Java-instanceof vs运行时泛型类的转换：

Question

In the code extracted from Java Complete Reference by Herbert Schildt:

class Gen<T> {
  T obj;

  Gen(T o) {
    ob = o;
  }

  T getob() {
    return ob; 
  }
}

class Gen2<T> extends Gen<T> {
  Gen2(T o) {
    super(o);
  }
}

class Test {
  public static void main(String args[]) {
    Gen2<Integer> obj = new Gen2<Integer>(99);
  }
}

he mentions that instanceof can't verify if a object is from a typed generic class at runtime because no Generic information is available:

if (obj instanceof Gen2<Integer>) // illegal, doesn't compile

you can only use

if (obj instanceof Gen2<?>) // legal

However, you can still Cast the same object to (Gen) as long as it is compatible:

(Gen<Integer>) obj // legal

but:

(Gen<Long>) obj // illegal

Isn't this a Java contradiction? If Java knows that obj can be cast to a Gen at runtime, why doesn't it knows that obj is an instanceof Gen class/subclass?

Answer 1

I read the book, confused by this too, I found this, maybe helpful: https://docs.oracle.com/javase/tutorial/java/generics/restrictions.html#cannotCast

However, in some cases the compiler knows that a type parameter is always valid and allows the cast. For example:

 List<String> list1 = ...; ArrayList<String> list2 = (ArrayList<String>)list1; // OK