Spring, Paypal : Using sample code to make a test payment

I am working on a Spring-MVC application in which I would like to integrate paypal payment mechanism. As you can expect I have to use the java-api-sdk by Paypal and the sameples seem good, but I am having trouble integrating those samples in my code.

As an initial task I would like to integrate the sample java file into my project and make a test payment. I have already created an app in Paypal and have the client-id and secret for a sandbox to go on with. I need to understand how to use their code to make a test payment using PaymentWithPayPalServlet.

The github link for the project is here

I am interested in using the below code to make a test payment and once successful, I will modify it to suit my requirements once I understand how to call it from Controller.

PaymentWithPayPalServlet.java :

public class PaymentWithPayPalServlet extends HttpServlet {

private static final long serialVersionUID = 1L;

private static final Logger LOGGER = Logger
        .getLogger(PaymentWithPayPalServlet.class);
Map<String, String> map = new HashMap<String, String>();

public void init(ServletConfig servletConfig) throws ServletException {

    InputStream is = PaymentWithPayPalServlet.class
            .getResourceAsStream("resources/sdk_config.properties");
    try {
        PayPalResource.initConfig(is);
    } catch (PayPalRESTException e) {
        LOGGER.fatal(e.getMessage());
    }

}

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    doPost(req, resp);
}

// ##Create
// Sample showing to create a Payment using PayPal
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    createPayment(req, resp);
    req.getRequestDispatcher("response.jsp").forward(req, resp);
}

public Payment createPayment(HttpServletRequest req, HttpServletResponse resp) {
    Payment createdPayment = null;
    // ###AccessToken
    // Retrieve the access token from
    // OAuthTokenCredential by passing in
    // ClientID and ClientSecret
    APIContext apiContext = null;
    String accessToken = null;
    try {
        accessToken = GenerateAccessToken.getAccessToken();

        // ### Api Context
        // Pass in a `ApiContext` object to authenticate
        // the call and to send a unique request id
        // (that ensures idempotency). The SDK generates
        // a request id if you do not pass one explicitly.
        apiContext = new APIContext(accessToken);
        // Use this variant if you want to pass in a request id
        // that is meaningful in your application, ideally
        // a order id.
        /*
         * String requestId = Long.toString(System.nanoTime(); APIContext
         * apiContext = new APIContext(accessToken, requestId ));
         */
    } catch (PayPalRESTException e) {
        req.setAttribute("error", e.getMessage());
    }
    if (req.getParameter("PayerID") != null) {
        Payment payment = new Payment();
        if (req.getParameter("guid") != null) {
            payment.setId(map.get(req.getParameter("guid")));
        }

        PaymentExecution paymentExecution = new PaymentExecution();
        paymentExecution.setPayerId(req.getParameter("PayerID"));
        try {
            createdPayment = payment.execute(apiContext, paymentExecution);
            ResultPrinter.addResult(req, resp, "Executed The Payment", Payment.getLastRequest(), Payment.getLastResponse(), null);
        } catch (PayPalRESTException e) {
            ResultPrinter.addResult(req, resp, "Executed The Payment", Payment.getLastRequest(), null, e.getMessage());
        }
    } else {

        // ###Details
        // Let's you specify details of a payment amount.
        Details details = new Details();
        details.setShipping("1");
        details.setSubtotal("5");
        details.setTax("1");

        // ###Amount
        // Let's you specify a payment amount.
        Amount amount = new Amount();
        amount.setCurrency("USD");
        // Total must be equal to sum of shipping, tax and subtotal.
        amount.setTotal("7");
        amount.setDetails(details);

        // ###Transaction
        // A transaction defines the contract of a
        // payment - what is the payment for and who
        // is fulfilling it. Transaction is created with
        // a `Payee` and `Amount` types
        Transaction transaction = new Transaction();
        transaction.setAmount(amount);
        transaction
                .setDescription("This is the payment transaction description.");

        // ### Items
        Item item = new Item();
        item.setName("Ground Coffee 40 oz").setQuantity("1").setCurrency("USD").setPrice("5");
        ItemList itemList = new ItemList();
        List<Item> items = new ArrayList<Item>();
        items.add(item);
        itemList.setItems(items);

        transaction.setItemList(itemList);


