BillingFlowParams.Builder setSkuDetails for testing static Google Play Billing responses

you should get SkuDetails from BillingClient.querySkuDetailsAsync, the sample code may seems like this:

private BillingClient mBillingClient;

// ....

mBillingClient = BillingClient.newBuilder(this).setListener(new PurchasesUpdatedListener() {
    @Override
    public void onPurchasesUpdated(int responseCode, @Nullable List<Purchase> purchases) {
        if (responseCode == BillingClient.BillingResponse.OK
                && purchases != null) {

            // do something you want

        } else if (responseCode == BillingClient.BillingResponse.USER_CANCELED) {
        } else {
        }
    }
}).build();


mBillingClient.startConnection(new BillingClientStateListener() {
    @Override
    public void onBillingSetupFinished(@BillingClient.BillingResponse int billingResponseCode) {        

if (billingResponseCode == BillingClient.BillingResponse.OK) {
        // The billing client is ready. You can query purchases here.

List<String> skuList = new ArrayList<>();
        skuList.add("android.test.purchased");

SkuDetailsParams skuDetailsParams = SkuDetailsParams.newBuilder()
                    .setSkusList(skuList).setType(BillingClient.SkuType.INAPP).build();
mBillingClient.querySkuDetailsAsync(skuDetailsParams,
                    new SkuDetailsResponseListener() {
 @Override
  public void onSkuDetailsResponse(int responseCode, List<SkuDetails> skuDetailsList) {

  BillingFlowParams flowParams = BillingFlowParams.newBuilder()
                                    .setSkuDetails(skuDetailsList.get(0))
                                    .build();
  int billingResponseCode = billingClient.launchBillingFlow(SkuActivity.this, flowParams);
  if (billingResponseCode == BillingClient.BillingResponse.OK) {
                                // do something you want
                            }
                        }
                    });
                 }
               }


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

You can also take a look to https://developer.android.com/google/play/billing/billing_library_overview

I'm posting a step by step process for in-app billing, which I had tested recently after the release of Billing Library version 4.0

Google help resource can be found here

First things first: Update your build.gradle

dependencies {
    def billing_version = "4.0.0"

    implementation "com.android.billingclient:billing:$billing_version"
}

Now, comes the coding part: BillingActivity.java

private BillingClient mBillingClient;

// ...


submitButton.setOnClickListener(v -> {
    mBillingClient = BillingClient.newBuilder(this).enablePendingPurchases().setListener(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);
                }
                // Perform your Successful Purchase Task here 
                
                Snackbar.make(findViewById(android.R.id.content), "Great!! Purchase Flow Successful! :)", Snackbar.LENGTH_LONG).show();
                dismiss();
            } else if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.USER_CANCELED) {
                // Handle an error caused by a user cancelling the purchase flow.
                Snackbar.make(findViewById(android.R.id.content), "User Cancelled the Purchase Flow!", Snackbar.LENGTH_LONG).show();
            } else {
                // Handle any other error codes.
                Snackbar.make(findViewById(android.R.id.content), "Error! Purchase Task was not performed!", Snackbar.LENGTH_LONG).show();
            }
        }
    }).build();

    mBillingClient.startConnection(new BillingClientStateListener() {

        @Override
        public void onBillingSetupFinished(@NonNull BillingResult billingResult) {

            if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK) {
                // The BillingClient is ready. You can query purchases here.

                List<String> skuList = new ArrayList<>();
                skuList.add("billing_template_1");
                skuList.add("billing_template_2");
                SkuDetailsParams.Builder params = SkuDetailsParams.newBuilder();
                params.setSkusList(skuList).setType(BillingClient.SkuType.INAPP);
                mBillingClient.querySkuDetailsAsync(params.build(),
                        new SkuDetailsResponseListener() {
                            @Override
                            public void onSkuDetailsResponse(BillingResult billingResult,
                                                             List<SkuDetails> skuDetailsList) {
                                // Process the result.
                                // Retrieve a value for "skuDetails" by calling querySkuDetailsAsync().
                                BillingFlowParams billingFlowParams = BillingFlowParams.newBuilder()
                                        .setSkuDetails(skuDetailsList.get(0))
                                        .build();
                                int responseCode = mBillingClient.launchBillingFlow(activity, billingFlowParams).getResponseCode();
                            }
                        });
            }
        }

        @Override
        public void onBillingServiceDisconnected() {
            // Try to restart the connection on the next request to Google Play by calling the startConnection() method.
            Toast.makeText(BillingActivity.this, "Service Disconnected!", Toast.LENGTH_SHORT).show();
        }
    });
});
        
void handlePurchase(Purchase purchase) {
    ConsumeParams consumeParams =
            ConsumeParams.newBuilder()
                    .setPurchaseToken(purchase.getPurchaseToken())
                    .build();

    ConsumeResponseListener listener = new ConsumeResponseListener() {
        @Override
        public void onConsumeResponse(BillingResult billingResult, @NonNull String purchaseToken) {
            if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK) {
                // Handle the success of the consume operation.
            }
        }
    };
    mBillingClient.consumeAsync(consumeParams, listener);
}
    
    
private void dismiss() {
        Intent intent = new Intent();
        intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        finish();
    }

FYI:

• This is just an updated version to @Renkuei answer posted.
• Don't forget .enablePendingPurchase()
• Don't forget skuList.add("billing_template_1") which has to be done from your Developer Console. So, billing_template_1 is the name of your Monetise > Products > In-app products > Prouduct ID