3: DECLARATIONS in Java

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CHAPTER 3: DECLARATIONS
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Example 31
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A JavaBean
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public class Light { // Properties: private int noOfWatts; private String location; private boolean indicator;
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// wattage // placement // on or off
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// Setters public void setNoOfWatts(int noOfWatts) { thisnoOfWatts = noOfWatts; } public void setLocation(String location) { thislocation = location; } public void setIndicator(boolean indicator) { thisindicator = indicator; } // Getters public int getNoOfWatts() { return noOfWatts; } public String getLocation() { return location; } public boolean isIndicator() { return indicator; } }
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Naming Patterns for the Event Model
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A listener is an object that is interested in being notified when a particular event takes place The origin of this event is usually an object called the source, which notifies interested listeners when the event occurs In this setup, a listener can be added to or removed from the list of listeners notified by a source about the occurrence of a particular event This setup is the basis of the event model which is depicted in Figure 31 The JavaBean specification stipulates naming patterns for the event model to facilitate its use by builder tools to assemble event-based applications Figure 31 shows where the naming patterns for handling events of type X are applied: An event class with the name XEvent, that extends the javautilEventObject class
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public class XEvent extends javautilEventObject { public XEvent(Object source) { super(source); } }
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A listener interface with the name XListener, that specifies the specific method to be called in a listener when an event of the type XEvent occurs The listener interface extends the javautilEventListener interface
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public interface XListener extends javautilEventListener { public void methodAInXListener(XEvent ev); }
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A listener interested in XEvents must implement the XListener interface, and must be registered with the source in order to be informed about XEvents
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32: JAVABEANS STANDARD Figure 31 The Event Model
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addXListener(listener)
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source addXListener(XListener l) removeXListener(XListener l) methodAInXListener( ) : XEvent
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event X
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interface javautilEventObject
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listener methodAInXListener(XEvent e)
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interface XListener methodAInXListener(XEvent e)
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interface javautilEventListener
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A listener interested in XEvent events is registered with the source using the addXListener() method The listener must implement the XListener interface in order to recieve events of type XEvent The listener is informed about events of type XEvent via the methodAInXListener() in the XListener interface
public class ListenerObject implements XListener { public void methodAInXListener(XEvent e) { /* */ } }
A source for XEvent, that implements the methods addXListener() and removeXListener() These methods are used to add or remove a listener interested in XEvents, respectively The parameter of these methods is of the type XListener
public class SourceObject { public synchronized void addXListener(XListener listener) { /* */ } public synchronized void removeXListener(XListener listener) { /* */ } }
Note that there are no naming patterns defined for the names of the source and the listener classes Neither is there any standard convention for naming the methods specified in the listener interface
CHAPTER 3: DECLARATIONS
33 Method Declarations
The general syntax of a method declaration is <method modifiers> <formal type parameter list> <return type> <method name> (<formal parameter list>) <throws clause> // Method header
{ // Method body
<local variable declarations> <nested local class declarations> <statements>
In addition to the name of the method, the method header can specify the following information: scope or accessibility modifier (see Section 49, p 138) additional method modifiers (see Section 410, p 146) a formal type parameter list, if the declaration is for a generic method (see Section 148, p 697) the type of the return value, or void if the method does not return any value (see Section 64, p 228) a formal parameter list (see below) checked exceptions thrown by the method are specified in a throws clause (see Section 69, p 257) The formal parameter list is a comma-separated list of parameters for passing information to the method when the method is invoked by a method call (see Section 37, p 81) An empty parameter list must be specified by ( ) Each parameter is a simple variable declaration consisting of its type and name: <parameter modifier> <type> <parameter name> The parameter names are local to the method (see Section 46, p 131) The parameter modifier final is discussed in Section 37 on page 89 The signature of a method comprises the method name and the formal parameter list only The method body is a block containing the local declarations and the statements of the method Local variable declarations are discussed in Section 23 on page 31, and nested local class declarations in Section 84 on page 371 Like member variables, member methods can be characterized as: instance methods static methods, which are discussed in Section 410, p 148