Java Language Interview Question and Answers

11. What is an abstract class?
  Abstract class is a class which contain one or more abstract methods, which has to be implemented by sub classes. An abstract class can contain no abstract methods also i.e. abstract class may contain concrete methods.
12. what are class variables
  Class variables are global to a class and belong to the entire set of objects that class creates. Only one memory location is created for each variable.
13. What is the Collection interface?
  The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates.
14. What must a class do to implement an interface?
  The class must provide all of the methods in the interface and identify the interface in its implements clause.
15. What is the Collections API?
  The Collections API is a set of classes and interfaces that support operations on collections of objects.
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