("10001"));
            System.out.println(parseBinary("101111111"));
        } catch (BinaryFormatException ex) {
            ex.getMessage();
        }
    }

    public static int parseBinary(String binaryString)
            throws BinaryFormatException {
        int value = 0;

        for (int i = 0; i < binaryString.length(); i++) {
            char ch = binaryString.charAt(i);

            if (ch != '0' && ch != '1') {
                throw new BinaryFormatException(String message);
                value = 0;
            } else
                value = value * 2 + binaryString.charAt(i) - '0';
        }

        return value;
    }

    class BinaryFormatException extends Exception {
        public BinaryFormatException(String message) {
            super(message);
        }
    }
}

Answer 1

It looks like you've defined BinaryFormatException as an inner class to your public class binaryToDecimal. That means that you need an instance of binaryToDecimal to have an instance of BinaryFormatException. However, you are in the static context of the parseBinary method. There is no instance of binaryToDecimal.

You have two choices:

1. Declare the BinaryFormatException class static.
2. Move the BinaryFormatException class code outside of the binaryToDecimal class.

Answer 2

Because BinaryFormatException is an inner class of BinaryToDecimal, so it needs an outer instance to create one. However, parseBinary is a static method, and it runs without a instance of the class.

Put the word "static" before "class BinaryFormat Exception" to fix this.

Answer 3

Thank you for your responses. This is the answer I finally came up with a few minutes ago that seems to work properly.

    public class binaryToDecimal extends parseBinary{

       public static void main( String[] args ){
           try {
               System.out.println(parseBinary( "10001"  ));
               System.out.println(parseBinary( "141111111" ));
           } catch (BinaryFormatException ex) {
               ex.printStackTrace();
           }
          }
    }

class parseBinary{
    String binaryString = "";
   public static int parseBinary(String binaryString ) throws BinaryFormatException
      {
      int value = 0;

      for (int i = 0; i < binaryString.length(); i++)
         {
         char ch = binaryString.charAt(i);

         if (ch != '0' && ch != '1'){
            //value = value * 2 + binaryString.charAt(i) - '0';
         throw new BinaryFormatException("Invalid format at: " + ch + " in binaryString parameter.");
         }else{         
            value = value * 2 + binaryString.charAt(i) - '0';
         }

      }
            return value;

   }
}
class BinaryFormatException extends Exception{
    public BinaryFormatException(String message){
        super(message);
    }
}