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Figure 5. Program output for Line01 for a 400x150 world.

Missing image

Draw the BLUE line

The code in Listing 3 initializes several variables and then calls a method named drawLine to implement the steps listed above to draw the BLUE line shown in Figure 3 .

Listing 3 . Draw the BLUE line. double xScale = 1.0*world.getWidth()/2; double yScale = 1.0*world.getHeight()/2;//Draw a line in BLUE. turtle.setPenColor(Color.BLUE);double slope = 1.0; double yIntercept = 0.0;drawLine(xScale,yScale,slope,yIntercept);

The method named drawLine

Once again, I will put the run method on hold while we examine the code that actually draws the line as shown in Listing 4 .

Listing 4 . The method named drawLine. void drawLine(double xScale,double yScale, double slope,double yIntercept){double yVal = 0; int row = 0;int col = 0; double xVal = -1.0;for(int cnt=0; cnt<=100;cnt++,xVal += 0.02){ //Get a y-value for a given x-value.yVal = function(xVal,slope,yIntercept);//Scale the x and y values to match the plotting surface col = (int)(xVal*xScale);row = (int)(yVal*yScale); //Move to the first point without drawing a line because the// pen is not down. Translate the origin to the center in the // process.turtle.moveTo(col + world.getWidth()/2, row + world.getHeight()/2);//Lower the pen in order to draw a line from each point to the// next point. turtle.penDown();}//end for loop }//end drawLine method

If you examine the code in Listing 4 along with values assigned to the variables in Listing 3 , you should be able to see the correlation between the code and the steps given earlier . In particular, you should be able to see how this code produces the BLUE lineshown in Figure 3 .

Note that the code in Listing 4 calls the method named function (shown in Listing 2 ) to get the values that define the line for the given slope and the given y-intercept value.

Repeat the process to draw two more lines

Returning to the run method, the code in Listing 5 repeats the process twice to draw the GREEN line and the BLACK line shown in Figure 3 for different slope and y-intercept values.

Listing 5 . Repeat the process to draw two more lines. //Draw another line in GREEN. turtle.penUp();turtle.setPenColor(Color.GREEN); slope = -0.5;yIntercept = 0.5; drawLine(xScale,yScale,slope,yIntercept);//Draw another line in BLACK.turtle.penUp(); turtle.setPenColor(Color.BLACK);slope = 2.0; yIntercept = -0.5;drawLine(xScale,yScale,slope,yIntercept); }//end run method

Listing 5 also signals the end of the run method and the end of the program.

A parabola

The program named Parabola01 shown in Listing 16 produces the graphic output shown in Figure 6 .

Figure 6. Graphic output from the program named Parabola01.

Missing image

The method named function for Parabola01

The method named function for the program named Parabola01 is shown in Listing 6 .

This method evaluates and returns the y-value for each incoming x-value for a parabola with no offsets centered at the origin as defined by the following equation:

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Source:  OpenStax, Object-oriented programming (oop) with java. OpenStax CNX. Jun 29, 2016 Download for free at https://legacy.cnx.org/content/col11441/1.201
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