BigInteger and MutableBigInteger contain a certain amount of very similar code. For example, the BigInteger methods add(int[],int[]), multiplyToLen(), and subtract(int[],int[]) are very similar to the MutableBigInteger methods add(), multiply(), and subtract(), respectively. For each of these operations it might, e.g., be feasible to create a static method which accepts int array parameters and to invoke this static method from the respective analogs in BigInteger and MutableBigInteger.