19.11        (ComplexMatrix) Use the Complex  class  introduced  in Programming Exercise 13.17 to develop the ComplexMatrix class for performing matrix opera- tions involving complex numbers. The ComplexMatrix class should extend the GenericMatrix class and implement the add, multiple, and zero methods. You need to modify GenericMatrix and replace every occurrence of Number by Object, because Complex is not a subtype of Number. Write a test program that creates the following two matrices and displays the result of addition and multiplica- tion of the matrices by invoking the printResult method.

 

4.     To allow duplicate elements to be stored in a collection, you need to use a list. A list not only can store duplicate elements but also allows the user to specify where they are stored. The user can access elements by an index.

 

5.     Two types of lists are supported: ArrayList and LinkedList. ArrayList is a resizable-array implementation of the List interface. All the methods in ArrayList are defined in List. LinkedList is a linked-list implementation of the List interface. In addition to implementing the List interface, this class provides the methods for retrieving, inserting, and removing elements from both ends of the list.

 

6.     Comparator can be used to compare the objects of a class that doesn’t implement

Comparable.

 

7.     The Vector class extends the AbstractList class. Starting with Java 2, Vector has been the same as ArrayList, except that the methods for accessing and modifying the vector are synchronized. The Stack class extends the Vector class and provides several methods for manipulating the stack.

 

8.     The Queue interface represents a queue. The PriorityQueue class implements Queue

for a priority queue.