在运行时确定泛型的转换类型

Question

My question is exactly similar to the one here . Just that, I want to do it in Java.

Suppose, I want to crate a list as:

List<a_type> myList = new ArrayList<a_type>();

The problem is that what a_type would be, is decided on the base of a condition. The two possible types are totally unrelated, having no superclass but Object in common. My problem will be solved if I can store the Type in a variable requiredType and use it as List<requiredType> . I am not sure if I'm just being greedy or foolish or ignorant in wanting this, but that's the situation.

Although not important, but the scenario that explains why I need it is explained here .

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PS: Please do not mention type-erasure. Kindly consider the situation as it is. I could have very well myself mentioned it in the question. If you think there are design issues, kindly read the situation here and suggest alternatives.

Answer 1

That's not possible in Java, because of the type-erasure and due to the fact that generics is only limited to compiletime.

eg

List<Integer> integers = new ArrayList<Integer>();

During compiletime, when the the type-erasing is done becomes

List integers = new ArrayList();

The generics is only a way to tell the compiler, that the list should only deal with objects of type Integer , and let's it point it out if you have somewhere tried to add something other than Integer for type safety.

Answer 2

The only effective (if “risky”) way I could see to do so, would be something akin to:

     List<?> myList = new ArrayList<?> ();

     /* … */

     final Object o = myList.get (0);

     if (o instanceof OneType) {
           OneType o1 = (OneType)o;
           /* … */
     } else if (o instanceof OtherType) {
           OtherType o2 = (OtherType)o;
           /* … */
     } else {
           throw new RuntimeException ("unexpected type found in list…");
     }

You will probably want to litter that code with @SuppressWarnings

Possibly a better(?) idea would be to implement at least a “marker interface” (without perhaps any methods) in both types, and use that as the commonality.

     public interface CanBeAddedToDualTypedList { };
     public class OneType implements CanBeAddedToDualTypeList { … };
     public class OtherType implements CanBeAddedToDualTypeList { … };


     List<CanBeAddedToDualTypedList> myList = 
                     new ArrayList<CanBeAddedToDualTypedList> ()

Edit I also put an answer on your related question, showing a concrete example of using a common interface for that specific case.

Answer 3

Java generic are not available at runtime so not possible with java

Search for "Type Erasure" on google and you will understand it better.

Answer 4

You can store the a_type.class object while that wont help you with generics, it can be used to add additional runtime type checks class.cast(...) .

Generics do not support runtime typing, the generic type itself does not exist at runtime. C# has a runtime class for each Generic type List<String> is different from List<int> . In java this is not the case both List<String> and List<Integer> degrade to List at runtime.

AFAIK (never tried it) extending a Generic class stores the type somewhere accessible to reflection class StringList extends List<String>{} , the StringList remembers the Generic type used to extend List.