BillingClient billingClient = BillingClient.newBuilder(getApplicationContext()).setListener(new PurchasesUpdatedListener() {
    @Override
    public void onPurchasesUpdated(int responseCode, @Nullable List<Purchase> purchases) {
        if(responseCode == BillingClient.BillingResponse.OK) {
            //Do something
        }
    }
}).build();
billingClient.startConnection(new BillingClientStateListener() {
    @Override
    public void onBillingSetupFinished(int responseCode) {
        if(responseCode == BillingClient.BillingResponse.OK) {
            //Response is OK and working fine
        }
    }

    @Override
    public void onBillingServiceDisconnected() {
        //Do something
    }
});

billingClient.queryPurchaseHistoryAsync(BillingClient.SkuType.INAPP, new PurchaseHistoryResponseListener() {
    @Override
    public void onPurchaseHistoryResponse(int responseCode, List<Purchase> purchasesList) {
        if(responseCode == BillingClient.BillingResponse.OK) {
            //Always returning 0 size() of purchasesList
            Toast.makeText(getApplicationContext(), "There are " + purchasesList.size() + " items you've purchased.", Toast.LENGTH_LONG).show();
        }
    }
});