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{ return Math.sqrt(dx*dx + dy*dy); } When the Triangle class calls the draw method, it calls this new version of draw2ndLine and draws a line to the new third point. Further, it returns that new point to the draw method so it will draw the closing side of the triangle correctly. //draws 2nd line using saved new point public Point draw2ndLine(Graphics g, Point b, Point c) { g.drawLine(b.x, b.y, newc.x, newc.y); return newc; } The Triangle Drawing Program The main program simple creates instances of the triangles you want to draw. Then, it adds them to a Vector in the TPanel class. public TriangleDrawing() { super(”Draw triangles”); TPanel tp = new TPanel(); t = new stdTriangle(new Point(10,10), new Point(150,50), new Point(100, 75)); t1 = new stdTriangle(new Point(150,100), new Point(240,40), new Point(175, 150)); tp.addTriangle(t); //add to triangle list tp.addTriangle(t1); //in the TPanel getContentPane().add(tp); setSize(300, 200); setBackground(Color.white); setVisible(true); } It is the paint routine in this class that actually draws the triangles. class TPanel extends Jpanel { Vector triangles; public TPanel() { triangles = new Vector(); //list of triangles } //——————————————-public void addTriangle(Triangle t) { triangles.addElement(t); //add more to list } //——————————————-// draw all the triangles

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super(a, b, c); } public Point draw2ndLine(Graphics g, Point a, Point b) { g.drawLine(a.x, a.y, b.x, b.y); return b; } } Drawing an Isoceles Triangle This class computes a new third data point that will make the two sides equal and length and saves that new point inside the class. public class IsocelesTriangle extends Triangle { Point newc; int newcx, newcy; int incr; public IsocelesTriangle(Point a, Point b, Point c) { super(a, b, c); double dx1 = b.x - a.x; double dy1 = b.y - a.y; double dx2 = c.x - b.x; double dy2 = c.y - b.y; double side1 = calcSide(dx1, dy1); double side2 = calcSide(dx2, dy2); if (side2 < side1) incr = -1; else incr = 1; double slope = dy2 / dx2; double intercept = c.y - slope* c.x; //move point c so that this is an isoceles triangle newcx = c.x; newcy = c.y; while(Math.abs(side1 - side2) > 1) { newcx += incr; //iterate a pixel at a time newcy = (int)(slope* newcx + intercept); dx2 = newcx - b.x; dy2 = newcy - b.y; side2 = calcSide(dx2, dy2); } newc = new Point(newcx, newcy); } //————————————-// calculate length of side private double calcSide(double dx, double dy)

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} //————————————–public void drawLine(Graphics g, Point a, Point b) { g.drawLine(a.x, a.y, b.x, b.y); } //————————————–// this routine has to be implemented //for each triangle type. abstract public Point draw2ndLine(Graphics g, Point a, Point b); //————————————–public void closeTriangle(Graphics g, Point c) { //draw back to first point g.drawLine(c.x, c.y, p1.x, p1.y); } } This Triangle class saves the coordinates of three lines, but the draw routine draws only the first and the last lines. The all important draw2ndLine method that draws a line to the third point is left as an abstract method. That way the derived class can move the third point to create the kind of rectangle you wish to draw. This is a general example of a class using the Template pattern. The draw method calls two concrete base class methods and one abstract method that must be overridden in any concrete class derived from Triangle. Another very similar way to implement the case triangle class is to include default code for the draw2ndLine method. public Point draw2ndLine(Graphics g, Point a, Point b) { g.drawLine(a.x, a.y, b.x, b.y); return b; } In this case, the draw2ndLine method becomes a Hook method that can be overridden for other classes. Drawing a Standard Triangle To draw a general triangle with no restrictions on its shape, we simple implement the draw2ndLine method in a derived stdTriangle class: public class stdTriangle extends Triangle { public stdTriangle(Point a, Point b, Point c) {

