我有下面的Json文件，我正在尝试读取：

{
  "billingInformation": {
    "taxes": {
      "gst": 2.5,
      "hst": 7.8
    },
    "billTo": {
      "name" : "Mike",
      "address" : "123, Lake Shore Drive, California",
      "phoneNumber" : "601 855 1249"
    },
    "salesAgent": {
      "name" : "Charlotte Thompson",
      "agentCode" : 44551
    },
    "items": {
      "item": [
        {
          "hsnCode": "5112",
          "description": "TV Set",
          "originCountry": "US",
          "quantity": 1,
          "unitPrice": 150.00
        }
      ],
      "currency": "USD"
    }
  }
}

我使用了Gson提供的直接对象Map：

result = new String(Files.readAllBytes(Paths.get(path)));
BillingInformation billingInformation = gson.fromJson(result, BillingInformation.class);

但结果总是：

class BillingInformation {
    taxes: null
    billTo: null
    salesAgent: null
    items: null
}

我怎样才能得到其他对象内部的数据？