code: public BigInteger optBigInteger(int index, BigInteger defaultValue) {
          Object val = this.opt(index);
          return JSONObject.objectToBigInteger(val, defaultValue);
      }
docs: Get the optional BigInteger value associated with an index. The
  defaultValue is returned if there is no value for the index, or if the
  value is not a number and cannot be converted to a number.
  
  @param index
  The index must be between 0 and length() - 1.
  @param defaultValue
  The default value.
  @return The value.