CSCI 1913: Introduction to Algorithms-Computer Laboratory 6

Data Structures, and Program Design 
0.  Introduction.

In this lab assignment, you will extend some simple Java classes that represent plane figures from elementary geometry. The object of this assignment is not to do anything useful, but rather to demonstrate how methods can be inherited by extending classes.

1.  Theory.

A polygon is a closed plane figure with three or more sides, all of which are line segments. The perimeter of a polygon is the sum of the lengths of its sides. A rectangle is a polygon with exactly four sides that meet at 90° angles. Like a polygon, it has a perimeter. It also has an area, the product of its base and height. A square is a rectangle whose sides are all the same length. Like a rectangle, it has a perimeter and an area.

      Polygons, rectangles, and squares make up an is-a hierarchy. The hierarchy gets its name because a square is-a rectangle, and a rectangle is-a polygon. Is-a hierarchies can be easily modeled by Java classes using the extends keyword.

2. Implementation.

The following is the source code for a Java class whose instances represent polygons. The file Polygon.java on Canvas contains a copy of this source code. You will need it to complete the laboratory assignment.

class Polygon  

{    private int[] sideLengths;  

  

  public Polygon(int sides, int ... lengths)  

  {      int index = 0;  

    sideLengths = new int[sides];      for (int length: lengths)  

    {  

      sideLengths[index] = length;        index += 1;  

    }  

  }  

  

  public int side(int number)  

  {  

    return sideLengths[number];  

  }  

  

  public int perimeter()  

  {      int total = 0;  

    for (int index = 0; index < sideLengths.length; index += 1)  

    {  

      total += side(index);  

    }  

    return total;  

  }  

}

The class Polygon uses a private array called sideLengths to store the lengths of a polygon’s sides. The array’s length, sideLengths.length, is the number of sides that the polygon has. The class Polygon also has a has a public constructor and two public methods. To keep things simple, they do not check their arguments for correctness, as they would if Polygon was part of a real program.       The constructor takes four or more arguments and returns an instance of Polygon that represents a polygon. The first argument is the number of sides that the polygon has. The remaining arguments are the lengths of those sides. For example, the Java statement:

Polygon triangle = new Polygon(3, 3, 4, 5);

declares the variable triangle and sets it to an instance of Polygon that represents a triangle (because the first argument says it has 3 sides). The lengths of the triangle’s sides are 3, 4, and 5.

      The three dots ‘...’ in the constructor mean that it can take zero or more extra integer arguments after its first argument. The forloop with the colon visits the extra arguments one at a time. Don’t worry if those parts of Java are unfamiliar. You don’t have to know how the constructor works, only how to call it.

      The method side returns the length of a polygon’s side. Sides are numbered starting from 0. For example, the expression triangle.side(0) returns 3, the expression triangle.side(1) returns 4, and the expression triangle.side(2) returns 5.

      The method perimeter returns a polygon’s perimeter, the sum of the lengths of its sides. For example, the expression triangle.perimeter() returns 3 + 4 + 5 = 12.

      For this assignment, you must write a class called Rectangle. As its name suggests, each instance of Rectangle must represent a rectangle. Along with a constructor, Rectangle must provide two methods, called area and perimeter. The method area must return the integer area of the rectangle, and the method perimeter must return the integer perimeter of the rectangle.

      You must also write another class, called Square. As its name suggests, each instance of Square must represent a square. Along with a constructor, Square must provide two methods, called area and perimeter. The method area must return the integer area of the square, and the method perimeter must return the integer perimeter of the square.

      The following driver program shows examples of how the constructors and methods of Rectangle and Square must work.

class Shapes  

{    public static void main(String[] args)  

  {  

    Rectangle wreck = new Rectangle(3, 5);  //  Make a 3 × 5 rectangle.  

    System.out.println(wreck.area());       //  Print its area, 15.  

    System.out.println(wreck.perimeter());  //  Print its perimeter, 16.  

  

    Square nerd = new Square(7);            //  Make a 7 × 7 square.  

    System.out.println(nerd.area());        //  Print its area, 49.  

    System.out.println(nerd.perimeter()     //  Print its perimeter, 28.    }  

}

Your classes Rectangle and Square must use the extends keyword, so they will inherit methods from other classes. Also, each class must inherit as many of its methods as possible from those other classes. YOU WILL LOSE POINTS FOR DEFINING A METHOD INSIDE A

CLASS, IF IT COULD HAVE BEEN INHERITED FROM ANOTHER CLASS!

      Here’s a hint about how to write the constructors for Rectangle and Square. Suppose that a class Triangle extends the class Polygon. Then Polygon is the superclass of Triangle. The keyword super can be used to call the constructor that belongs to a superclass. For example, Triangle’s constructor, which takes the lengths of a triangle’s three sides, might look like this.

public Triangle(int a, int b, int c)  

{    super(3, a, b, c);  }

It uses Polygon’s constructor to make a polygon with 3 sides, whose lengths are a, b, and c. If super is used in this way, then it must be the first statement in the constructor. You don’t have to write Triangle—this was only an example!