code: public static Protos.PaymentRequest.Builder createPaymentRequest(NetworkParameters params,
              @Nullable Coin amount, Address toAddress, @Nullable String memo, @Nullable String paymentUrl,
              @Nullable byte[] merchantData) {
          return createPaymentRequest(params, ImmutableList.of(createPayToAddressOutput(amount, toAddress)), memo,
                  paymentUrl, merchantData);
      }
docs: Create a payment request with one standard pay to address output. You may want to sign the request using
  {@link #signPaymentRequest}. Use {@link Protos.PaymentRequest.Builder#build} to get the actual payment
  request.
  
  @param params network parameters
  @param amount amount of coins to request, or null
  @param toAddress address to request coins to
  @param memo arbitrary, user readable memo, or null if none
  @param paymentUrl URL to send payment message to, or null if none
  @param merchantData arbitrary merchant data, or null if none
  @return created payment request, in its builder form