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Chapter 4 Events |
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Chapter 4 Events |
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An instance of the Event class is a platform-independent representation that encapsulates the specifics of an event that happens within the Java 1.0 model. It contains everything you need to know about an event: who, what, when, where, and why the event happened. Note that the Event class is not used in the Java 1.1 event model; instead, Java 1.1 has an AWTEvent class, with subclasses for different event types.
When an event occurs, you decide whether or not to process the event. If you decide against reacting, the event passes through your program quickly without anything happening. If you decide to handle the event, you must deal with it quickly so the system can process the next event. If handling the event requires a lot of work, you should move the event-handling code into its own thread. That way, the system can process the next event while you go off and process the first. If you do not multithread your event processing, the system becomes slow and unresponsive and could lose events. A slow and unresponsive program frustrates users and may convince them to find another solution for their problems.
Event contains ten instance variables that offer all the specific information for a particular event. Instance variables
The arg field contains some data regarding the event, to be interpreted by the recipient. For example, if the user presses Return within a TextField, an Event with an id of ACTION_EVENT is generated with the TextField as the target and the string within it as the arg. See a description of each specific event to find out what its arg means.
The clickCount field allows you to check for double clicking of the mouse. This field is relevant only for MOUSE_DOWN events. There is no way to specify the time delta used to determine how quick a double-click needs to be, nor is there a maximum value for clickCount. If a user quickly clicks the mouse four times, clickCount is four. Only the passage of a system-specific time delta will reset the value so that the next MOUSE_DOWN is the first click. The incrementing of clickCount does not care which mouse button is pressed.
The evt field does not appear to be used anywhere but is available if you wish to pass around a linked list of events. Then your program can handle this event and tell the system to deal with the next one (as demonstrated in the following code), or you can process the entire chain yourself.
public boolean mouseDown (Event e, int x, int y) {
System.out.println ("Coordinates: " + x + "-" + y);
if (e.evt != null)
postEvent (e.evt);
return true;
}
The id field of Event contains the identifier of the event. The system-generated events are the following Event constants:
| WINDOW_DESTROY | MOUSE_ENTER |
| WINDOW_EXPOSE | MOUSE_EXIT |
| WINDOW_ICONIFY | MOUSE_DRAG |
| WINDOW_DEICONIFY | SCROLL_LINE_UP |
| KEY_PRESS | SCROLL_LINE_DOWN |
| KEY_RELEASE | SCROLL_PAGE_UP |
| KEY_ACTION | SCROLL_PAGE_DOWN |
| KEY_ACTION_RELEASE | SCROLL_ABSOLUTE |
| MOUSE_DOWN | LIST_SELECT |
| MOUSE_UP | LIST_DESELECT |
| MOUSE_MOVE | ACTION_EVENT |
As a user, you can create your own event types and store your own unique event ID here. In Java 1.0, there is no formal way to prevent conflicts between your events and system events, but using a negative IO is a good ad-hoc method. It is up to you to check all the user events generated in your program in order to avoid conflicts among user events.
For keyboard-related events, the key field contains the integer representation of the keyboard element that caused the event. Constants are available for the keypad keys. To examine key as a character, just cast it to a char. For nonkeyboard-related events, the value is zero.
The modifiers field shows the state of the modifier keys when the event happened. A flag is set for each modifier key pressed by the user when the event happened. Modifier keys are Shift, Control, Alt, and Meta. Since the middle and right mouse key are indicated in a Java event by a modifier key, one reason to use the modifiers field is to determine which mouse button triggered an event. See Working With Mouse Buttons in Java 1.0 for an example.
The target field contains a reference to the object that is the cause of the event. For example, if the user selects a button, the button is the target of the event. If the user moves the mouse into a Frame, the Frame is the target. The target indicates where the event happened, not the component that is dealing with it.
The when field contains the time of the event in milliseconds. The following code converts this long value to a Date to examine its contents:
Date d = new Date (e.when);
The x and y fields show the coordinates where the event happened. The coordinates are always relative to the top left corner of the target of the event and get translated based on the top left corner of the container as the event gets passed through the containing components. (See the previous Identifying the Target for an example of this translation.) It is possible for either or both of these to be outside the coordinate space of the applet (e.g., if user quickly moves the mouse outside the applet).
Numerous constants are provided with the Event class. Several designate which event happened (the why). Others are available to help in determining the function key a user pressed (the what). And yet more are available to make your life easier.