        // The Payment creation API requires a list of
        // Transaction; add the created `Transaction`
        // to a List
        List<Transaction> transactions = new ArrayList<Transaction>();
        transactions.add(transaction);

        // ###Payer
        // A resource representing a Payer that funds a payment
        // Payment Method
        // as 'paypal'
        Payer payer = new Payer();
        payer.setPaymentMethod("paypal");

        // ###Payment
        // A Payment Resource; create one using
        // the above types and intent as 'sale'
        Payment payment = new Payment();
        payment.setIntent("sale");
        payment.setPayer(payer);
        payment.setTransactions(transactions);

        // ###Redirect URLs
        RedirectUrls redirectUrls = new RedirectUrls();
        String guid = UUID.randomUUID().toString().replaceAll("-", "");
        redirectUrls.setCancelUrl(req.getScheme() + "://"
                + req.getServerName() + ":" + req.getServerPort()
                + req.getContextPath() + "/paymentwithpaypal?guid=" + guid);
        redirectUrls.setReturnUrl(req.getScheme() + "://"
                + req.getServerName() + ":" + req.getServerPort()
                + req.getContextPath() + "/paymentwithpaypal?guid=" + guid);
        payment.setRedirectUrls(redirectUrls);


        // Create a payment by posting to the APIService
        // using a valid AccessToken
        // The return object contains the status;
        try {
            createdPayment = payment.create(apiContext);
            LOGGER.info("Created payment with id = "
                    + createdPayment.getId() + " and status = "
                    + createdPayment.getState());
            // ###Payment Approval Url
            Iterator<Links> links = createdPayment.getLinks().iterator();
            while (links.hasNext()) {
                Links link = links.next();
                if (link.getRel().equalsIgnoreCase("approval_url")) {
                    req.setAttribute("redirectURL", link.getHref());
                }
            }
            ResultPrinter.addResult(req, resp, "Payment with PayPal", Payment.getLastRequest(), Payment.getLastResponse(), null);
            map.put(guid, createdPayment.getId());
        } catch (PayPalRESTException e) {
            ResultPrinter.addResult(req, resp, "Payment with PayPal", Payment.getLastRequest(), null, e.getMessage());
        }
    }
    return createdPayment;
}
}

I am also posting contents of response.jsp, presuming I get the response there.

response.jsp

  <script
            src="http://ift.tt/1qRgvOJ"></script>
    <script
            src="http://ift.tt/1ri5oOa"></script>
    <script
            src="//cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.2.0/js/scrollspy.min.js"></script>
    <script>
        $(document)
                .ready(
                function () {
                    $("#accordion .panel-collapse:last").collapse('toggle');

                    $(document.body)
                            .append(
                            '<footer class="footer"> <div class="container"> <div class="footer-div"> <ul class="footer-links"> <li> <a href="http://ift.tt/16bIwYE" target="_blank"><i class="fa fa-github"></i> Github</a></li><li> <a href="http://ift.tt/12Ptpko" target="_blank"><i class="fa fa-book"></i> REST API Reference</a> </li><li> <a href="http://ift.tt/1D7uHn0" target="_blank"><i class="fa fa-exclamation-triangle"></i> Report Issues </a> </li></ul> </div></div></footer>');

                    $(".prettyprint").each(function () {
                        if ($(this).html().trim() != '') {
                            try {
                                $(this).html(syntaxHighlight(JSON.stringify(JSON.parse($(this).html().trim()), null, 2)));
                            } catch (e) {
                                return false;
                            }
                        } else {
                            $(this).html('No Data');
                        }
                    });

                });

