Android微信支付的”统一下单“接口如何调用

我现在已经将统一下单所需要的参数构建成了xml的内容格式，然后换成String输出。
然后微信的调用接口是这个：https://api.mch.weixin.qq.com/pay/unifiedorder

/**
     * 构造出xml文件的内容，用于发送给“统一下单”获取到prepay_id
     * 
     * @return XML的String输出
     */
    private String buildXMLUnifiedOrder() {

        mOutTradeNo = getIntent().getStringExtra("orderNo");
        List> paramList = new ArrayList>();

        paramList.add(new TwoTuple("appid", PathCommonDefines.WECHAT_APP_ID));
        paramList.add(new TwoTuple("body", mBody));
        paramList.add(new TwoTuple("mch_id", PathCommonDefines.WECHAT_PARTNER_ID));
        paramList.add(new TwoTuple("nonce_str", mNoncestr));
        paramList.add(new TwoTuple("notify_url", mNotifyUrl));
        paramList.add(new TwoTuple("out_trade_no", mOutTradeNo));
        paramList.add(new TwoTuple("spbill_create_ip", ipAddressUtil.getWifiLocalIPAddress()));
        paramList.add(new TwoTuple("total_fee", mTotalFee));
        paramList.add(new TwoTuple("trade_type", "APP"));
        paramList.add(new TwoTuple("sign", mSign));

        StringBuilder xmlBuilder = new StringBuilder();
        xmlBuilder.append("");
        for (TwoTuple paramTuple : paramList) {
            xmlBuilder.append("<").append(paramTuple.first).append(">");
            xmlBuilder.append(paramTuple.second);
            xmlBuilder.append("");

        }
        xmlBuilder.append("");

        return xmlBuilder.toString();

    }


/**
     * 
     * 两个元素的元组，用于在一个方法里返回两种类型的值
     *
     * @param 
     * @param 
     */
    public class TwoTuple {

        public final A first;
        public final B second;

        public TwoTuple(A a, B b) {
            first = a;
            second = b;
        }

    }