When the system generates an event, it calls a handler method for it. To deal with the event, you have to override the appropriate method. The different event type sections describe which methods you override. Key constants
These constants are set when a user presses a key. Most of them correspond to function and keypad keys; since such keys are generally used to invoke an action from the program or the system, Java calls them action keys and causes them to generate a different Event type (KEY_ACTION) from regular alphanumeric keys (KEY_PRESS).
Table 4.2 shows the constants used to represent
keys and the event type that uses each constant. The values, which are
all declared public static final int,
appear in the key variable
of the event instance. A few keys represent ASCII characters that have
string equivalents such as \n.
Black stars (
) mark the constants that are new in Java 1.1; they can be used
with the 1.0 event model, provided that you are running Java 1.1. Java
1.1 events use a different set of key constants defined in the KeyEvent
class.
| Constant | Event Type | Constant | Event Type | |
|---|---|---|---|---|
| HOME | KEY_ACTION | F9 | KEY_ACTION | |
| END | KEY_ACTION | F10 | KEY_ACTION | |
| PGUP | KEY_ACTION | F11 | KEY_ACTION | |
| PGDN | KEY_ACTION | F12 | KEY_ACTION | |
| UP | KEY_ACTION | PRINT_SCREEN |
KEY_ACTION | |
| DOWN | KEY_ACTION | SCROLL_LOCK |
KEY_ACTION | |
| LEFT | KEY_ACTION | CAPS_LOCK |
KEY_ACTION | |
| RIGHT | KEY_ACTION | NUM_LOCK |
KEY_ACTION | |
| F1 | KEY_ACTION | PAUSE |
KEY_ACTION | |
| F2 | KEY_ACTION | INSERT |
KEY_ACTION | |
| F3 | KEY_ACTION | ENTER (\n) |
KEY_PRESS | |
| F4 | KEY_ACTION | BACK_SPACE (\b) |
KEY_PRESS | |
| F5 | KEY_ACTION | TAB (\t) |
KEY_PRESS | |
| F6 | KEY_ACTION | ESCAPE |
KEY_PRESS | |
| F7 | KEY_ACTION | DELETE |
KEY_PRESS | |
| F8 | KEY_ACTION |
Modifiers are keys like Shift, Control, Alt, or Meta. When a user presses any key or mouse button that generates an Event, the modifiers field of the Event instance is set. You can check whether any modifier key was pressed by ANDing its constant with the modifiers field. If multiple modifier keys were down at the time the event occurred, the constants for the different modifiers are ORed together in the field.
public static final int ALT_MASK
public static final int CTRL_MASK
public static final int META_MASK
public static final int SHIFT_MASK
When reporting a mouse event, the system automatically sets the modifiers field. Since Java is advertised as supporting the single-button mouse model, all buttons generate the same mouse events, and the system uses the modifiers field to differentiate between mouse buttons. That way, a user with a one- or two-button mouse can simulate a three-button mouse by clicking on his mouse while holding down a modifier key. Table 4.3 lists the mouse modifier keys; an applet in Working With Mouse Buttons in Java 1.0 demonstrates how to differentiate between mouse buttons.
| Mouse Button | Modifier Key |
|---|---|
| Left mouse button | None |
| Middle mouse button | ALT_MASK |
| Right mouse button | META_MASK |
For example, if you have a three-button mouse, and click the right button, Java generates some kind of mouse event with the META_MASK set in the modifiers field. If you have a one-button mouse, you can generate the same event by clicking the mouse while depressing the Meta key.
NOTE:
If you have a multibutton mouse and do an Alt+right mouse or Meta+left mouse, the results are platform specific. You should get a mouse event with two masks set.
The component peers deliver separate key events when a user presses and releases nearly any key. KEY_ACTION and KEY_ACTION_RELEASE are for the function and arrow keys, while KEY_PRESS and KEY_RELEASE are for the remaining control and alphanumeric keys.
The peers deliver the KEY_ACTION event when the user presses a function or keypad key. The default Component.handleEvent() method calls the keyDown() method for this event. If the user holds down the key, this event is generated multiple times. If you are using the 1.1 event model, the interface method KeyListener.keyPressed() handles this event.
The peers deliver the KEY_ACTION_RELEASE event when the user releases a function or keypad key. The default handleEvent() method for Component calls the keyUp() method for this event. If you are using the 1.1 event model, the KeyListener.keyReleased() interface method handles this event.
The peers deliver the KEY_PRESS event when the user presses an ordinary key. The default Component.handleEvent() method calls the keyDown() method for this event. Holding down the key causes multiple KEY_PRESS events to be generated. If you are using the 1.1 event model, the interface method KeyListener.keyPressed() handles this event.
The peers deliver KEY_RELEASE events when the user releases an ordinary key. The default handleEvent() method for Component calls the keyUp() method for this event. If you are using the 1.1 event model, the interface method KeyListener.keyReleased() handles this event.
