TheAlgorithms/Java · error · NullPointerException

Input cannot be null

Error message

Input cannot be null

What it means

Thrown by RomanToInteger.romanToInt as a NullPointerException when the input string is null. Note: this is a NullPointerException, not an IllegalArgumentException — the Javadoc explicitly documents this. The check precedes the toUpperCase() call, which would otherwise throw its own NullPointerException.

Source

Thrown at src/main/java/com/thealgorithms/conversions/RomanToInteger.java:73

     * Converts a Roman numeral string to its integer equivalent.
     * Steps:
     * <ol>
     *     <li>Iterate over the string from right to left.</li>
     *     <li>For each character, convert it to an integer value.</li>
     *     <li>If the current value is greater than or equal to the max previous value, add it.</li>
     *     <li>Otherwise, subtract it from the sum.</li>
     *     <li>Update the max previous value.</li>
     *     <li>Return the sum.</li>
     * </ol>
     *
     * @param roman the Roman numeral string
     * @return the integer value of the Roman numeral
     * @throws IllegalArgumentException if the input contains invalid Roman characters
     * @throws NullPointerException if the input is {@code null}
     */
    public static int romanToInt(String roman) {
        if (roman == null) {
            throw new NullPointerException("Input cannot be null");
        }

        roman = roman.toUpperCase();
        int sum = 0;
        int maxPrevValue = 0;
        for (int i = roman.length() - 1; i >= 0; i--) {
            int currentValue = romanSymbolToInt(roman.charAt(i));
            if (currentValue >= maxPrevValue) {
                sum += currentValue;
                maxPrevValue = currentValue;
            } else {
                sum -= currentValue;
            }
        }

        return sum;
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Check for null before calling romanToInt and handle the missing-value case.
2. Use Objects.requireNonNull(roman, "roman numeral input") if null should be treated as a programming error.
3. If null should map to zero, check and return 0 yourself before calling the method.

Example fix

// before
int result = RomanToInteger.romanToInt(romanStr); // romanStr may be null

// after
if (romanStr == null) {
    throw new IllegalArgumentException("Roman numeral input must not be null");
    // or: return 0; // if null should mean zero
}
int result = RomanToInteger.romanToInt(romanStr);

if (roman == null) {
    throw new IllegalArgumentException("Roman numeral input must not be null");
}
int result = RomanToInteger.romanToInt(roman);

static boolean isNotNullRoman(String s) {
    return s != null;
}

try {
    int result = RomanToInteger.romanToInt(roman);
} catch (NullPointerException e) {
    // input was null; provide a default or log the error
    logger.warn("Null Roman numeral input");
}

When it happens

Trigger: Calling romanToInt(null). Passing a variable that was not populated from a config, database, or API response. Supplying a null field from a JSON deserialization where the key was absent.

Common situations: A database column or config property for a Roman numeral is null. A JSON field is missing and the deserializer maps it to null. A function parameter is conditionally set but the condition was not met.

Related errors

• IPv4 address is empty.
• Input cannot be null or empty
• Input cannot be null or empty
• Unknown Roman symbol: {}
• Unit cannot be null.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/20cd7e2d4421b66c. Report an issue: GitHub.