Restrict Java Generic to a Class

Question

In C#, this is possible:

public interface IMyCoolInterface<TEntity> where TEntity : class
{
}

So the generic is confined / restricted to a class.

Is this possible with Java?

Answer 1

Generic parameters are always classes in Java. There's no other option. The language doesn't allow primitives to be parameters, and there's no equivalent to C#'s struct .

Answer 2

You can restrict it even further if you want the interface to be of a certain base type by adding the extends keyword within your generic

public interface MyGenericInterface<T extends List<?>> {

  T convert(String value);

}

Now every class that implements MyGenericInterface must use an object that extends List in its implementation

public class MyClass implements MyGenericInterface<ArrayList<String>> {

  @Override
  public ArrayList<String> convert(String value) {
    return new ArrayList<>();
  }
}