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protected method in the derived class, but Hook methods are intended to be overridden, while Concrete methods are not. 4. Finally, a Template class may contain methods which themselves call any combination of abstract, hook and concrete methods. These methods are not intended to be overridden, but describe an algorithm without actually implementing its details. Design Patterns refers to these as Template methods. Sample Code Let s consider a simple program for drawing triangles on a screen. We ll start with an abstract Triangle class, and then derive some special triangle types from it. Standard Abstract triangle Isoceles triangle Right triangle triangle Our abstract Triangle class illustrates the Template pattern: public abstract class Triangle { Point p1, p2, p3; //————————————–public Triangle(Point a, Point b, Point c) { //save p1 = a; p2 = b; p3 = c; } //————————————–public void draw(Graphics g) { //This routine draws a general triangle drawLine(g, p1, p2); Point current = draw2ndLine(g, p2, p3); closeTriangle(g, current);

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int w = getWidth(); int h = getHeight(); xfactor = (0.9f * w) / (maxX - minX); yfactor = (0.9f * h)/ (maxY - minY); xpmin = (int)(0.05f * w); ypmin = (int)(0.05f * h); xpmax = w - xpmin; ypmax = h - ypmin; repaint(); //this causes the actual plot } //————————————- protected int calcx(float xp) { return (int)((xp-minX) * xfactor + xpmin); } protected int calcy(float yp) { int ypnt = (int)((yp-minY) * yfactor); return ypmax - ypnt; } } Thus, these methods all belonged in a base PlotPanel class without any actual plotting capabilities. Note that the plot method sets up all the scaling constants and just calls repaint. The actual paint method is deferred to the derived classes. Since the JPanel class always has a paint method, we don t want to declare it as an abstract method in the base class, but we do need to override it in the derived classes. Kinds of Methods in a Template Class A Template has four kinds of methods that you can make use of in derive classes: 1. Complete methods that carry out some basic function that all the subclasses will want to use, such as calcx and calcy in the above example. These are called Concrete methods. 2. Methods that are not filled in at all and must be implemented in derived classes. In Java , you would declare these as abstract methods, and that is how they are referred to in the pattern description. 3. Methods that contain a default implementation of some operations, but which may be overridden in derived classes. These are called Hook methods. Of course this is somewhat arbitrary, because in Java you can override any public or

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THE TEMPLATE PATTERN Whenever you write a parent class where you leave one or more of the methods to be implemented by derived classes, you are in essence using the Template pattern. The Template pattern formalizes the idea of defining an algorithm in a class, but leaving some of the details to be implemented in subclasses. In other words, if your base class is an abstract class, as often happens in these design patterns, you are using a simple form of the Template pattern. Motivation Templates are so fundamental, you have probably used them dozens of times without even thinking about it. The idea behind the Template pattern is that some parts of an algorithm are well defined and can be implemented in the base class, while other parts may have several implementations and are best left to derived classes. Another main theme is recognizing that there are some basic parts of a class that can be factored out and put in a base class so that they do not need to be repeated in several subclasses. For example, in developing the PlotPanel classes we used in the Strategy pattern examples, we discovered that in plotting both line graphs and bar charts we needed similar code to scale the data and compute the x-and y pixel positions. public class PlotPanel extends JPanel { float xfactor, yfactor; int xpmin, ypmin, xpmax, ypmax; float minX, maxX, minY, maxY; float x[], y[]; Color color; //——————————————-public void setBounds(float minx, float miny, float maxx, float maxy) { minX=minx; maxX= maxx; minY=miny; maxY = maxy; } //——————————————- public void plot(float[] xp, float[] yp, Color c) { x = xp; //copy in the arrays y = yp; color = c; //and color //compute bounds and scaling factors

