More OOP: Exercise 2

Using everything you've learned so far about creating OOP 
classes, create a new class called Cylinder:

Cylinder
----------------------
- radius : double
- height : double
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+ Cylinder()
+ Cylinder(radius, height)
+ getArea() : double 
+ getVolume() : double 
+ displayCylinder() : String

Make sure you include appropriate accessor and mutator methods.

The default radius is 1; the default height is 1.
Both radius and height must be more than 0.
If either radius or height receives an invalid value, 
use the default value.

The surface area of a cylinder = 2 x PIr^2 + height x 2PIr
(2 x circle area + area of rectangle -> height * circumference)
The volume of a cylinder is PIr^2 x height

The displayCylinder() method returns a string representation 
of a cylinder in the form:
Cylinder: radius=r.r, height=h.h
(where r.r is the radius h.h is the height, both formatted 
to 1 decimal place)

Test your Cylinder in a separate Main class:
- create a default instance of cylinder and display it on the screen
- change the cylinder's radius to 2 and height to 1.5
- display the cylinder's area and volume
- create a second cylinder with a radius of 2.5 and height of .5
- display the second cylinder on the screen
- set the second cylinder's radius to a negative value
- display the second cylinder on the screen again
- set the second cylinder's height to a negative value
- display the second cylinder on the screen again