Question

I’m referring to the paradigm in Item 34 in Effective Java by Joshua Bloch.  I would like to take the method he’s using which is to have each related enum implement a base interface, and initialize an EnumMap from the “sub-enums.” See the code section below.  I’m getting a syntax error which I don’t understand.  I’m not set on this method of implementation, but I would like to understand why it won’t work.

Note that this example assumes each class definition is in its own file.

public interface BaseEnum { 
    ... 
}

public enum EnumOps1 implements BaseEnum { 
    ... 
}

public class Widget {
    public Widget() {
         regMap = new EnumMap<EnumOps1, WidgetData>(EnumOps1.class);

         for (EnumOps1 op : EnumOps1.values()) {
             regMap.put(op, getWidgetData(op.key()));  // line with syntax error
         }
    }

    protected Map<? extends BaseEnum, WidgetData> regMap;
} 

Syntax error detail:

method put in interface java.util.Map<K,V> cannot be applied to given types
required: capture#1 of ? extends BaseEnum, WidgetData
found: EnumOps1, WidgetData

Answer 1

It’s a problem with your wildcarding. You should declare your map as Map<BaseEnum, WidgetData> and also your EnumMap as ex. HashMap<BaseEnum, WidgetData>

There’s plenty of discussion of why this is true on SO, but see What is PECS (Producer Extends Consumer Super)? for example.

Edit

Sadly, you’re right – you can’t use an EnumMap there. This is because you are trying to use an interface, and EnumMap stipulates (as it requires a type T extends Enum<T>) that it must be an Enum only.

Your choices basically boil down to

1) Use an EnumMap<EnumOps1,...> and lose out on the polymorphism

2) Use a HashMap<BaseEnum,...> and everything works fine but you have to use a non-Enum map.

3) Use wildcards as you are trying, but you’ll run into the PECS restrictions I linked earlier e.g. you can add or remove elements but not both (super vs extends)