action() This is called when an interface element is manipulated, such as a Button being in Java

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action() This is called when an interface element is manipulated, such as a Button being
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mouseDown() Called when the mouse button is pressed over a component mouseDrag() Called when the mouse is moved over a component while the mouse button is
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mouseEnter() Called when the pointer enters a component mouseExit() Called when the pointer leaves a component mouseUp() Called when the mouse button is released
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The majority of the events are self-explanatory and are related to the operation of the mouse and mouse-pointer The method which most often causes confusion is action(), which is invoked by the Java system when a user interface element is manipulated The reason that confusion arises is that if the event handling is implemented within an instance of the Frame class which is parenting several user interface components, then all events caused by all of the components are sent to the action() method in the Frame It is then the responsibility of this method to determine which component has sent the event and deal with the event accordingly Of course, it is possible to have each user interface component handle events directly, but since a subclass of each class must be created for every type of element that handles events differently, the class hierarchy can soon become huge and difficult to maintain In the next section we will illustrate a simple solution to this problem Handling events using the action() method The approach that we use is to break the event down into the type of element that sent the event and then process the specific event to determine which element of that type was responsible This approach provides a reasonable and flexible approach to coding the method It retains event control in a high-level window, without pushing the code down to the subclasses and bloating the class hierarchy with large numbers of classes which exist only to handle events; this is more an issue when writing an interface for an applet, where each class must be separately loaded over the network a situation where a large number of classes can significantly delay the loading and execution of your code We have written a simple applet with an interface similar to the one in the examples so far in this section The code looks like this: import javaappletApplet; import javaawt*;
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public class evui extends Applet { Panel botPanel; public void init() { setLayout(new BorderLayout()); botPanel = new Panel(); add("South",botPanel); botPaneladd(new TextField("TextField")); botPaneladd(new Button("Button 1")); botPaneladd(new Button("Button 2")); botPaneladd(new CheckBox("Checkbox")); Choice c = new Choice(); caddItem("Choice Item 1"); caddItem("Choice Item 2"); caddItem("Choice Item 3"); botPaneladd(c); } public boolean action(Event evt, Object arg) { if (evttarget instanceof Button) { String str = (String) arg; if (strequals("Button 1")) { showStatus("First Button!!"); } else { if (strequals("Button 2")) { showStatus("Second Button!!"); } else { return false; } } return true; } else { if (evttarget instanceof CheckBox) { showStatus("CheckBox: " + (Boolean) arg); } else { if (evttarget instanceof TextField) { showStatus("TextArea: " + (String) arg); } else { return false; } } return true; } } } This example simply illustrates how to handle events Before discussing it, it is worth restating that it involves an applet and that more details on applets can be found in 10 All you will need to know about applets to understand this example is that it inherits from Component and that the method init is executed when an applet is started up We start by building a limited interface with two Buttons, a TextField and a CheckBox in a Panel Since this is an applet it inherits from Component and so has methods for add() and action() When an event is sent from any of the embedded components the default event handling methods called by handleEvent return a Boolean false This means that the events rise through the interface hierarchy until they are sent to the Applet handleEvent method, which calls our action() method as shown in the
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code above When an event is sent to this method the code tries to establish what type of object has caused the event to be sent by using the instanceOf operator against the target variable of the event This variable contains a reference to the object which sent the event Once the type of object has been determined the code then handles the individual objects The arg parameter is of type Object and what it contains will depend on the type of element that has been executed For example, if a button has been pressed it will contain a string which identifies the button, if a checkbox has been manipulated it will contain a Boolean value which will be true or false depending on whether the checkbox has been set or not Once the type of user interface element has been determined a cast is needed to extract the value of arg The arg parameter is used to see which button has been pressed The arg parameter is an arbitrary one This means that it is set according to the object which caused the event You can see how this operates when the CheckBox is altered The parameter which was cast to a String for the Buttons must be regarded as a Boolean for the CheckBox Similarly, the TextField uses the arg parameter to pass the text it holds If there were more interface components to be handled then it would make sense to pass off the event to another method responsible for handling all components of a given type, so that the method handleButtonEvent() is called once action() has identified that a button has caused the event to be sent through the system When writing code to handle events, it is worth making a big effort to select an approach which concentrates the handlers in a limited number of components without creating huge methods which will prove difficult to maintain The event mechanism is very flexible and can be used to create useful event handlers with only minimal coding, but the way that the handlers are placed against components is of paramount importance for the long-term maintenance of the code
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