java - Can I instantiate a superclass and have a particular subclass be instantiated based on the parameters supplied

Question

I am using Google's GSON package http://code.google.com/p/google-gson/

I am converting JSON to Java.

I have this fragment of code where I do the conversion.

Gson gson = new Gson();
Type collectionType = new TypeToken<Collection<QueryProperty>>() {}.getType();
Collection<QueryProperty> queryProperties = gson.fromJson(query, collectionType);

My QueryProperty class has these fields (with getters/setters):

int id;
String uri;
String name;
Set<QueryValue> values;
String selected;

The QueryValue class had these fields (with getters/setters) perviously:

int id;
String uri;
String name;

I now want to be able to have different types of QueryValue.

I want to add a NumericQueryValue class (subclass of QueryValue), so I can pass in a set of bounds with which to query the db.

double lower;
double upper;

And want to make a new ResourceQueryValue (subclass of QueryValue) which looks the same as QueryValue used to:

int id;
String uri;
String name;

Is there anyway I can make this work. That is have a generic QueryValue class, but have the correct subclass returned depending on the parameters that the JSON supplies.

If I have not been clear please do let me know.

Answer 1

Gson does not currently have a simple mechanism for polymorphic deserialization, other than implementing custom deserialization processing. The next release looks like it will provide a built-in solution.

Previous StackOverflow.com Questions And Answers (Some With Examples) On This Topic:

• Deserialising a generic with unknown compile time type where a field indicates the type
• Parse JSON with no specific structure for a field with GSON
• json object serialization/deserialization using google gson
• Polymorphism with gson