java - Explanation of the get-put principle

Question

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 value null, which belongs to every reference type.

• You cannot get anything out from a type declared with an super wildcard except for a value of type Object, 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.

Answer 1

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.)