Primitive Data and Definite Loops - Building Java Programs: A Back to Basics Approach - page 113

Table 2.10 Analysis of Different Height Figures

Dashes in Spaces in Dots in Spaces in Dots in

Subheight

drawLine drawTop drawTop drawBottom DrawBottom

3 6 line - 1 6 - 2 * line 3 - line 2 * (line - 1)

4 8 line - 1 8 - 2 * line 4 - line 2 * (line - 1)

the height is determined by the width and vice versa. Consequently, we only need to
introduce a single class constant. Let's use the height of the hourglass halves:

public static final int SUB_HEIGHT = 4;

We've called the constant SUB_HEIGHT rather than HEIGHT because it refers to the
height of each of the two halves, rather than the figure as a whole. Notice how we use
the underscore character to separate the different words in the name of the constant.
So, how do we modify the original program to incorporate this constant? We look
through the program for any magic numbers and insert the constant or an expression
involving the constant where appropriate. For example, both the drawTop and
drawBottom methods have a for loop that executes 3 times to produce 3 lines of
output. We change this to 4 to produce 4 lines of output, and more generally, we
change it to SUB_HEIGHT to produce SUB_HEIGHT lines of output.
In other parts of the program we have to update our formulas for the number of
dashes, spaces, and dots. Sometimes we can use educated guesses to figure out how
to adjust such a formula to use the constant. If you can't guess a proper formula, you
can use the table technique to find the appropriate formula. Using this new output
with a subheight of 4, you can update the various formulas in the program. Table 2.10
shows the various formulas.
We then go through each formula and figure out how to replace it with a new formula
involving the constant. The number of dashes increases by 2 when the subheight
increases by 1, so we need a multiplier of 2. The expression 2 * SUB_HEIGHT produces
the correct values. The number of spaces in drawTop does not change with the sub-
height, so the expression does not need to be altered. The number of dots in drawTop
involves the number 6 for a subheight of 3 and the number 8 for a subheight of 4. Once
again we need a multiplier of 2, so we use the expression 2 * SUB_HEIGHT - 2 *
line . The number of spaces in drawBottom involves the value 3 for a subheight of 3
and the value 4 for a subheight of 4, so the generalized expression is SUB_HEIGHT -
line . The number of dots in drawBottom does not change when subheight changes.
Here is the new version of the program with a class constant for the subheight. It
uses a SUB_HEIGHT value of 4 , but we could change this to 3 to produce the smaller
version or to some other value to produce yet another version of the figure.

1 public class DrawFigure2 {
2 public static final int SUB_HEIGHT = 4;
3
4 public static void main(String[] args) {
5 drawLine();