Question

Write an inheritance hierarchy for classes Quadrilateral, Trapezoid, Parallelogram, Rectangle and Square. Use Quadrilateral as the superdass of the hierarchy. Create and use a Point class to represent the points in each shape. Make the hierarchy as deep (i.e., as many levels) as possible. Specify the instance variables and methods for each class. The private instance variables of Quadrilateral should be the \(x\) -y coordinate pairs for the four endpoints of the Quadrilateral. Write a program that instantiates objects of your dasses and outputs each object's area (except Quadri lateral).

Short answer

The hierarchy is Point, Quadrilateral, Trapezoid, Parallelogram, Rectangle, and Square. Point class has x, y coordinates; Quadrilateral has Points for vertices; Trapezoid computes area with the trapezoidal formula; Parallelogram, Rectangle, and Square use base times height (Rectangle and Square inherit the area method). A test program creates instances and prints areas for all but Quadrilateral.

Step-by-step solution

Define Point Class

Create a Point class to encapsulate the coordinates of a point. This class will have two private instance variables to represent the x and y coordinates, a constructor to initialize these variables, and getter methods to retrieve the values of these variables.

Define Quadrilateral Class

Define the Quadrilateral class as the superclass for all shapes in the hierarchy. This class will have four private instance variables of type Point to represent the four vertices of the quadrilateral. Include a constructor to initialize these Points. Since the area cannot be directly determined for a generic Quadrilateral, an area computation method is not defined in this class.

Define Trapezoid Class

Define the Trapezoid class that extends Quadrilateral. Include specific instance variables if needed for trapezoid properties, such as the lengths of the parallel sides. Implement a method to calculate the area of a trapezoid using trapezoidal formula. The constructor should call the super constructor to set the points.

Define Parallelogram Class

Define the Parallelogram class that extends Quadrilateral. It inherits the vertices from the superclass. Implement a method to calculate the area of the parallelogram that could use, for example, the base times the height. The constructor should call the super constructor to set the points.

Define Rectangle Class

Define the Rectangle class that extends Parallelogram since a rectangle is a specific type of parallelogram with right angles. This class will inherit all properties of a parallelogram and the area method will be adequately defined by the area of the parallelogram method since the logic is the same (base times height).

Define Square Class

Define the Square class that extends Rectangle. Since a square is a rectangle with all sides equal, you won't need additional instance variables, and you can use the Rectangle's method to calculate the area (side times side). The constructor should validate that all sides are equal.

Write a Test Program

Create a test program where objects of your classes are instantiated with given points. Call the area computation methods for the Trapezoid, Parallelogram, Rectangle, and Square objects, and print out the results. The Quadrilateral class objects shouldn't have their areas calculated or printed.

Key concepts

Object-Oriented Programming

Object-Oriented Programming (OOP) is a programming paradigm centered around objects rather than actions. In Java, OOP is foundational, influencing the way programs are structured and written. Understanding OOP concepts is vital to design effective Java programs and creates an organized, modular, and reusable codebase.

A cornerstone of OOP is the use of classes, which are blueprints for creating objects. These objects can have attributes (data fields) and behaviors (methods or functions). The OOP philosophy emphasizes encapsulation, inheritance, and polymorphism. Encapsulation means that object data is often kept private within the class and is accessible through public methods. Inheritance allows a new class to absorb properties of an existing class, reducing redundancy. Polymorphism enables a single interface to be used for different underlying data types.

The exercise provided utilizes inheritance to create a class hierarchy where different geometric shapes inherit from a more general Quadrilateral class. Thus, changing the Quadrilateral class affects all shapes in the hierarchy, illustrating the power of OOP's structure and maintenance efficacy.

Java Class Design

Java Class Design is a meticulous process that determines how a class is structured to encapsulate data and behaviors. In the context of the exercise, the class design involves the inheritance hierarchy for geometric shapes. Proper class design is fundamental here, as it eases understanding and managing complex relationships between classes.

When designing a class, it's crucial to consider the appropriate access level for variables and methods, whether it's private, protected, or public. Using the least access necessary hides implementation details and preserves class invariants. The design also should aim to make classes extensible where appropriate. For example, the Quadrilateral class in the exercise needs to be a versatile superclass, potentially abstract, to allow for multiple specific shapes that derive from it without being able to instantiate Quadrilateral directly because it represents a general concept.

In Java, the constructor is a key feature of class design, initializing new objects with essential data. The exercise employs constructors extensively, propagating points for each specific shape type through the hierarchy.

Geometry in Java

Geometry in Java leverages the concept of inheritance in Object-Oriented Programming to encapsulate the properties of geometric shapes. Java can model complex geometric relationships through class hierarchies, making coding these relationships more intuitive.

For instance, each geometric class might define a method to calculate its area. In the case of the shapes in the exercise, the Square class can inherit the method to calculate the area from the Rectangle class, because a square is a special case of a rectangle. This approach reduces duplication of code and centralizes the logic for area calculation, thus making it easy to manage and modify if needed.

The 'Point' class in the solution is another concept of geometry in Java. It represents the coordinates in a plane, which the shapes utilize to define their vertices. Since all shapes are composed of points, this class is fundamental across the object hierarchy.

Encapsulation

Encapsulation, in Java, is a principle that binds together the data and functions that manipulate the data and keeps both safe from outside interference and misuse. Data encapsulation leads to data abstraction, which provides a protective shield that prevents the data from being accessed by the code outside this shield.

Technically, encapsulation means all the internal representation of an object is hidden from the view outside the object’s definition. In the solution, the 'Point' class encapsulates the x and y coordinates with private accessors, and these can only be accessed through getter methods, providing a controlled way of accessing the variables. Similarly, the quadrilateral's vertices are private, ensuring that they can't be changed arbitrarily, preserving the integrity of the shape's definition.

Encapsulation is fundamental in class design as it ensures an object's internal state cannot be tampered with directly, promotes decoupling, and increases the flexibility and maintainability of the code.