How to add In App Purchase Subscription to Android?

Question

I'm trying to implement in app purchase subscription to my android java app and following the tutorials from this link

https://developer.android.com/google/play/billing/integrate#java.

I created a new test project and following the steps, but not sure what do I have to put in the following two fields.

 BillingClient client = ...
 AcknowledgePurchaseResponseListener acknowledgePurchaseResponseListener = ...

public class MainActivity extends AppCompatActivity {

private PurchasesUpdatedListener purchasesUpdatedListener = new PurchasesUpdatedListener() {
    @Override
    public void onPurchasesUpdated(BillingResult billingResult, List<Purchase> purchases) {
        if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK
                && purchases != null) {
            for (Purchase purchase : purchases) {
                handlePurchase(purchase);
            }
        } else if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.USER_CANCELED) {
            // Handle an error caused by a user cancelling the purchase flow.
        } else {
            // Handle any other error codes.
        }
    }

    void handlePurchase(Purchase purchase) {
        // Purchase retrieved from BillingClient#queryPurchases or your PurchasesUpdatedListener.
        //Purchase purchase = ...;

        // Verify the purchase.
        // Ensure entitlement was not already granted for this purchaseToken.
        // Grant entitlement to the user.

        ConsumeParams consumeParams =
                ConsumeParams.newBuilder()
                        .setPurchaseToken(purchase.getPurchaseToken())
                        .build();

        ConsumeResponseListener listener = new ConsumeResponseListener() {
            @Override
            public void onConsumeResponse(BillingResult billingResult, String purchaseToken) {
                if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK) {
                    // Handle the success of the consume operation.
                    BillingClient client = ...
                    AcknowledgePurchaseResponseListener acknowledgePurchaseResponseListener = ...

                    void handlePurchase(Purchase purchase) {
                        if (purchase.getPurchaseState() == Purchase.PurchaseState.PURCHASED) {
                            if (!purchase.isAcknowledged()) {
                                AcknowledgePurchaseParams acknowledgePurchaseParams =
                                        AcknowledgePurchaseParams.newBuilder()
                                                .setPurchaseToken(purchase.getPurchaseToken())
                                                .build();
                                client.acknowledgePurchase(acknowledgePurchaseParams, acknowledgePurchaseResponseListener);
                            }
                        }
                    }

                }
            }
        };

        billingClient.consumeAsync(consumeParams, listener);
    }
};

private BillingClient billingClient = BillingClient.newBuilder(this)
        .setListener(purchasesUpdatedListener)
        .enablePendingPurchases()
        .build();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    billingClient.startConnection(new BillingClientStateListener() {
        @Override
        public void onBillingSetupFinished(BillingResult billingResult) {
            if (billingResult.getResponseCode() ==  BillingClient.BillingResponseCode.OK) {
                // The BillingClient is ready. You can query purchases here.
                List<String> skuList = new ArrayList<>();
                skuList.add("4");
                SkuDetailsParams.Builder params = SkuDetailsParams.newBuilder();
                params.setSkusList(skuList).setType(BillingClient.SkuType.INAPP);
                billingClient.querySkuDetailsAsync(params.build(),
                        new SkuDetailsResponseListener() {
                            @Override
                            public void onSkuDetailsResponse(BillingResult billingResult,
                                                             List<SkuDetails> skuDetailsList) {
                                // Process the result.
                                for (SkuDetails skuDetails : skuDetailsList) {
                                    if (skuDetails.equals("4")) {
                                        BillingFlowParams billingFlowParams = BillingFlowParams.newBuilder()
                                                .setSkuDetails(skuDetails)
                                                .build();
                                        int responseCode = billingClient.launchBillingFlow(MainActivity.this, billingFlowParams).getResponseCode();
                                    }
                                }
                            }
                        });

            }
        }
        @Override
        public void onBillingServiceDisconnected() {
            // Try to restart the connection on the next request to
            // Google Play by calling the startConnection() method.
        }
    });

}

}