Extending Generics to Subclasses

[Brendan Thompson]

Hi I have a question regarding how to restrict a subclass to only implementing some of the superclass' generics.

GenericOrder can take ComputerPart, Peripheral, Service, Cheese, and Fruit classes. I want the subclass ComputerOrder to only be able to take ComputerPart, Peripheral, and Service classes. Eclipse gives me a warning with the code I've got now so I'm not sure if I have it right or not. This is the Eclipse warning:

Multiple markers at this line
- The type parameter ComputerPart is hiding the type
ComputerPart
- The type parameter Service is hiding the type Service
- The type parameter Peripheral is hiding the type Peripheral

import java.util.ArrayList;

public abstract class GenericOrder<ComputerPart, Peripheral, Service, Cheese, Fruit>
{
	public ArrayList orders;
	
	public abstract void placeOrder();
}

public class ComputerOrder<ComputerPart, Peripheral, Service> extends 
GenericOrder<ComputerPart, Peripheral, Service, Cheese, Fruit>
{

	@Override
	public void placeOrder() 
	{
	}
}

[Matthew Brown]

I don't think you've got the right idea about generics. This line:

public abstract class GenericOrder<ComputerPart, Peripheral, Service, Cheese, Fruit> 

doesn't create a class restricted to those 5 types. It declares a class that has 5 generic types, with ComputerPart, Peripheral, Service, Cheese, Fruit just used as the generic placeholders. Here it's not referring to the actual classes ComputerPart etc. You might as well have written:

public abstract class GenericOrder<T1, T2, T3, T4, T5> 

And following on from that, any class that extends GenericOrder needs to provide classes for all 5 generic types (or be generic itself).

There's no way using generics to have a class operating on 5 specific random classes and no others. That's not what they are for.

[Brendan Thompson]

Thanks for clearing that up. Generics are still a bit confusing to me so I'll try and read a bit more about them.