extends
The wildcard declaration of List foo3 means that any of these are legal assignments:
List foo3 = new ArrayList(); // Number "extends" Number (in this context)
List foo3 = new ArrayList(); // Integer extends Number
List foo3 = new ArrayList(); // Double extends Number
Reading - Given the above possible assignments, what type of object are you guaranteed to read from List foo3:
- You can read a
Number because any of the lists that could be assigned to foo3 contain a Number or a subclass of Number.
- You can't read an
Integer because foo3 could be pointing at a List.
- You can't read a
Double because foo3 could be pointing at a List.
Writing - Given the above possible assignments, what type of object could you add to List foo3 that would be legal for all the above possible ArrayList assignments:
- You can't add an
Integer because foo3 could be pointing at a List.
- You can't add a
Double because foo3 could be pointing at a List.
- You can't add a
Number because foo3 could be pointing at a List.
You can't add any object to List because you can't guarantee what kind of List it is really pointing to, so you can't guarantee that the object is allowed in that List. The only "guarantee" is that you can only read from it and you'll get a T or subclass of T.
super
Now consider List .
The wildcard declaration of List foo3 means that any of these are legal assignments:
List foo3 = new ArrayList(); // Integer is a "superclass" of Integer (in this context)
List foo3 = new ArrayList(); // Number is a superclass of Integer
List foo3 = new ArrayList
Reading - Given the above possible assignments, what type of object are you guaranteed to receive when you read from List foo3:
- You aren't guaranteed an
Integer because foo3 could be pointing at a List or List.
- You aren't guaranteed a
Number because foo3 could be pointing at a List.
- The only guarantee is that you will get an instance of an
Object or subclass of Object (but you don't know what subclass).
Writing - Given the above possible assignments, what type of object could you add to List foo3 that would be legal for all the above possible ArrayList assignments:
- You can add an
Integer because an Integer is allowed in any of above lists.
- You can add an instance of a subclass of
Integer because an instance of a subclass of Integer is allowed in any of the above lists.
- You can't add a
Double because foo3 could be pointing at an ArrayList.
- You can't add a
Number because foo3 could be pointing at an ArrayList.
- You can't add an
Object because foo3 could be pointing at an ArrayList.
PECS
Remember PECS: "Producer Extends, Consumer Super".
"Producer Extends" - If you need a List to produce T values (you want to read Ts from the list), you need to declare it with ? extends T, e.g. List. But you cannot add to this list.
"Consumer Super" - If you need a List to consume T values (you want to write Ts into the list), you need to declare it with ? super T, e.g. List. But there are no guarantees what type of object you may read from this list.
If you need to both read from and write to a list, you need to declare it exactly with no wildcards, e.g. List.
Example
Note this example from the Java Generics FAQ. Note how the source list src (the producing list) uses extends, and the destination list dest (the consuming list) uses super:
public class Collections {
public static void copy(List dest, List src) {
for (int i = 0; i < src.size(); i++)
dest.set(i, src.get(i));
}
}
Also see
How can I add to List data structures?
