Getting "NumberFormatException" input string="" error and I don't know how to solve it [duplicate]

Question

I'm trying to solve this question, but the NumberFormatException always happens. Using try-catch doesn't give me what I want because it always results in printing out "invalid input!" Please help me.

The following is the code:

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

public class BTECtry2 {

    public static int binarySearch(double[] arr2, int L, int R, double x) {
        if (L <= R) {
            int mid = L + (R - L) / 2;
            if (arr2[mid] == x) {
                return mid;
            }
            if (arr2[mid] > x) {
                return binarySearch(arr2, L, mid - 1, x);
            } else {
                return binarySearch(arr2, mid + 1, R, x);
            }

        }
        return -1;
    }

    public static void insertionSort(double[] arr) {
        int x = arr.length;
        for (int i = 0; i < x; i++) {
            int y = i;
            Double money;
            for (int j = i; j < x; j++) {
                if (arr[j] < arr[y]) {
                    y = j;
                }

            }
            if (y != i) {
                money = arr[y];
                arr[y] = arr[i];
                arr[i] = money;
            }
        }

    }

    public static void main(String[] args) throws IOException {
        int[] USDcoin = new int[15];
        double[] JDcoin = new double[15]; // size is 15 for both arrays because there are 15 values in the txt file 
        String coins = "USD.txt";

        BufferedReader reader = new BufferedReader(new FileReader(coins));

        String y;
        int i = 0;
        while ((y = reader.readLine()) != null) {
            int temp = Integer.parseInt(y);
            USDcoin[i] = temp;
            JDcoin[i] = temp * (0.71);
            i += 1;

        }
        while (true) {
            System.out.println("P. to display the array");
            System.out.println("D. to sort the array");
            System.out.println("S. search for a number in array");
            System.out.println("Q. Quit the program");
            System.out.print("Enter your choice from the list above: ");
            Scanner input = new Scanner(System.in);
            String list;
            list = input.next();

            switch (list) {
                case "P" -> {

                    for (int j = 0; j < JDcoin.length; j++) {
                        System.out.println(JDcoin[j]);

                    }

                }

                case "D" -> {
                    insertionSort(JDcoin);
                    for (int j = 0; j < JDcoin.length; j++) {
                        System.out.println(JDcoin[j]);
                    }
                }

                case "S" -> {
                    Double value;
                    System.out.println(" Enter number : ");
                    value = input.nextDouble();
                    int w = binarySearch(JDcoin, 0, JDcoin.length, value);
                    if (w == -1) {
                        System.out.println(" Coin searched for is not found! ");
                    } else {
                        System.out.println(" Coin searched for is found! ");
                    }

                }
                case "Q" -> {
                    reader.close();
                    System.exit(0);

                }

                default -> {

                    System.out.println(" Invalid input! ");
                }

            }

        }

    }

}

The "USD.txt" file is just a txt file with a random list of number as follows:

200
100
400
350
800
750
250
200

Answer 1

I can see a few problems with your code. First, the part where you read in the file and parse the string around the line:

int temp = Integer.parseInt(y);

You should validate the contents of y before passing it into the parser:

• is it empty?
• does it match a number? (see regular expressions for that matter)
• maybe eliminate leading or trailing whitespaces (e.g. with .trim())

Then the switch-case of the user input has the following flaws: it is missing the 'break;' statements at the end of each case. This leads to unwanted behaviour. Look up how a switch-case block is executed espacially what a 'fallthrough' is. Also a switch-case used with a String input is case sensitive. So in your case, when the user inputs "p" (lowercase) it will give you " Invalid input! ", because it does not match your case "P" (uppercase).

Hope this helps a bit and gets you going.