created: 09/23/99; revised 07/04/2011

Fill in the Blanks

picture of a box 1. Design of the Box Class. Objects of this class will represent a box (such as a cardboard box.) The sides of a box are rectangles. A Box will have three essential characteristics: width, height, and length. From these characteristics other values may be calculated: volume and total surface area.

Fill in the blanks in the following design for the class:

2. Checking the Design. To check the design, write a small program that uses the class. Write a program that creates a Box object with sides of 2.5, 5.0, and 6.0 inches. Write out its surface area and volume.


class BoxTester
{

  public static void main ( String[] args )
  {
     Box box = new   ;

     System.out.println( "Area: "    + box. +
                         "  volume: " + box. );

  }
}

3. Skeleton of the Class. Fill in the blanks that give the over-all design of the class.

class Box
{
  //  

  //  

  //  

}

4. Fill in Instance Variables. Fill in the data type of each instance variable.

class Box
{
  // Instance Variables
   length ;
   width ;
   height ;

  // Constructors

  // Methods

}

The variables should be floating point because integer dimensions would be too much of a restriction. Nearly always, if you use floating point you should use double.

5. Complete the Constructors. Constructors initialize the instance variables of the objects being constructed.

class Box
{
  // Instance Variables
  double length ;
  double width  ;
  double height ;

  // Constructors
  ( double , double , double   )
  {
    this.width   =   ;
    this.height  =  ;
    this.length  =  ;
  }

  // initialize all sides to the same value
  ( double  )
  {
    width   =  ;
    height  =  ;
    length  =  ;
  }


  // Methods

}

When an instance variable and a parameter use the same identifier, you specify the instance variable of the object by saying this.identifier as in the above.

6. Complete a Method. The specification for the volume( ) method says it looks like this:

// calculate the volume of the box
double  volume()

The formula to use is: volume = product of all three sides

class Box
{
  // Instance Variables
  double length ;
  double width  ;
  double height ;


  // Constructors
  Box ( double width, double height, double length )
  {
    this.width  = width ;
    this.height = height ;
    this.length = length ;
  }

  Box ( double side )
  {
    width  = side ;
    height = side ;
    length = side ;
  }

  // Methods
  double volume()
  {
    return  *  * ;
  }

}

(Of course, the three instance variables could be used in any order in the arithmetic expression.)

7. Complete the other Method. The specification for the area() method says it looks like this: double area(). There are three pairs of sides to a box. Opposite sides have the same area, so the calculation looks like this: 2 * (sum of area of each unique side)

class Box
{
  // Instance Variables
  double length ;
  double width  ;
  double height ;


  // Constructors
  Box ( double width, double height, double length )
  {
    this.width  = width ;
    this.height = height ;
    this.length = length ;
  }

  Box ( double side )
  {
    width  = side ;
    height = side ;
    length = side ;
  }

  // Methods
  double volume()
  {
    return width * height * length ;
  }

  double area()
  {
    return 2 * (  *  +  
                  *  +  
                  *  ) ;
  }

}

8. Testing Class. At this point all the coding for the definition of Box is done. Say that the small test program (class BoxTester) is in a file called BoxTester.java and that the complete definition of the Box class is in a file called Box.java.

class BoxTester
{

  public static void main ( String[] args )
  {

     // create a box with sides= 2.5, 3.0, and 5.0

     Box box1 = new  Box( 2.5, 3.0, 5.0 );

     System.out.println( "Box1 Area: "    + box1.area() + 
                         "  Volume: " + box1.volume()  );

     // create a box with all sides = 3.0

     Box box2 =   Box (  );

     System.out.println( "Box2 Area: "    + box2.area() + 
                         "  Volume: " + box2.volume()  );


  }
}

9. Compile each File. Each of the two files must be compiled:

C:\MyFiles>javac   

C:\MyFiles>javac

10. Run the Program. The java bytecode interpreter must be started with the *.class file that contains the main() method:

C:\MyFiles>java

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