Polymorphism: Why use "List list = new ArrayList" instead of "ArrayList list = new ArrayList"?

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Why should the interface for a Java class be prefered?

When should I use

List<Object> list = new ArrayList<Object>();

ArrayList inherits from List, so if some features in ArrayList aren't in List, then I will have lost some of the features of ArrayList, right? And the compiler will notice an error when trying to access these methods?