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 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.

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.)