List is an interface, which means that it defines a group or set of classes that are required to act as is defined by the "List" interface.
As Boatski said, the following is a legal declaration:
However, if you wish to instantiate or create an Object that is of type List<String>, which is read "A list of base type String", for reference, you need to create an object of one of the types that implement the List interface.
As in: AbstractList
List<String> myList = new ArrayList<String>();
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