The key to understanding why this works is rather simple: if you can only take items from a collection, then using a collection of Strings and reading Objects from it is fine.

In other words, the wildcard with an extends-bound ( upper bound) makes the type covariant. In return for this limitation, you get the desired behavior: Collection is a subtype of Collection.

This means that you can safely read E's from items (elements of this collection are instances of a subclass of E), but cannot write to it as you don't know what objects comply with that unknown subtype of E. I don't know why this design wasn't chosen in Java (given its close to relation to Smalltalk), but it may have something to do with simplifying the type system (note that most languages with classes-as-objects tend to be dynamic languages).The wildcard type argument ? extends E indicates that this method accepts a collection of objects of E or a subtype of E, not just E itself. In that case, 'class methods' are actually just normal instance methods of the class's metaclass. In Ruby, classes are instances of the Class class (and yes, that means that Class is an instance of itself). in Smalltalk classes are instances of a parallel hierarchy of metaclasses. Classes aren't objects, but objects are objects (duh!), so the question should rather be: why would a language not use companion objects?Īnother aspect is simplicity: why have two things, objects with instance members and classes with static members when you can just have objects with instance members?Īn alternative that is used in many Smalltalk-derived languages, is to make classes themselves objects. In an object-oriented language, something not being an object is an extremely crippling restriction.

Kotlin with keyword