At runtime List<Double>, List<? extends Number>, List<?> and List<Object> are all identical. The generic parameter is not compiled at all. All the magic with generics is all compile time fun. This also means that if you have a List that is empty you have no idea what the generic parameter is!
Try not to think of the generic parameter as a "Subtype", the generic parameter really means "the class uses a generic parameter" so in this case "the list uses a Number". A good example of this is the HashMap source, if you have a look into the inner workings of it, it is actually storing an array of Entry, and the entries all have keys and values stored on them. When you look at more complex uses of generics you occasionally see this sort of use.
In the situation of a List the generic parameter means that the list stores that type of object, it could be that the object never stores an object of the type of the generic parameter at all! Like this:
public class DummyIterator<O> implements Iterator<O>{
public boolean hasNext() {
return false;
}
public O next() {
return null;
}
}
What does List<? extends Number> actually mean? Well it is pretty much the same as List<Number> for most uses. Keep in mind though that by saying ? you are pretty much saying 'I don't care about the type' which shows itself in this situation:
List<Double> doubles = new ArrayList<Double>();
List<? extends Number> numbers = doubles;
numbers.add(new Double(1)); //COMPILE ERROR
Number num = numbers.get(0);
So we can't add a Double to a <? extends Number>. But for this example:
List<Double> doubles = new ArrayList<Double>();
List<Number> numbers = doubles; //COMPILE ERROR
numbers.add(new Integer(1));
Number num = numbers.get(0);
You can't assign the List<Double> to a List<Number> which makes sense as you are specifically telling it, that lists use only Number types
So where should you use a ?? well really anywhere you could say "I don't care about the generic parameter" so for instance:
boolean equalListSizes(List<?> list1, List<?> list2) {
return list1.size() == list2.size();
}
You would use the ? extends Number type of format only where you are not modifying the object using the generic parameter. so for instance:
Number firstItem(List<? extends Number> list1) {
return list1.get(0);
}
Instead of using the ? and ? extends Number formats try using generics on the class / method instead, in most cases it makes your code more readable as well!:
<T extends Number> T firstItem(List<T> list1) {
return list1.get(0);
}
Class:
class Animal{}
class Dog extends Animal{}
class AnimalHouse<A extends Animal> {
List<A> animalsInside = new ArrayList<A>();
void enterHouse(A animal){
animalsInside.add(A);
}
A leaveHouse() {
return animalsInside.remove(0);
}
}
AnimalHouse<Dog> ah = new AnimalHouse<Dog>();
ah.enterHouse(new Dog());
Dog rufus = ah.leaveHouse();
As a bonus thought around generics you can also parameterise methods to return a particular class. A good example of this is the any() method in junit and the empty list collection:
Dog rufus = Matchers.<Dog>any();
List<Dog> dogs = Collections.<Dog>emptyList();
This syntax allows you to specify the return type of an object. Sometimes quite useful to know (makes some casting redundant)!
Double和Integer都存储在相同的List<? extends Number>列表中。List<Double>和List<Integer>都是List<? extends Number>，但是List<? extends Number>不等于List<Number>。 - aioobeDog可能是继承自Animal的。 - aioobe