Exercise 2

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3D Shapes

Topic Coverage

  • Composition (has-a) and inheritance (is-a) relationships
  • Interfaces

Problem Description

In this lab, we are going to explore different design options for modeling 3D objects. For instance, a cuboid is a box-shaped object having six flat sides with all right-angled corners. From the three sides of the cuboid, one can compute its volume. Moreover, a solid cuboid object is made up of a specific material, and by using the density of the material, the mass can be found.

Task

By using object-oriented modeling and applying the abstraction principle, design classes/interfaces to facilitate the creation of a 3D shape and a solid 3D shape, starting with a cuboid and solid cuboid. We then extend this concept to deal with other possible 3D shapes such as a sphere.

Level 1

Create an immutable class Cuboid with a constructor that takes in three parameters in order: height, width and length of type double. Include the method volume() that returns the volume. The string representation of a Cuboid is in the form:

cuboid [height x width x length]

with height, width and length formatted to two decimal places.

jshell> new Cuboid(2.0, 2.0, 2.0) $.. ⇒ cuboid [2.00 x 2.00 x 2.00]

jshell> new Cuboid(2.0, 2.0, 2.0).volume() $.. ⇒ 8.0

Level 2

Now create an immutable class SolidCuboid with a constructor that takes in four parameters in order: height, width, length and density of type double. Include the method mass() that returns the mass. The string representation of a SolidCuboid is in the form:

solid-cuboid [height x width x length] with a mass of mass

jshell> new SolidCuboid(2.0, 2.0, 2.0, 0.5) $.. ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

jshell> new SolidCuboid(2.0, 2.0, 2.0, 0.5).volume() $.. ⇒ 8.0

jshell> new SolidCuboid(2.0, 2.0, 2.0, 0.5).mass() $.. ⇒ 4.0

jshell> Cuboid cuboid = new SolidCuboid(2.0, 2.0, 2.0, 0.5) cuboid ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

jshell> cuboid.volume() $.. ⇒ 8.0

jshell> cuboid.mass() | Error: | cannot find symbol | symbol: method mass() | cuboid.mass() | ^---------^

Level 3

In the same way, create a Sphere and SolidSphere with constructors that take in a radius, as well as a density in the case of a SolidSphere. The following sample run depicts the implementation details.

jshell> new Sphere(2.0) $.. ⇒ sphere [2.00]

jshell> new Sphere(2.0).volume() $.. ⇒ 33.510321638291124

jshell> new SolidSphere(2.0, 0.5) $.. ⇒ solid-sphere [2.00] with a mass of 16.76

jshell> new SolidSphere(2.0, 0.5).volume() $.. ⇒ 33.510321638291124

jshell> new SolidSphere(2.0, 0.5).mass() $.. ⇒ 16.755160819145562

jshell> Sphere sphere = new SolidSphere(2.0, 0.5) sphere ⇒ solid-sphere [2.00] with a mass of 16.76

jshell> sphere.volume() $21 ⇒ 33.510321638291124

jshell> sphere.mass() | Error: | cannot find symbol | symbol: method mass() | sphere.mass() | ^---------^

Level 4

Include the Shape3D interface and establish the following is-a relationships:

  • Cuboid is a Shape3D
  • Sphere is a Shape3D

jshell> Cuboid c = new Cuboid(2.0, 2.0, 2.0) c ⇒ cuboid [2.00 x 2.00 x 2.00]

jshell> SolidCuboid sc = new SolidCuboid(2.0, 2.0, 2.0, 0.5) sc ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

jshell> Sphere s = new Sphere(2.0) s ⇒ sphere [2.00]

jshell> SolidSphere ss = new SolidSphere(2.0, 0.5) ss ⇒ solid-sphere [2.00] with a mass of 16.76

jshell> Shape3D shape = c shape ⇒ cuboid [2.00 x 2.00 x 2.00]

jshell> shape = sc shape ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

jshell> shape = s shape ⇒ sphere [2.00]

jshell> shape = ss shape ⇒ solid-sphere [2.00] with a mass of 16.76

jshell> shape.volume() $.. ⇒ 33.510321638291124

jshell> c = sc c ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

jshell> s = ss s ⇒ solid-sphere [2.00] with a mass of 16.76

jshell> new Shape3D() | Error: | Shape is abstract; cannot be instantiated | new Shape3D() | ^---------^

Level 5

Notice that it is not easy to model the four classes, Cuboid, SolidCuboid, Sphere, and SolidSphere. In particular, the mass method has the same implementation repeated in both the SolidCuboid and SolidSphere classes. This violates the abstraction principle.

We have all been taught that a solid has a definite volume and shape. We introduce a SolidImpl class with the following relationships:

  • SolidImpl has a Shape3D
  • SolidImpl has a density

jshell> SolidImpl solid = new SolidImpl(new Sphere(1.0), 0.5) solid ⇒ SolidImpl

jshell> solid.volume() $.. ⇒ 4.1887902047863905

jshell> solid.mass() $.. ⇒ 2.0943951023931953

jshell> Shape3D shape = solid shape ⇒ SolidImpl

Level 6

The class SolidImpl contains the implementation for computing the mass. It is a good candidate to be inherited from the SolidCuboid and SolidSphere classes. However, this poses a problem as these two classes are already inheriting from Cuboid and Sphere respectively!

To circumvent the issue, we can use delegation to replace the SolidImpl class, while still appearing to allow multiple inheritance.

  • Create an interface, let’s call it Solid, with the method specifications for mass and volume. The SolidImpl class can now be an implementation of Solid.
  • Let the SolidCuboid class inherit from Cuboid and implement the Solid interface.
  • Within SolidCuboid, delegate the computation of mass to a SolidImpl property in the class.

jshell> SolidCuboid solidcuboid = new SolidCuboid(2.0, 2.0, 2.0, 0.5) solidcuboid ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

jshell> solidcuboid.volume() $.. ⇒ 8.0

jshell> solidcuboid.mass() $.. ⇒ 4.0

jshell> Cuboid cuboid = solidcuboid cuboid ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

jshell> solidcuboid.volume() $.. ⇒ 8.0

jshell> solidcuboid.mass() $.. ⇒ 4.0

jshell> Shape3D shape = solidcuboid shape ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

jshell> shape.volume() $.. ⇒ 8.0

jshell> shape.mass() | Error: | cannot find symbol | symbol: method mass() | shape.mass() | ^--------^

jshell> Solid solid = solidcuboid solid ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

jshell> solid.volume() $.. ⇒ 8.0

jshell> solid.mass() $.. ⇒ 4.0

jshell> shape = cuboid shape ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

jshell> shape = solid shape ⇒ solid-cuboid [2.00 x 2.00 x 2.00] with a mass of 4.00

Do the same for the SolidSphere.