Explanation of the get-put principle
I have read O'Reilly book, in that I came to know this get-put principle.
- Use an
extends
wildcard when you only get values out of a structure.- Use a
super
wildcard when you only put values into a structure.- And don't use a wildcard when you both want to get and put from/to a structure.
Exceptions are:
You cannot put anything into a type declared with an
extends
wildcard except for the valuenull
, which belongs to every reference type.You cannot get anything out from a type declared with an
super
wildcard except for a value of typeObject
, which is a super type of every reference type.
Can anyone help me to explore this rule at depth? If possible, please put them hierarchical manner.
Consider a bunch of bananas. This is a Collection<? extends Fruit>
in that it's a collection of a particular kind of fruit - but you don't know (from that declaration) what kind of fruit it's a collection of. You can get an item from it and know it will definitely be a fruit, but you can't add to it - you might be trying to add an apple to a bunch of bananas, which would definitely be wrong. You can add null
to it, as that will be a valid value for any kind of fruit.
Now consider a fruitbowl. This is a Collection<? super Banana>
, in that it's a collection of some type "greater than" Banana
(for instance, Collection<Fruit>
or Collection<TropicalFruit>
). You can definitely add a banana to this, but if you fetch an item from the bowl you don't know what you'll get - it may well not be a banana. All you know for sure is that it will be a valid (possibly null
) Object
reference.
(In general, for Java generics questions, the Java Generics FAQ is an excellent resource which contains the answer to almost anything generics-related you're likely to throw at it.)