Java generic types and methods

Question

I want to create a function that gets an List parameter and creates a certain jsonResult. I know in advance that every T object of mine will have a getId() function. So my method

private static <T> JsonObject createJsonAarrayResult(List<T> items, Class<T> type){ JsonObject jsonResult = new JsonObject(); JsonArray jsonResultItem = new JsonArray(); for (T item : items){ JsonObject jsonArrayItem = new JsonObject(); jsonResultItem.addProperty("id", item.getId()); } return jsonResult; }

The line that calls item.getId() of course gives me an error. How can I bypass it. I am using type to also to pass the class type to use it for casting. But I still get an error. Is there a way to use T but and call methods from specific object?

Denis Lukenich · Accepted Answer · 2015-12-08 09:07:09Z

For example every of your T will implement an interface of abstract class which has this function has getID().

E.g.:

public interface InterfaceID { String getID(); }

Then make sure that each of your types you pass to this method actually use this interface (or any other Interface/Class which you may already have) you can declare the generic with extends:

 private static <T extends InterfaceID> JsonObject createJsonAarrayResult(List<T> items){ .... // The Class<T> parameter is not needed in your example and can be removed.

From now on the getID() function is available.

Bai Ethan · Accepted Answer · 2015-07-16 09:12:19Z

You get an error because the java compiler does not know the T has a method like get Id. You can write an interface like:

public interface SomeInterface { long getId(); }

Then modify your interface like this.

private static <T> JsonObject createJsonAarrayResult(List<SomeInterface> items, Class<T> type){ ... }

Don't forget to let your class implements SomeInterface.

Also, you can just use

<T extends SomeInterface>

to fix that problem.

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