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Adapters in Java
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In a broad sense, a number of adapters are already built into the Java language In this case, the Java adapters serve to simplify an unnecessarily complicated event interface One of the most commonly used of these Java adapters is the WindowAdapter class One inconvenience of Java is that windows do not close automatically when you click on the Close button or window Exit menu item The general solution to this problem is to have your main Frame window implement the WindowListener interface and leave all of the Window events empty except for windowClosing
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public void mainFrame extends Frame implements WindowListener { //frame listens for window events public void mainFrame() { addWindowListener(this); } public void windowClosing(WindowEvent wEvt) { Systemexit(0); //exit on System exit box click } public void windowClosed(WindowEvent wEvt) {} public void windowOpened(WindowEvent wEvt) {} public void windowIconified(WindowEvent wEvt) {} public void windowDeiconified(WindowEvent wEvt) {} public void windowActivated(WindowEvent wEvt) {} public void windowDeactivated(WindowEvent wEvt) {} }
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As you can see, this code is awkward and hard to read The Window Adapter class is provided to simplify this procedure This class contains empty implementations of all seven of the previous WindowEvents You then need only to override the windowClosing event and insert the appropriate exit code One such simple program is shown next
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//illustrates using the WindowAdapter class //make an extended window adapter which //This class closes the frame //when the closing event is received class WindAp extends WindowAdapter { public void windowClosing(WindowEvent e) { Systemexit(0); } } //============== public class Closer extends Frame { public Closer() { WindAp windap = new WindAp(); addWindowListener(windap); setSize(new Dimension(100,100)); setVisible(true); } static public void main(String argv[]) { new Closer(); } }
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You can, however, make a much more compact but less readable version of the same code by using an anonymous inner class
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//create window listener for window close click addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e) {Systemexit(0);} });
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Adapters like these are common in Java when a simple class can be used to encapsulate a number of events They include ComponentAdapter, ContainerAdapter, FocusAdapter, KeyAdapter, MouseAdapter, and MouseMotionAdapter
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lightbulb Thought Question
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1 How would you write a class adapter to make the JFC JTable look like a two-column list box
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Programs on the CD-ROM
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Program \Adapter\TwoLists\Twolistjava \Adapter\JTwoLists\JTwoListjava A class adapter version \Adapter\JtwoClassList\ JTwoClassListjava Illustrates reflection methods Description An AWT version of two lists An object adapter version
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\Adapter\showMethodsjava In addition, the Window adapter is illustrated in both the JTwoList and JTwoClassList programs on the accompanying CD-ROM
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10 The Bridge Pattern
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At first sight, the Bridge pattern looks much like the Adapter pattern in that a class is used to convert one kind of interface to another However, the Adapter pattern is intended to make one or more classes' interfaces look the same as that of a particular class By contrast, the Bridge pattern is designed to separate a class's interface from its implementation so that you can vary or replace the implementation without changing the client code The participants in the Bridge pattern are
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the Abstraction, which defines the class's interface, the Refined Abstraction, which extends and implements that interface, the Implementor, which defines the interface for the implementation classes, and the ConcreteImplementors, which are the implementation classes
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Suppose that you have a program that displays a list of products in a window The simplest interface for that display is a JList box But once a significant number of products have been sold, you might want to display the products in a table along with their sales figures Since we have just discussed the adapter pattern, you think immediately of the class-based adapter, where we adapt the fairly elaborate interface of the JList to our simpler needs in this display In simple programs, this will work fine, but as we'll see, there are limits to that approach Further, let's suppose that we need to produce two kinds of displays from our product data, a customer view that is just the list of products we've mentioned and an executive view that also shows the number of units shipped We'll display the product list in an ordinary JList box and the executive view in a JTable display These two displays are the implementations of the display classes, as shown in Figure 101 Figure 101 Two displays of the same information using a Bridge pattern
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Now, we want to define a single, simple interface that remains the same regardless of the type and complexity of the actual implementation classes We'll start by defining an abstract Bridger class
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public abstract class Bridger extends JPanel { public abstract void addData(Vector v); }
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This class is so simple that it receives only a Vector of data and passes it on to the display classes
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On the other side of the bridge are the implementation classes, which usually have a more elaborate and somewhat lower-level interface Here we'll have them add the data lines to the display one at a time
public interface visList { public void addLine(String s); public void removeLine(int num); }
Implicit in the definition of these classes is some mechanism for determining which part of each string is the name of the product and which part is the quantity shipped In this simple example, we separate the quantity from the name with two dashes and parse these apart within the list classes The Bridge between the interface on the left and the implementation on theright is the listBridge class, which instantiates one or the other of the list display classes Note that it extends the Bridger class for the use of the application program
public class listBridge extends Bridger { protected visList list; public listBridge(visList jlist) { setLayout(new BorderLayout()); list = jlist; add("Center", (JComponent)list); } public void addData(Vector v) { for (int i = 0; i < vsize(); i++) { String s = (String)velementAt (i); listaddLine (s); } } }
Then, at the top programming level, we just create instances of a table and a list using the listBridge class
//add in customer view as list box listBridge lList = new listBridge(new productList()); lListadd Data (prod); pleftadd("North", new JLabel("Customer view")); pleftadd("Center," lList); //add in executive view as table listBridge lTable = new listBridge(new productTable()); lTableaddData (prod); prightadd("North", new JLabel("Executive view")); prightadd("Center", lTable);