Generics 和工厂方法

Question

I have two simple classes that extends an interface:

public interface Product 
{

}

public class ProductA implements Product
{

}

public class ProductB implements Product
{

}

I have two 'services' classes: ServiceA and ServiceB. Each one works with one of the Product classes defined before. Both implements Service interface.

public interface Service<T extends Product>
{
   public void print (T product); 
}

public class ServiceA implements Service<ProductA>
{
  public void print (ProductA product)
  {
      System.out.println("Product A");
  } 
}


public class ServiceB implements Service<ProductB>
{
  public void print (ProductB product)
  {
      System.out.println("Product B");
  }

}

I would like to developed a factory to instantiate my services:

[BEFORE I FOUND SOLUTION]

public class FactoryService
{
   public static Service<? extends Product> getService (String serviceType)
   {
      Service<? extends Product> s = null;
    
      if ("1".equals(serviceType))              
          s =  new ServiceA();
      else if ("2".equals(serviceType))     
          s = new ServiceB();           
    
      return s;
  }
}

[SOLUTION]

public static <T> T getService (Type targetType)
{
    T service = null;
    
    if (!targetType.getClass().isInstance(Product.class))
        throw new RuntimeException();
        
    if (ProductA.class.getTypeName().equals(targetType.getTypeName()))
        service = (T) new ServiceA();
    else if (ProductB.class.getTypeName().equals(targetType.getTypeName()))
        service = (T) new ServiceB();
    
    return service;     
}

When I tried to use my factory I get compile errors:

[BEFORE I FOUND SOLUTION]

public static void main(String[] args)
{       
    Product pA = new ProductA();
    Product pB = new ProductB();
    Service<? extends Product> service = FactoryService.getService("1");
    service.print(pA);                      
}

[SOLUTION]

public static void main(String[] args)
{       
    Product pA = new ProductA();
    Product pB = new ProductB();

    Service<Product> service = FactoryService.getService(pA.getClass());
    service.print(pA);
    
    service = FactoryService.getService(pB.getClass());
    service.print(pB);
    
// No compilation errors
    
}

The error says:

The method print(capture#5-of? extends Product) in the type Service<capture#5-of? extends Product> is not applicable for the arguments (Product).

How can I solve this problem?

Thanks

Answer 1

When you want to use generics in declaring a type, you shouldn't use <? extends Type> <? extends Type> as Jesper pointed out. Instead you should use <Type> . In your case, replace Service<? extends Product> Service<? extends Product> with Service<Product> .
Now you'll get another error:

Type mismatch: cannot convert from ServiceA to Service<Product>

The solution I suggest is instead of defining ServiceA and ServiceB , to just use the Service#print method and check the generic type, then do what is necessary. The FactoryService#getService method is not needed in this case.

Spoiler for my solution (try without it first):

public class Service<T extends Product> {
public void print(T product) {
if (product instanceof ProductA) {
System.out.println("Product A");
} else if (product instanceof ProductB) {
System.out.println("Product B");
}
}
}