Java PECS
extends
The wildcard declaration of?List<? extends Number> foo3?means that any of these are legal assignments:
List<? extends Number> foo3 = new ArrayList<Number>(); // Number "extends" Number (in this context) List<? extends Number> foo3 = new ArrayList<Integer>(); // Integer extends Number List<? extends Number> foo3 = new ArrayList<Double>(); // Double extends NumberReading?- 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<Double>.
- You can't read a?Double?because?foo3?could be pointing at a?List<Integer>.
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<Double>.
- You can't add a?Double?because?foo3?could be pointing at a?List<Integer>.
- You can't add a?Number?because?foo3?could be pointing at a?List<Integer>.
You can't add any object to?List<? extends T>?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 <? super T>.
The wildcard declaration of?List<? super Integer> foo3?means that any of these are legal assignments:
List<? super Integer> foo3 = new ArrayList<Integer>(); // Integer is a "superclass" of Integer (in this context) List<? super Integer> foo3 = new ArrayList<Number>(); // Number is a superclass of Integer List<? super Integer> foo3 = new ArrayList<Object>(); // Object is a superclass of IntegerReading?- 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<Number>or?List<Object>.
- You aren't guaranteed a?Number?because?foo3?could be pointing at a?List<Object>.
- 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<Integer>.
- You can't add a?Number?because?foo3?could be pointing at an?ArrayList<Integer>.
- You can't add an?Object?because?foo3?could be pointing at an?ArrayList<Integer>.
PECS
Remember?PECS:?"Producer Extends, Consumer Super".
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"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<? extends Integer>. But you cannot add to this list.
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"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<? super Integer>. But there are no guarantees what type of object you may read from this list.
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If you need to both read from and write to a list, you need to declare it exactly with no wildcards, e.g.?List<Integer>.
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 <T> void copy(List<? super T> dest, List<? extends T> src) {for (int i = 0; i < src.size(); i++) dest.set(i, src.get(i)); } }Also see?How can I add to List<? extends Number> data structures?
From:https://stackoverflow.com/questions/4343202/difference-between-super-t-and-extends-t-in-java
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