Exercise P4.4 of Page 174:

Write a program that prompts the user for two numbers, then prints

• The sum
• The average
• The distance (absolute value of the difference)
               (Go to Math class to search an absolute function)
• The maximum (the larger of the two)

To do so, implement a class

public class Pair
{
  /**
     Constructs a pair.
     @param aFirst the first value of the pair
     @param aSecond the second value of the pair
  */
  public Pair(double aFirst, double aSecond)
  {
     ...
  }
  /**
     Computes the sum of the valies of this pair.
     @return the sum of the first and second values
  */
  public double getSum()
  {
     ...
  }
  ...
}

Then implement s class PairTester that constructs a Pair object,
invokes its methods, and prints the results.