Java abstract base class

List is declared as an abstract interface , not a class. If your class implements a List, you are expected to provide implementations for all of the abstract List methods. If you don’t, you will get errors like: – The type MyList must implement the inherited abstract method.

List is an interface because of how general it is. It does not assume anything about implementation. AbstractSequentialList and AbstractList on the other hand do assume certain ways of element access. Hence they’re implemented as abstract classes .

In computer science, a list or sequence is an abstract data type that represents a finite number of ordered values, where the same value may occur more than once.

Since List is an interface you need to instantiate a concrete implementation of the interface in order to use it. You can choose between the following List implementations in the Java Collections API: java. util.