I'm new to Java, and I'm trying to figure out one thing about generics. If I declare a method like

public <T> List<T> toList(final T... arr) { ... }

Can I return both ArrayList and LinkedList? Or for example, if I have declare a method like

public <T> T[] toArray(final List<T> l) { ... }

Can I pass both ArrayList and LinkedList as argument and it'll works good?
If this is right, does it works with all objects too? So, if I create a class and I extend it more times, can I use the top class as arg of method, but the pass its subclasses when I call it?

CodePudding user response：

It's not about generics only. You can assign any object to a variable of its parent class.

Object o = 5;    // It's valid

If you pass a LinkedList or ArrayList to the toArray()method, it doesn't matter. It will be automatically converted to List. Similarly, if you return a LinkedList from the toArray() method, it doesn't matter. It will be converted to List.

But one thing to keep in mind is if you pass a LinkedList, it will get converted to List and you will be able to use only the methods of the List interface.

List list = new LinkedList();
list.addFirst(1);              // Invalid