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?

Answer 1

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

Eg:

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.

Answer 2

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.