15.10.3. Lower Bounded Wildcards

Lower Bounded Wildcards: <? super T> Allows writing items of type T or its supertypes.

Lower Bounded Wildcards are a feature in Java Generics that permit you to write (i.e., add) objects of a specific type (T) or its supertypes into a collection. This means that if you have a class hierarchy, such as Animal and Dog (where Dog is a subclass of Animal), you can write objects of type Dog to a collection that expects Animal or any of its superclasses. Essentially, by using <? super T>, you are saying, 'I can add items of type T or anything that is a more general type than T.'

Think of a box that can hold any type of fruit. If you state that this box can hold 'fruit' (the superclass), then you can put in apples, oranges, or even bananas. The box won't allow you to take out any specific type later (as that's not the purpose), but you can add any type of fruit into it. This is similar to how lower bounded wildcards work in Java.

Used primarily when you need to add elements to a collection.

Lower Bounded Wildcards are typically used in scenarios where you want to add elements to a collection but you want to ensure those elements are of a certain type or a supertype. For example, if you have a method that takes a List<? super Dog> as a parameter, it allows you to add Dog objects to that list, but it doesn't give any guarantees about the specific type of elements that can be read from the list. This is particularly useful in certain design patterns where you need flexibility in the types of objects you're dealing with.

Imagine a workshop where the toolbox is meant to hold any kind of tools. If the workshop manager says, 'You can place any type of tool or any older/legacy type of tools into this toolbox,' you know you can freely add hammers and screwdrivers without worrying about the specific type of tools in it. This flexibility mirrors the usage of lower bounded wildcards.

1. They restrict the type only for adding elements.
2. You cannot retrieve items in a type-safe way, as the actual type is not known.

There are two crucial aspects of lower bounded wildcards to keep in mind. Firstly, they constrain the kind of objects you can add to the collection, but they do not enforce constraints on retrieving those objects. This means while you can add specific types, you cannot guarantee what type you will get back unless you cast it, which can introduce runtime errors. Secondly, this wildcard is excellent for producer-consumer scenarios where you only need to put items into a collection, and you are not concerned about extracting them in a specific type.

If you think of a library that accepts old books and new books but doesn’t sort them when adding to a shelf, you can put any book on the shelf, but when someone asks for a book back later, you can't guarantee its particular title or genre without searching through the shelf. This situation demonstrates how lower bounded wildcards function in collections.

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