Extract Class from Parameterized Interface using default method

Question

How can I use a Java 8 default method in an interface to extract the Class of a Parameterized Type instead of using an abstract class?

Option 1 (Fails):

public interface EpicCoolInterface<T> {

 default Class<T> getParameterizedTypeClass() {
    return T.class; //doesn't work
}

Option 2 (Fails):

public interface EpicCoolInterface<T> {

 default Class<T> getParameterizedTypeClass() {
    return (Class<T>) ((ParameterizedType) getClass().getGenericInterfaces()[0])
    .getActualTypeArguments()[0];
    //java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType
}

Third attempt (successful but no interface):

public abstract class CoolAbstractClass<T> {

  private Class<T> clazz;

  public CoolAbstractClass() {
    try {
      this.clazz = (Class<T>) ((ParameterizedType) getClass().getGenericSuperclass())
          .getActualTypeArguments()[0];
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  }

  public Class<T> getType() {
    return clazz;
  }
}

Dean Xu · Accepted Answer · 2017-12-12 06:19:36Z

Actually, what you need is generic type inference.

Although your third attempt works, it only be applicable to specify situation(the class directly extends the abstract class).

You can use my utility class GenericUtil's method

Type[] getGenericTypes(Type sourceType, Class<?> targetClass)

You can find the source code and javadoc on github.

For your question, you can define your inteerface like this:

  public static interface EpicCoolInterface<T> {
    // Return Type rather than Class, because T not always be a class.
    // You can do type check and return Class<T> with force typecast.
    default Type getParameterizedTypeClass() {
      return GenericUtil.getGenericTypes(getClass(), EpicCoolInterface.class)[0];
    }
  }

And let's test our code:

  public static void main(String[] args) {
    EpicCoolInterface<Integer> a = new EpicCoolInterface<Integer>() {
    };
    System.out.println(a.getParameterizedTypeClass());
    EpicCoolInterface<EpicCoolInterface<Integer>> b = new EpicCoolInterface<EpicCoolInterface<Integer>>() {
    };
    System.out.println(b.getParameterizedTypeClass());
    EpicCoolInterface<EpicCoolInterface<?>> c = new EpicCoolInterface<EpicCoolInterface<?>>() {
    };
    System.out.println(c.getParameterizedTypeClass());
  }

it output:

class java.lang.Integer
xdean.stackoverflow.java.reflection.Q46360416.xdean.stackoverflow.java.reflection.Q46360416$EpicCoolInterface<java.lang.Integer>
xdean.stackoverflow.java.reflection.Q46360416.xdean.stackoverflow.java.reflection.Q46360416$EpicCoolInterface<?>

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