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Consequences of the Strategy Pattern Strategy allows you to select one of several algorithms dynamically. These algorithms can be related in an inheritance hierarchy or they can be unrelated as long as they implement a common interface. Since the Context switches between strategies at your request, you have more flexibility than if you simply called the desired derived class. This approach also avoids the sort of condition statements than can make code hard to read ad maintain. On the other hand, strategies don t hide everything. The client code must be aware that there are a number of alternative strategies and have some criteria for choosing among them. This shifts an algorithmic decision to the client programmer or the user. Since there are a number of different parameters that you might pass to different algorithms, you have to develop a Context interface and strategy methods that are broad enough to allow for passing in parameters that are not used by that particular algorithm. For example the setPenColor method in our PlotStrategy is actually only used by the LineGraph strategy. It is ignored by the BarGraph strategy, since it sets up its own list of colors for the successive bars it draws.

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} protected int calcy(float yp) { int ypnt = (int)((yp-minY) * yfactor); return ypmax - ypnt; } } The two derived classes simply implement the paint method for the two kinds of graphs. Here is the one for the Line plot. public class LinePlotPanel extends PlotPanel { public void paint(Graphics g) { int xp = calcx(x[0]); //get first point int yp = calcy(y[0]); g.setColor(Color.white); //flood background g.fillRect(0,0,getWidth(), getHeight()); g.setColor(Color.black); //draw bounding rectangle g.drawRect(xpmin, ypmin, xpmax, ypmax); g.setColor(color); //draw line graph for(int i=1; i< x.length; i++) { int xp1 = calcx(x[i]); //get n+1st point int yp1 = calcy(y[i]); g.drawLine(xp, yp, xp1, yp1); //draw line xp = xp1; //copy for next loop yp = yp1; } } } The final two plots are shown below:

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Plot Panel LinePlot BarPlot Panel Panel The base PlotPanel class contains the common code for scaling the data to the window. public class PlotPanel extends JPanel { float xfactor, yfactor; int xpmin, ypmin, xpmax, ypmax; float minX, maxX, minY, maxY; float x[], y[]; Color color; //——————————————-public void setBounds(float minx, float miny, float maxx, float maxy) { minX=minx; maxX= maxx; minY=miny; maxY = maxy; } //——————————————- public void plot(float[] xp, float[] yp, Color c) { x = xp; //copy in the arrays y = yp; color = c; //and color //compute bounds and sclaing factors int w = getWidth() - getInsets().left getInsets(). right; int h = getHeight() - getInsets().top getInsets(). bottom; xfactor = (0.9f * w) / (maxX - minX); yfactor = (0.9f * h)/ (maxY - minY); xpmin = (int)(0.05f * w); ypmin = (int)(0.05f * h); xpmax = w - xpmin; ypmax = h - ypmin; repaint(); //this causes the actual plot } //————————————-protected int calcx(float xp) { return (int)((xp-minX) * xfactor + xpmin);

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context.readData(”data.txt”); //read the data context.plot(); //plot the data } } The Line and Bar Graph Strategies The two strategy classes are pretty much the same: they set up the window size for plotting and call a plot method specific for that display panel. Here is the Line graph Strategy: public class LinePlotStrategy extends PlotStrategy { LinePlotPanel lp; public LinePlotStrategy() { super(”Line plot”); lp = new LinePlotPanel(); getContentPane().add(lp); } //————————————- public void plot(float[] xp, float[] yp) { x = xp; y = yp; //copy in data findBounds(); //sets maxes and mins setSize(width, height); setVisible(true); setBackground(Color.white); lp.setBounds(minX, minY, maxX, maxY); lp.plot(xp, yp, color); //set up plot data repaint(); //call paint to plot } } Drawing Plots in Java Since Java GUI is event-driven, you don t actually write a routine that draws lines on the screen in direct response to the plot command event. Instead you provide a panel whose paint event carries out the plotting when that event is called. The repaint() method shown above ensures that it will be called right away. We create a PlotPanel class based on JPanel and derive two classes from it for the actual line and bar plots:

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