Consequences of the Observer Pattern in Java

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Consequences of the Observer Pattern
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The Observer pattern promotes abstract coupling to Subjects A Subject doesn't know the details of any of its Observers This has the potential disadvantage of successive or repeated updates to the Observers when there is a series of incremental changes to the data If the cost of these updates is high, introducing some sort of change management might be necessary so that the Observers are not notified too soon or too often When one client changes the underlying data, you need to decide which object will initiate the notification of the change to the other Observers If the Subject notifies all of the Observers when it is changed, each client is not responsible for remembering to initiate the notification However, this can result in a number of small successive updates being triggered If the clients tell the Subject when to notify the other clients, this cascading notification can be avoided; however, the clients are left with the responsibility to tell the Subject when to send the notifications If one client "forgets," the program simply won't work properly Finally, you can specify the kind of notification to send by defining a number of update methods for the Observers to receive, depending on the type or scope of change In some cases, the clients will thus be able to ignore some of these notifications
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1 Rewrite the Watch2Windows program in this chapter to use the javautilObserver interface 2 Write a program using the Observable class and Observable interface
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Programs on the CD-ROM
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Program Description \Observer\Watch2Windowsjava Illustrates the Observer pattern between a main window and two observing windows \Observer\Ltest\Ltestjava A simple example of how a JList observes a JListData object
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23 The State Pattern
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The State pattern allows you to have an object represent the state of your application and to switch application states by switching objects For example, you can have an enclosing class switch between a number of related contained classes and pass method calls on to the current contained class Design Patterns suggests that the State pattern switches between internal classes in such a way that the enclosing object appears to change its class In Java, at least, this is a bit of an exaggeration, but the actual purpose to which the classes are put can change significantly
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Many programmers havwe had the experience of creating a class that performs slightly different computations or displays different information based on the arguments passed into the class This often leads to some sort of switch or if-else statement inside of the class that determines which behavior to carry out It is this inelegance that the State partern seeks to replace
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Let's consider the case of a drawing program similar to the one we develoed for the Momento class in 21 This program has toolbar buttoms for Selectr, Rectangle, Fill, Circle, and Clear, as shown in Figure 231 Figure 231 A simple drawing program for illustrating the State pattern
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Each toolbar button does something different when it is selected and the mouse is clicked or dragged across the screen Thus the state of the graphical editor affects the behavior that the program should exhibit This suggests some sort of design using the State pattern Initially, we might design our program like this, with a Mediator managing the actions of five command buttons, as shown in Figure 232
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Figure 232 One possible interaction between the classes needed to support the simple drawing program
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However, this initial design puts the entire burden of maintaining the state of the program on the Mediator, when the main purpose of a Mediator is to coordinate activities between various controls, such as the buttons Keeping the state of the buttons and the desired mouse activity inside the Mediator can make it unduly complicated as well as lead to a set of if or switch tests, which make the program difficult to read and maintain Further, this set of large, monolithic conditional statements might have to be repeated for each action that the Mediator interprets, such as mouseUp, mouseDrag, rightClick, and so on This, too, makes the program very hard to read and maintain Instead, let's analyze the expected behavior for each button 1 If the Pick button is selected, then clicking inside a drawing element should cause it to be highlighted or appear with handles If the mouse is dragged and a drawing element is already selected, the element should move on thescreen 2 If the Rect button is selected, then clicking on the screen should cause a new rectangle drawing element to be created 3 If the Fill button is selected and a drawing element is already selected, then that element should be filled with the current color If no drawing is selected, then clicking inside a drawing should cause it to be filled with the current color 4 If the Circle button is selected, then clicking on the screen should cause a new circle drawing element to be created 5 If the Clear button is selected, then all of the drawing elements are removed Several of these actions share some common threads Four use the mouse click event to cause actions, and one uses the mouse drag event to cause an action Thus we really want to create a system that can help us redirect these events based on which button is currently selected Let's create a State object that handles mouse activities
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public class State { public void mouseDown(int x, int y) { } public void mouseUp(int x, int y) { } public void mouseDrag(int x, int y) { } }
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We include the mouseUp event in case we need it later Because none of the cases described need all of these events, we give the base class empty methods rather than create an abstract base class Then we create four derived State classes forPick, Rect, Circle, and Fill, and put instances of all of them inside a StateManager class that sets the current state and executes methods on that State object In Design Patterns, this StateManager class is called a Context object, as illustrated in Figure 233 Figure 233 A StateManager class that keeps track of the current state
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A typical State object simply overrides those event methods that it must handle specially For example, following is the complete Rectangle state object:
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public class RectState extends State { private Mediator med; //save the Mediator here public RectState(Mediator md) { med = md; } //create a new Rectangle where mouse clicks public void mouseDown(int x, int y) { medaddDrawing(new visRectangle(x, y));
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The RectState object tells the Mediator to add a rectangle to the drawing list Similarly, the Circle state object tells the Mediator to add a circle to the drawing list
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public class CircleState extends State { private Mediator med; //save the Mediator public CircleState(Mediator md) { med = md; } //draw a circle where the mouse clicks public void mouseDown(int x, int y) { medaddDrawing(new visCircle(x, y)); } }
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The only tricky button is the Fill button because we have defined two actions forit: 1 If an object is already selected, fill it 2 If the mouse is clicked inside an object, fill the object To carry out these tasks, we need to add the select method to the base State class This method is called when each toolbar button is selected
public class State { public void mouseDown(int x, int y) { } public void mouseUp(int x, int y) { } public void mouseDrag(int x, int y) { } public void select (Drawing d, Color c) { } }
The Drawing argument is either the currently selected Drawing or null if none is selected, and the color is the current fill color In this program, we have arbitrarily set the fill color to red So the Fill state class is as follows:
public class FillState extends State { private Mediator med; //state the Mediator private Color color; //save the current color public FillState(Mediator md) { med = md; } //fill the drawing if selected public void select(Drawing d, Color c) { color = c; if (d!= null) { dsetFill(c); //fill that drawing } } //fill the drawing if you click inside of one public void mouseDown (int x, int y) { Vector drawings = medgetDrawings(); for (int i = 0; i < drawingssize(); i++) { Drawing d = (Drawing)drawingselementAt(i); if (dcontains(x, y)) dsetFill(color); //fill the drawing }