another java generic question

Question

I have the following class:

interface Able{/* ... */}
class A implements Able{/* ... */}

and I have

Map

        

Answer 1

A good way to understand the issue is to read what the wildcard means:

Map as;

"A map with keys of type String and values of one type that extends Able."

The reason why add operations are not allowed is because they "open the door" to introduce different types in the collection, which would conflict with the typing system. e.g.

class UnAble implements Able;
Map unableMap = new HashMap();
Map ableMap = unableMap;
ableMap.put("wontwork",new A()); // type mismatch: insert an A-type into an Unable map 

A correct use of the the wildcard construction would be:

Result processAble(Map) { ... read records & do something ... } 

Map ableMap = new HashMap;
ableMap.put("willwork",new A());
processAble(as);
processAble(unableMap); // from the definition above