public interface QuerySkuDetailsListener {
        void onSuccess(List<SkuDetails> skuDetailsList);

        void onErrorProductsHaveDifferentTypes();

        void onBillingClientError(int error_code);
    }

    public void querySkuDetails(final QuerySkuDetailsListener querySkuDetailsListener, final Product... products) {
        if (products.length < 1) {
            //ERROR_NO_PRODUCTS_TO_QUERY
            throw new NullPointerException();
        }

        String querySkuType = products[0].getSkuType();

        SkuDetailsParams.Builder params = SkuDetailsParams.newBuilder();
        List<String> skuList = new ArrayList<>();
        for (Product product : products) {
            skuList.add(product.getSku());
            if (!product.getSkuType().equals(querySkuType)) {
                //ERROR_SKU_TYPE_CANT_BE_DIFFERENT
                querySkuDetailsListener.onErrorProductsHaveDifferentTypes();
                return;
            }
        }
        params.setSkusList(skuList).setType(querySkuType);

        billingClient.querySkuDetailsAsync(params.build(), new SkuDetailsResponseListener() {
            @Override
            public void onSkuDetailsResponse(BillingResult billingResult, List<SkuDetails> skuDetailsList) {
                if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK && skuDetailsList != null) {
                    querySkuDetailsListener.onSuccess(skuDetailsList);
                }
                else {
                    //ERROR QUERYING SKU DETAILS
                    querySkuDetailsListener.onBillingClientError(billingResult.getResponseCode());
                }
            }
        });
    }

private void initiatePurchaseFlow(final Activity activity, @Nullable final Product oldProduct, final Product product, final JsonObject metadata, final InitiatePurchaseListener initiatePurchaseListener) {

        //Check if billingClient is ready
        if (billingClient.isReady()) {
            querySkuDetails(new QuerySkuDetailsListener() {
                    @Override
                    public void onSuccess(List<SkuDetails> skuDetailsList) {
                        // Process the result.
                        BillingFlowParams.Builder flowParamsBuilder = BillingFlowParams.newBuilder();
                        flowParamsBuilder.setSkuDetails(skuDetailsList.get(0));
                        if (null != oldProduct) {
                            flowParamsBuilder.setOldSku(oldProduct.getSku());
                            flowParamsBuilder.setReplaceSkusProrationMode(BillingFlowParams.ProrationMode.IMMEDIATE_WITH_TIME_PRORATION);
                        }
                        BillingFlowParams flowParams = flowParamsBuilder.build();
                        int responseCode = billingClient.launchBillingFlow(activity, flowParams).getResponseCode();
                        if (responseCode == BillingClient.BillingResponseCode.OK) {
                            //SUCCESS, OK
                            ...Rest of the irrelevant code...
                        }
                        else {
                            //ERROR_IN_LAUNCHING_BILLING_FLOW
                            dispenseInitiatePurchaseFlowError(activity, initiatePurchaseListener, InitiatePurchaseErrorCodes.ERROR_IN_LAUNCHING_BILLING_FLOW, getBillingClientErrorDescription(responseCode));
                        }
                    }

                    @Override
                    public void onErrorProductsHaveDifferentTypes() {
                        //ERROR_IN_QUERYING_SKU_DETAILS_PRODUCTS_HAVE_DIFFERENT_TYPES
                        dispenseInitiatePurchaseFlowError(activity, initiatePurchaseListener, InitiatePurchaseErrorCodes.ERROR_IN_QUERYING_SKU_DETAILS_PRODUCTS_HAVE_DIFFERENT_TYPES, getBillingClientErrorDescription(0));
                    }

                    @Override
                    public void onBillingClientError(int error_code) {
                        //ERROR_IN_QUERYING_SKU_DETAILS
                        dispenseInitiatePurchaseFlowError(activity, initiatePurchaseListener, InitiatePurchaseErrorCodes.ERROR_IN_QUERYING_SKU_DETAILS, getBillingClientErrorDescription(error_code));
                    }
                }, product);
        }
        else {
            //ERROR_BILLING_CLIENT_IS_NOT_READY
            //Trying to reconnect once
            billingClient.startConnection(new BillingClientStateListener() {
                @Override
                public void onBillingSetupFinished(BillingResult billingResult) {
                    if (billingResult.getResponseCode() == BillingResponse.OK) {
                        //launching again
                        initiatePurchaseFlow(activity, oldProduct, product, metadata, initiatePurchaseListener);
                    }
                }
                @Override
                public void onBillingServiceDisconnected() {
                    // showing an error dialog
                }
            });
        }
    }