        /* http://ift.tt/1vsN6cv */
        function syntaxHighlight(json) {
            json = json.replace(/&/g, '&').replace(/</g, '&lt;').replace(/>/g,
                    '&gt;');
            return json
                    .replace(
                    /("(\\u[a-zA-Z0-9]{4}|\\[^u]|[^\\"])*"(\s*:)?|\b(true|false|null)\b|-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?)/g,
                    function (match) {
                        var cls = 'number';
                        if (/^"/.test(match)) {
                            if (/:$/.test(match)) {
                                cls = 'key';
                                if (match == '"id":') {
                                    console.log("Matched ID" + match);
                                    cls = 'key id';
                                }
                            } else {
                                cls = 'string';
                            }
                        } else if (/true|false/.test(match)) {
                            cls = 'boolean';
                        } else if (/null/.test(match)) {
                            cls = 'null';
                        }
                        return '<span class="' + cls + '">' + match
                                + '</span>';
                    });
        }
    </script>
</head>
<body>
<%
    List<String> responses = (List<String>) request
            .getAttribute("responses");
    List<String> requests = (List<String>) request
            .getAttribute("requests");
    List<String> messages = (List<String>) request
            .getAttribute("messages");
    List<String> errors = (List<String>) request.getAttribute("errors");
%>
<div class="row header">
    <div class="col-md-5 pull-left">
        <br/> <a href="index.html"><h1 class="home">&#10094;&#10094;
        Back to Samples</h1></a><br/>
    </div>
    <br/>

    <div class="col-md-4 pull-right">
        <img
                src="http://ift.tt/1D7uHn2"
                class="logo" width="300"/>
    </div>
</div>

<div class="panel-group" id="accordion" role="tablist"
     aria-multiselectable="true">
    <%
        for (int i = 0; i < responses.size(); i++) {
            String resp = responses.get(i);
            String req = requests.get(i);
            String message = messages.get(i);
            String error = errors.get(i);
            String errorClass = (error != null) ? "error" : null;
    %>
    <div class="panel panel-default">
        <div class="panel-heading <%= errorClass %>" role="tab" id="heading-<%= i + 1 %>">
            <h4 class="panel-title">
                <a data-toggle="collapse" data-parent="#accordion"
                   href="#step-<%= i + 1 %>" aria-expanded="false"
                   aria-controls="step-<%= i + 1 %>"><%= i + 1 %>. <%= message %> <% if (error != null) { %>
                    (Failed)<% } %></a>
            </h4>
        </div>
        <div id="step-<%= i + 1 %>" class="panel-collapse collapse"
             role="tabpanel" aria-labelledby="heading-<%= i + 1 %>">
            <div class="panel-body">
                <%
                    if (request.getAttribute("redirectURL") != null) {
                %>
                <div>
                    <a href=<%=(String) request.getAttribute("redirectURL")%>>Redirect
                        to PayPal to approve the payment</a>
                </div>
                <%
                    }
                %>
                <div class="row hidden-xs hidden-sm hidden-md">
                    <div class="col-md-6">
                        <h4>Request Object</h4>
                        <pre class="prettyprint "><%= req %></pre>
                    </div>
                    <div class="col-md-6">
                        <h4 class="<%= errorClass %>">Response Object</h4>
                        <% if (error != null) { %>
                        <p class="error"><i class="fa fa-exclamation-triangle"></i> <%= error %>
                        </p>
                        <% } %>
                        <pre class="prettyprint "><%= resp %></pre>
                    </div>
                </div>
                <div class="hidden-lg">
                    <ul class="nav nav-tabs" role="tablist">
                        <li role="presentation"><a href="#step-<%= i + 1 %>-request"
                                                   role="tab" data-toggle="tab">Request</a></li>
                        <li role="presentation" class="active"><a
                                href="#step-<%= i + 1 %>-response" role="tab" data-toggle="tab">Response</a></li>
                    </ul>
                    <div class="tab-content">
                        <div role="tabpanel" class="tab-pane"
                             id="step-<%= i + 1 %>-request">
                            <h4>Request Object</h4>
                            <pre class="prettyprint "><%= req %></pre>
                        </div>
                        <div role="tabpanel" class="tab-pane active"
                             id="step-<%= i + 1 %>-response">
                            <h4>Response Object</h4>
                            <pre class="prettyprint "><%= resp %></pre>
                        </div>
                    </div>
                </div>
            </div>
        </div>
    </div>
    <% } %>
</div>
</body>
</html>

I would love to understand what they are trying to do and why not some simple methods which can be called directly. Thanks a lot.