1. Write a pair of classes, Square1 and Rectangle1. Define Square1 as a subclass of Rectangle1. In addition to setters and getters, provide such methods as computeArea and computePerimeter. Specify the preconditions, postconditions and class invariants, if any, as comments (

The java program including classes is shown below.

import java.util.*;

import java.lang.*;

//base class

class Rectangle1

{

   //variables to hold dimensions and area

   static double l;

   static double b;

   static double a;

   //setter

   static void setWidth(double w)

   {

       b=w;

   }

   //getter

   static double getWidth()

   {

       return b;

   }

   //setter

   static void setLength(double h)

   {

       l=h;

   }

   //getter

   static double getLength()

   {

       return b;

   }

   //area calculation for rectangle

   static void computeArea()

   {

       a = getLength()*getWidth();

       System.out.println("Area of rectangle = " +a);

   }

}

//derived class

class Square1 extends Rectangle1

{

   //variables to hold dimensions and perimeter

   static double s;

   static double peri;

   //settter

   static void setSide(double d)

   {

       s=d;

   }

   //getter

   static double getSide()

   {

       return s;

   }

   //perimeter calculation for square

   static void computePerimeter()

   {

       peri = 2*(getSide()+getSide());

       System.out.println("Perimeter of square = "+peri);

   }

}

public class MyClass {

   public static void main(String args[]) {

     //object of derived class created

     Square1 ob = new Square1();

     ob.setLength(1);

     ob.setWidth(4);

     ob.setSide(1);

     ob.computeArea();

     ob.computePerimeter();

   }

}

Explanation:

1. The class for rectangle is defined having variables to hold the dimensions of a rectangle.

2. The getters and setters are included for both the dimensions.

3. The method to compute the area of the rectangle is defined.

4. The class for square is defined having variables to hold the side of a square.

5. The getter and setter are included for the dimension of the square.

6. The method to compute the perimeter of the square is defined.

7. The class Square1 is the derived class which inherits the class Rectangle1 through the keyword extends.

8. All the variables, getters, setters and methods are declared as static.

9. Another class is defined having only the main() method.

10. Inside main(), the object of the derived class, Square1, is created.

11. The setters and the methods are called through this object.

12. The value of the dimensions are passed as arguments to the setters.

13. The output is attached in an image.

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