通用通配符正确使用

Question

I have an object defined as follow:

protected Map<String, ? extends List<? extends LightGeometry>> geoms=new HashMap<String,List<LightGeometry>>();

I try to insert in an object that looks conform to the wildcard

ArrayList<LightGeometry> points=new ArrayList<LightGeometry>();
points.add((LightGeometry)new LightPoint(pt));
geoms.put("point", points);

The compiler throws an error that says:

The method put(String, capture#18-of ? extends List<? extends LightGeometry> ) in the type Map<String,capture#18-of ? extends List<? extends LightGeometry>> Map<String,capture#18-of ? extends List<? extends LightGeometry>> Map<String,capture#18-of ? extends List<? extends LightGeometry>> is not applicable for the arguments (String, ArrayList<LightGeometry>)

What am I missing?

EDIT: Why I am using the wildcard with the generic types It basically comes down to being able to assign the list (which I get through a service) to a the geoms object which is in another class, without having to sift through the list to cast.

public void onSuccess(Map<String, ArrayList<LightPolygon>> result) {
    // TODO Auto-generated method stub
    GWT.log("" + result.size());
    Log.debug("" + result.size());
    long startTime = System.currentTimeMillis();
    if (overlay != null) {
        overlay.setData(result);
        overlay.update();
        Log.debug("time to add features: "
                + (System.currentTimeMillis() - startTime));
    }
}

If I were to make the geoms variable a Map<String, List<LightGeometry>> then I get a cast error that says that I can't assign a Map<String,ArrayList<LightPolygon>> to a 'Map>`

Answer 1

A generic wildcard does not mean "Anything that extends List" It means "Some specific thing that extends List, but I don't know what that specific thing is."

Therefore it is illegal to add anything to the collection, because you don't know what the specific thing is! It could be a map of LinkedLists, and here you are putting an ArrayList in it, which would cause someone else to get class cast exception when they assume it's a LinkedList.

If you want a Map that holds any type of list, just declare it as a Map of Lists, not a Map of ? extends List.

--

tl;dr - it does what you want without the wildcards. Wildcards are usually used for specifying method signatures where it doesn't matter if you know what the types are. If you just want a collection that holds subclasses, you just specify the supertype.

example:

Map<String, List<Number>> map = new HashMap<String,List<Number>>();

List<Number> integers =new ArrayList<Number>();
integers.add(Integer.valueOf(1));
map.put("integers", integers);

List<Number> floats = new LinkedList<Number>(); 
floats.add(Float.valueOf(5.2f));
map.put("floats", floats);

No casting!