TheAlgorithms/Java · error · IllegalArgumentException

Must be a natural number

Error message

Must be a natural number

What it means

Thrown by CollatzConjecture.collatzConjecture when firstNumber < 1. The Collatz sequence is defined only over the natural numbers (positive integers): the rule halves even numbers and applies 3n+1 to odd numbers, terminating at 1. Zero and negative integers have no well-defined Collatz sequence, so the library rejects them before seeding the result list and the while loop.

Source

Thrown at src/main/java/com/thealgorithms/maths/CollatzConjecture.java:32

     * @param n current number of the sequence
     * @return next number of the sequence
     */
    public int nextNumber(final int n) {
        if (n % 2 == 0) {
            return n / 2;
        }
        return 3 * n + 1;
    }

    /**
     * Calculate the Collatz sequence of any natural number.
     *
     * @param firstNumber starting number of the sequence
     * @return sequence of the Collatz Conjecture
     */
    public List<Integer> collatzConjecture(int firstNumber) {
        if (firstNumber < 1) {
            throw new IllegalArgumentException("Must be a natural number");
        }
        ArrayList<Integer> result = new ArrayList<>();
        result.add(firstNumber);
        while (firstNumber != 1) {
            result.add(nextNumber(firstNumber));
            firstNumber = nextNumber(firstNumber);
        }
        return result;
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Pass a positive integer (>= 1) such as collatzConjecture(27).
2. Validate the input at the caller boundary: if (firstNumber < 1) reject with a user-facing message.
3. If parsing from a string, parse then range-check before invoking.

Example fix

// before
List<Integer> seq = cc.collatzConjecture(0);

// after
List<Integer> seq = cc.collatzConjecture(1);

if (firstNumber < 1) {
    // reject with user-facing message
    throw new IllegalArgumentException("Collatz start must be >= 1");
}
cc.collatzConjecture(firstNumber);

static boolean isNatural(int n) { return n >= 1; }

When it happens

Trigger: Calling collatzConjecture(0), collatzConjecture(-5), or passing a value parsed from input that was not bounds-checked. The check fires before any sequence generation, so it triggers on the very first call with a non-natural number.

Common situations: User-supplied or file-parsed integer not validated; off-by-one in a loop that starts at 0 and feeds the method; testing with edge cases without realising the domain is strictly positive.

Related errors

• Input must be a positive integer.
• Input must me positive.
• n must be positive.
• number is negative
• n must be positive and odd.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/0dd593b8f540de0f. Report an issue: GitHub.