Gson: How do I parse polymorphic values that can be either lists or strings?


I need to parse a JSON file that contains long list of customers. In the JSON file each customer may have one id as a string:

{ "cust_id": "87655", ... },

or a few ids as an array:

{ "cust_id": [ "12345", "45678" ], ... },

The Customer class is as below:

<pre class="lang-java prettyprint-override">public class Customer { @SerializedName("cust_id") @Expose private String custId; public String getCustId() { return custId; } public void setCustId(String custId) { this.custId = custId; } }

I parse the JSON using Gson:

Gson gson = new Gson() Customers customers1 = gson.fromJson(json, Customers.class)

and it fails with com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a string but was BEGIN_ARRAY when it attempts to parse the array.

The reason of failure is clear.

My question: what is the best way to handle both cases (when id is a string and when it is an array of strings), given <strong>I can not change the json file structure</strong>?


If you want to handle both scenarios you can use a custom deserializer. Of course, you have to change the "cust_id" variable to be a list or an array.


String json1 = "{\"cust_id\": \"87655\"}"; String json2 = "{\"cust_id\": [\"12345\", \"45678\"]}"; GsonBuilder gsonBuilder = new GsonBuilder(); gsonBuilder.registerTypeAdapter(Customer.class, new CustomerDeserializer()); Gson gson = gsonBuilder.create(); Customer customer1 = gson.fromJson(json1, Customer.class); System.out.println(customer1); Customer customer2 = gson.fromJson(json2, Customer.class); System.out.println(customer2);


public class Customer { @SerializedName("cust_id") private List<String> custId; public List<String> getCustId() { return custId; } public void setCustId(List<String> custId) { this.custId = custId; } }


public class CustomerDeserializer implements JsonDeserializer<Customer> { @Override public Customer deserialize(JsonElement jsonElement, Type typeOf, JsonDeserializationContext context) throws JsonParseException { Customer result = null; Gson gson = new Gson(); try { // try to deserialize by assuming JSON has a list result = gson.fromJson(jsonElement, Customer.class); } catch (JsonSyntaxException jse) { // error here means JSON has a single string instead of a list try { // get the single ID String custId = jsonElement.getAsJsonObject().get("cust_id").getAsString(); result = new Customer(); result.setCustId(Arrays.asList(new String[] {custId})); } catch (Exception e) { // more error handling here e.printStackTrace(); } } return result; }



Customer [custId=[87655]] Customer [custId=[12345, 45678]]


Try method overriding:

public class Customer { @SerializedName("cust_id") @Expose private String custId; public void setCustId(String custId) { this.custId = {custId}; } public String[] getCustId() { return custId; } @override public void setCustId(String[] custId) { this.custId = custId; } }

Now In the code all values of CUSTID will be arrays instead of strings


You can just simply specify all values as array, even if is just one value.

{ "cust_id": ["87655"], ... },

UPDATE: If you cannot change the json, you can bind every field in Customer class except custId and set it manually.

public class Customer { private String[] custId; public String getCustId() { return custId; } public void setCustId(String custId) { custId = new String[] {custId}; } public void setCustId(String[] custId) { this.custId = custId; } }

And then parse manually:

Gson gson = new Gson(); Customers customers = gson.fromJson(json, Customers.class); Object custId = new JSONObject(json).get("cust_id"); if (custId instanceof String) { customers.setCustId((String) custId); else if (custId instanceof JSONArray) { customers.setCustId(convertToStringArray(new JSONArray(custId))); }


Refer <a href="https://stackoverflow.com/a/44968691/3488928" rel="nofollow">this</a>.

Now the problem is that you will have to write your own code on the returned map to get the desired result.


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