Passing the Class in java of a generic list?

Question

I have a method for reading JSON from a service, I'm using Gson to do my serialization and have written the following method using type parameters.

public T getDeserializedJSON(Class<T> aClass,String url)
{
    Reader r = getJSONDataAsReader(url);
    Gson gson = new Gson();
    return gson.fromJson(r, aClass);
}

I'm consuming json which returns just an array of a type e.g.

[
 { "prop":"value" }
 { "prop":"value" }
]

I have a java class which maps to this object let's call it MyClass. However to use my method I need to do this:

RestClient<ArrayList<MyClass>> restClient = new RestClient<ArrayList<MyClass>>();
ArrayList<MyClass> results = restClient.getDeserializedJSON(ArrayList<MyClass>.class, url);

However, I can't figure out the syntax to do it. Passing just ArrayList.class doesn't work.

So is there a way I can get rid of the Class parameter or how do I get the class of the ArrayList of MyClass?

https://stackoverflow.com/questions/27000227/cannot-select-parameterized-type is a related question and ultimately helped me — Uncle Iroh, Oct 03 '19 at 20:24

score 36 · Accepted Answer · edited Jul 06 '20 at 11:22

36

You can use Bozho's solution, or avoid the creation of a temporary array list by using:

Class<List<MyClass>> clazz = (Class) List.class;

The only problem with this solution is that you have to suppress the unchecked warning with @SuppressWarnings("unchecked").

edited Jul 06 '20 at 11:22

Lii

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answered May 09 '10 at 15:00

Eyal Schneider

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"The only problem with this solution is that you have to suppress the unchecked warning" - I'm not sure about that. You can as well have this warning, right? – Line Oct 11 '17 at 12:27

score 10 · Answer 2 · answered May 09 '10 at 14:27

10

You can't. You'd have to use unsafe cast:

Class<List<MyClass>> clazz = 
   (Class<List<MyClass>>) new ArrayList<MyClass>().getClass();

answered May 09 '10 at 14:27

Bozho

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score 7 · Answer 3 · answered May 09 '10 at 15:51

7

As a follow up to this, I found this in the Gson docs.

Type listType = new TypeToken<List<String>>() {}.getType();

Which solves the problem of getting the type safely but the TypeToken class is specific to Gson.

answered May 09 '10 at 15:51

Rob Stevenson-Leggett

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1

Ok, but this actually didn't solve the generic problem! I look for something like `Type listType = new TypeToken>() {}.getType();` and the generic type `T` is passed as a method paramter like you did in your question – Khalid ElSayed Jan 30 '18 at 14:06

score 3 · Answer 4 · edited Nov 26 '19 at 22:37

3

If you are using SpringFramework you could use ParameterizedTypeReference as follows:

 restClient.getDeserializedJSON(ParameterizedTypeReference<List<MyClass>>(){},url);

edited Nov 26 '19 at 22:37

nonamer92

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answered Nov 26 '19 at 20:47

Luno Batista

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score 1 · Answer 5 · answered Jan 08 '21 at 12:03

I had a similar scenario but I have a workaround to use an array instead of ArrayList

MyClass[] results = restClient.getDeserializedJSON(MyClass[].class, url);

And here what deserialize method does

You can then proceed with list as follows:

List<MyClass> r_results= Arrays.stream(results).collect(Collectors.toList());

score 0 · Answer 6 · answered Feb 04 '21 at 11:57

0

Depending on how is your requirement, but if you are ok working with array there you can provide as Class: MyClass[].class

answered Feb 04 '21 at 11:57

Radu Linu

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Passing the Class in java of a generic list?

6 Answers6

Linked