In-app Billing Initalise

MainActivity.java

BillingClient mBillingClient;

mBillingClient = BillingClient.newBuilder(this).setListener(new PurchasesUpdatedListener() {
    @Override
    public void onPurchasesUpdated(int responseCode, @Nullable List<Purchase> purchases) {
        for(Purchase purchase: purchases) {
            //When every a new purchase is made
        }
    }
}).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.
        }
    }
    @Override
    public void onBillingServiceDisconnected() {
        // Try to restart the connection on the next request to
        // Google Play by calling the startConnection() method.
    }
});

add