code: public BigInteger getBigInteger(String key) throws JSONException {
          Object object = this.get(key);
          BigInteger ret = objectToBigInteger(object, null);
          if (ret != null) {
              return ret;
          }
          throw new JSONException("JSONObject[" + quote(key)
                  + "] could not be converted to BigInteger (" + object + ").");
      }
docs: Get the BigInteger value associated with a key.
  
  @param key
  A key string.
  @return The numeric value.
  @throws JSONException
  if the key is not found or if the value cannot
  be converted to BigInteger.