I'm trying to write a program to determine if the input value is irrational or not by determining the number of digits. why won't it compile?

Question

If the number of digits is infinite then I mark it as irrational and everything else is rational as it would be finite.

I tired input 3.14 but it crashed and didn't compile the output of irrational or rational.

import java.math.BigDecimal;

import java.util.Scanner;

public class non_terminating_decimals {
    public static void main(String[] args) {

        Scanner inputNumber = new Scanner(System.in);
        System.out.println("input number : ");
        BigDecimal inputnumber = inputNumber.nextBigDecimal();
        BigDecimal numerofDigits = input(new BigDecimal(String.valueOf(inputnumber)));

        BigDecimal infinity = BigDecimal.valueOf(Double.POSITIVE_INFINITY);

        if (numerofDigits == infinity) {

            System.out.println("Irrational");
        }
        else {

            System.out.println("Rational");
        }
    }

    static int integerDigits(BigDecimal number) {
        return number.signum() == 0 ? 1 : number.precision() - number.scale();
    }

    static BigDecimal input(BigDecimal number) {
        return BigDecimal.valueOf(0);
    }
}

Answer 1

Let's unpack this statement:

BigDecimal infinity = 
    BigDecimal.valueOf(Double.POSITIVE_INFINITY);

Double.POSITIVE_INFINITY is some number.

Looking at the documentation for BigDecimal.valueOf, we see it uses Double.toString() to do the conversion.

Looking at the documentation for that, we see that a value of positive infinity results in the string "Infinity".

Thus, we're effectively left with trying to evaluate

BigDecimal("Infinity");

And if we look at the documentation for that particular constructor, there's no suggestion it can handle non-numeric string arguments.