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Q17.What is transient variable?
Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.

Q18.What is Collection API?
The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.

Q19.Is Iterator a Class or Interface? What is its use?
Iterator is an interface which is used to step through the elements of a Collection.

Q20.What is similarities/difference between an Abstract class and Interface?
Differences are as follows:
• Interfaces provide a form of multiple inheritance. A class can extend only one other
class.
• Interfaces are limited to public methods and constants with no implementation.
Abstract classes can have a partial implementation, protected parts, static methods,
etc.
• A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
• Interfaces are slow as it requires extra indirection to to find corresponding method
in in the actual class. Abstract classes are fast.
Similarities:
• Neither Abstract classes or Interface can be instantiated.