第 49 条：检查参数的有效性

/**
* Returns a BigInteger whose value is (this mod m). This method
* differs from the remainder method in that it always returns a
* non-negative BigInteger.
*
* @param m the modulus, which must be positive
* @return this mod m
* @throws ArithmeticException if m is less than or equal to 0
*/
public BigInteger mod(BigInteger m) {
    if (m.signum() <= 0)
        throw new ArithmeticException("Modulus <= 0: " + m);
    ... // Do the computation
}

// Inline use of Java's null-checking facility
this.strategy = Objects.requireNonNull(strategy, "strategy");

// Private helper function for a recursive sort
private static void sort(long a[], int offset, int length) {
    assert a != null;
    assert offset >= 0 && offset <= a.length;
    assert length >= 0 && length <= a.length - offset;
    ... // Do the computation
}