Casting within a generic class hierarchy : Generic Class Hierarchies « Generics « Java Tutorial

1. Java Tutorial
2. Generics
3. Generic Class Hierarchies

You can cast one instance of a generic class into another only if the two are otherwise compatible and their type arguments are the same.

class Gen<T> {  
  T ob; 
    
  Gen(T o) {  
    ob = o;  
  }  
  
  T getObject() {  
    return ob;  
  }  
}  
 
class Gen2<T> extends Gen<T> { 
  Gen2(T o) { 
    super(o); 
  } 
} 
 
public class MainClass {  
  public static void main(String args[]) {  
    Gen<Integer> intObject = new Gen<Integer>(88); 
    Gen2<Long> longObject = new Gen2<Long>(99L);  
 
    //longObject = (Gen2<Long>)intObject;
 
  }  
}