Factory method to instantiate instance of objects used in a generic class

Question

I have the following classes.

public abstract class AbstractClass {

      protected String value1;

      public void setValue1(String value1) {
           this.value1 = value1;
      }

}

public abstract class ConcreteClass1 extends AbstractClass{

}

public abstract class ConcreteClass2 extends AbstractClass {

}

public class FactoryOfAbstractClass {

   public AbstractClass newInstance(Class responseType) {
      if (responseType == ConcreteClass1.class) {
         return new ConcreteClass1();
      } else if (responseType == ConcreteClass2.class) {
         return new ConcreteClass2();
      }
      return null;

}

public abstract class ServiceClass<T extends AbstractClass> {

   public T method1 (String arg1, String arg2, Class responseType) {

      //Some operations resulting in a variable called value1
      String value1= someOp();
      T result = FactoryOfAbstractClass.newInstance(responseType);
      result.setValue1(value1);
      return result;

   }

}

In line T result = FactoryOfAbstractClass.newInstance(responseType); I get a compilation error saying I need to typecast the object I'm getting to T. I don't understand the compilation error since my factory method is returning an instance of Abstract class and the service class's generic T extends AbstractClass .

The compilation issue goes away once I cast it to T. But I don't understand why we need to typecast it in the first place. Why is the typecasting needed?

The other option is to not use a Factory and instead go with responseType.newInstance() . But that internally uses reflections and I want to avoid them. Is there any other approach i can take for this?

Answer 1

I don't understand the compilation error since my factory method is returning an instance of Abstract class and the service class's generic T extends AbstractClass

Let's replace AbstractClass with Car for a more understandable explanation. Suppose a programmer create a ServiceClass<Mercedes> (Mercedes is a typeof Car, right?). FactoryOfAbstractClass.newInstance() 's return type is Car . So the compiler has no idea of the concrete type of Car returned by this method. It could be a Mercedes, but it could also be a Ford, or a Peugeot, or a Volskwagen. But you assign the result to T (which, in this case, is Mercedes ). So that can't compile without a cast.

Frankly, this factory is completely unnecessary. Since the caller is supposed to pass the class to use anyway, you could just take an instance of that class instead of a factory, and thus let the caller create the instance. Or you could take a Supplier<AbstractClass> as argument, and the caller would just have to pass ConcreteClass1::new . That would make the code simpler, safer, and more extensible (since you wouldn't be limited to only two known subclasses).