NumberFormatException

Why This Happens

Integer.parseInt(), Double.parseDouble(), and similar methods throw NumberFormatException when the input string does not represent a valid number. This happens with non-numeric characters, empty strings, extra whitespace, or numbers with wrong format.

The Problem

String input = "12.5";
int value = Integer.parseInt(input); // NumberFormatException

The Fix

String input = "12.5";
try {
    double value = Double.parseDouble(input.trim());
    int intValue = (int) value;
} catch (NumberFormatException e) {
    System.err.println("Invalid number: " + input);
}

Step-by-Step Fix

1. 1

   Identify the invalid input

   Read the exception message to see the exact string that failed to parse.

2. 2

   Check the parsing method

   Ensure you are using the right parsing method for the number format (parseInt for integers, parseDouble for decimals).

3. 3

   Validate or handle gracefully

   Trim whitespace, validate the format with a regex, or wrap in try-catch and provide a default value or error message.