In-app billing Purchase Item / Subscribe

MainActivity.java

/**
* To purchase an Product
*/


final List<String> skuList = new ArrayList<>();
skuList.add("product_1"); // SKU Id
SkuDetailsParams params = SkuDetailsParams.newBuilder()
        .setSkusList(skuList)
        .setType(BillingClient.SkuType.INAPP)
        .build();
billingClient.querySkuDetailsAsync(params,
        new SkuDetailsResponseListener() {
            @Override
            public void onSkuDetailsResponse(BillingResult billingResult,
                                             List<SkuDetails> skuDetailsList) {
                if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK && skuDetailsList != null) {
                    if(skuDetailsList.size() == 1) {
                        SkuDetails skuDetails = skuDetailsList.get(0);
                        BillingFlowParams.Builder builder = BillingFlowParams.newBuilder()
                                        .setSkuDetails(skuDetails).setType(BillingClient.SkuType.INAPP);
                        int responseCode = mBillingClient.launchBillingFlow(this, builder.build());
                    }
                }
            }
        });
        
/**
 * 
 * OR
 * 
 */
 
// Note: setOldSku is deprecated

BillingFlowParams.Builder builder = BillingFlowParams.newBuilder()
                .setOldSku("prodcut_id").setType(BillingClient.SkuType.INAPP);
int responseCode = mBillingClient.launchBillingFlow(this, builder.build());        



/**
* To purchase an Subscription
*/

final List<String> skuList = new ArrayList<>();
skuList.add("subscription_id"); // SKU Id
SkuDetailsParams params = SkuDetailsParams.newBuilder()
        .setSkusList(skuList)
        .setType(BillingClient.SkuType.SUBS)
        .build();
billingClient.querySkuDetailsAsync(params,
        new SkuDetailsResponseListener() {
            @Override
            public void onSkuDetailsResponse(BillingResult billingResult,
                                             List<SkuDetails> skuDetailsList) {
                if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK && skuDetailsList != null) {
                    if(skuDetailsList.size() == 1) {
                        SkuDetails skuDetails = skuDetailsList.get(0);
                        BillingFlowParams.Builder builder = BillingFlowParams.newBuilder()
                                        .setSkuDetails(skuDetails).setType(BillingClient.SkuType.SUBS);
                        int responseCode = mBillingClient.launchBillingFlow(this, builder.build());
                    }
                }
            }
        });
       
/**
 * 
 * OR
 * 
 */
 
// Note: setOldSku is deprecated

BillingFlowParams.Builder builder = BillingFlowParams.newBuilder()
                .setOldSku("subscription_id").setType(BillingClient.SkuType.SUBS);
int responseCode = mBillingClient.launchBillingFlow(this, builder.build());