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1 Use the Strategy pattern in a program to read in text and display it in a JTextField either in a monospaced font without wrapping or in a proportional font with wrapping set
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Program \Strategy\StrategyPlotjava Description Selects algorithms for drawing data as line or bar plots
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25 The Template Pattern
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In this chapter, we take up the Template pattern a very simple pattern that you will discover you use all the time Whenever you write a parent class and leave one or more of the methods to be implemented by derived classes, you are in essence using the Template pattern This pattern formalizes the idea of defining an algorithm in a class, while 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 We note that Design Patterns refers to this as the Template Method Pattern, but we shorten the name in this chapter for convenience
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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 the 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 idea 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 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
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public class PlotPanel extends JPanel { private float xfactor, yfactor; private int xpmin, ypmin, xpmax, ypmax; private float minX, maxX, minY, maxY; private float x[], y[]; private 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 the bounds and scaling factors int w = getWidth() - getInsets()left - getInsets()right; int h = getHeight() - getInsets()top getInsets()bottom; xfactor = (09f * w) /(maxX - minX); yfactor = (09f * h)/ (maxY - minY); xpmin = (int)(005f * w); ypmin = (int)(005f * h);
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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; } }
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Thus these methods all belong in a base PlotPanel class without any actual plotting capabilities Note that the plot method sets up all of the scaling constants and then 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
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Kinds of Methods in a Template Class
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As set out in Design Patterns, the Template pattern has four kinds of methods that you can make use of in derived classes: 1 Complete methods that carry out some basic function that all of the subclasses will want to use, such as calcx and calcy in the previous 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 protected method in the derived class But Hook methods are intended to be overridden, whereas Concrete methods are not 4 Finally, a Template class may contain methods that call any combination of abstract, hook, and concrete methods These methods arenot intended to be overridden but describe an algorithm without actually implementing its details Design Patterns refers to these as Template methods
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