Java Generics, Create an instance of Class<T>

Question

I am trying to write a generic method for deserializing json into my model. My problem is that I don't know how to get Class from the generic type T. My code looks something like this (and doesn't compile this way)

public class JsonHelper {

    public <T> T Deserialize(String json)
    {
        Gson gson = new Gson();

        return gson.fromJson(json, Class<T>);
    }
}

I tried something else, to get the type, but it throws an error I had the class as JsonHelper<T> and then tried this

Class<T> persistentClass = (Class<T>) ((ParameterizedType)getClass()
    .getGenericSuperclass())
    .getActualTypeArguments()[0];

The method signature looks like this

com.google.gson.Gson.fromJson(String json, Class<T> classOfT)

So, how can I translate along T so that when I call JsonHelper.Deserialize<MyObject>(json); I get an instance of the correct object?

I'm not sure it's possible. If it was, why wouldn't the GSon class do it directly and remote the Class<T> parameter? — JB Nizet
– JB Nizet, Commented Feb 18, 2011 at 19:50
Josh, I searched for and answer to the same problem and never got one. I ended up doing what the two answers below suggest. — jjnguy
– jjnguy, Commented Feb 18, 2011 at 19:55

erickson · Accepted Answer · 2011-02-18 19:50:18Z

13

You need to get a Class instance from somewhere. That is, your Deserialize() method needs to take a Class<T> as a parameter, just like the underlying fromJson() method.

Your method signature should look like Gson's:

<T> T Deserialize(String json, Class<T> type) ...

Your calls will look like this:

MyObject obj = helper.Deserialize(json, MyObject.class);

By the way, the convention to start method names with a lowercase letter is well established in Java.

answered Feb 18, 2011 at 19:50

erickson

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2 Comments

Josh Over a year ago

I come from a background primarily in .net and that is where I picked up that coding practice. :)

ColinD Over a year ago

@Josh: Of course it's understandable if someone coming from another language starts out using the style they're used to. That said, it's really best to learn and try to align your code style to the standards for the language you're working in.

StriplingWarrior · Accepted Answer · 2011-02-18 19:49:50Z

6

Unfortunately, the way Java handles generics, you cannot get the class like you're asking. That's why Google's stuff asks specifically for the class as an argument. You'll have to modify your method signature to do the same.

answered Feb 18, 2011 at 19:49

StriplingWarrior

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