NOTE:
If you want to capture arrow and keypad keys under the X Window System, make sure the key codes are set up properly, using the xmodmap command.
NOTE:
Some platforms generate events for the modifier keys by themselves, whereas other platforms require modifier keys to be pressed with another key. For example, on a Windows 95 platform, if Ctrl+A is pressed, you would expect one KEY_PRESS and one KEY_RELEASE. However, there is a second KEY_RELEASE for the Control key. Under Motif, you get only a single KEY_RELEASE.
Window events
Window events happen only for components that are children of Window. Several of these events are available only on certain platforms. Like other event types, the id variable holds the value of the specific event instance.
The peers deliver the WINDOW_DESTROY event whenever the system tells a window to destroy itself. This is usually done when the user selects the window manager's Close or Quit window menu option. By default, Frame instances do not deal with this event, and you must remember to catch it yourself. If you are using the 1.1 event model, the WindowListener.windowClosing() interface method handles this event.
The peers deliver the WINDOW_EXPOSE event whenever all or part of a window becomes visible. To find out what part of the window has become uncovered, use the getClipRect() method (or getClipBounds() in Java version 1.1) of the Graphics parameter to the paint() method. If you are using the 1.1 event model, the WindowListener.windowOpening() interface method most closely corresponds to the handling of this event.
The peers deliver the WINDOW_ICONIFY event when the user iconifies the window. If you are using the 1.1 event model, the interface method WindowListener.windowIconified() handles this event.
The peers deliver the WINDOW_DEICONIFY event when the user de-iconifies the window. If you are using the 1.1 event model, the interface method WindowListener.windowDeiconified() handles this event.
The WINDOW_MOVED event signifies that the user has moved the window. If you are using the 1.1 event model, the ComponentListener.componentMoved() interface method handles this event.
The component peers deliver mouse events when a user presses or releases a mouse button. Events are also delivered whenever the mouse moves. In order to be platform independent, Java pretends that all mice have a single button. If you press the second or third button, Java generates a regular mouse event but sets the event's modifers field with a flag that indicates which button was pressed. If you press the left button, no modifiers flags are set. Pressing the center button sets the ALT_MASK flag; pressing the right button sets the META_MASK flag. Therefore, you can determine which mouse button was pressed by looking at the Event.modifiers attribute. Furthermore, users with a one-button or two-button mouse can generate the same events by pressing a mouse button while holding down the Alt or Meta keys.
NOTE:
Early releases of Java (1.0.2 and earlier) only propagated mouse events from Canvas and Container objects. With the 1.1 event model, the events that different components process are better defined.
The peers deliver the MOUSE_DOWN event when the user presses any mouse button. This action must occur over a component that passes along the MOUSE_DOWN event. The default Component.handleEvent() method calls the mouseDown() method for this event. If you are using the 1.1 event model, the MouseListener.mousePressed() interface method handles this event.
The peers deliver the MOUSE_UP event when the user releases the mouse button. This action must occur over a component that passes along the MOUSE_UP event. The default handleEvent() method for Component calls the mouseUp() method for this event. If you are using the 1.1 event model, the interface method MouseListener.mouseReleased() handles this event.
The peers deliver the MOUSE_MOVE event whenever the user moves the mouse over any part of the applet. This can happen many, many times more than you want to track, so make sure you really want to do something with this event before trying to capture it. (You can also capture MOUSE_MOVE events and without losing much, choose to deal with only every third or fourth movement.) The default handleEvent() method calls the mouseMove() method for the event. If you are using the 1.1 event model, the interface method MouseMotionListener.mouseMoved() handles this event.
The peers deliver the MOUSE_DRAG event whenever the user moves the mouse over any part of the applet with a mouse button depressed. The default method handleEvent() calls the mouseDrag() method for the event. If you are using the 1.1 event model, the interface method MouseMotionListener.mouseDragged() handles this event.
The peers deliver the MOUSE_ENTER event whenever the cursor enters a component. The default handleEvent() method calls the mouseEnter() method for the event. If you are using the 1.1 event model, the interface method MouseListener.mouseEntered() handles this event.
The peers deliver the MOUSE_EXIT event whenever the cursor leaves a component. The default handleEvent() method calls the mouseExit() method for the event. If you are using the 1.1 event model, the interface method MouseListener.mouseExited() handles this event.
The peers deliver scrolling events for the Scrollbar component. The objects that have a built-in scrollbar (like List, ScrollPane, and TextArea) do not generate these events. No default methods are called for any of the scrolling events. They must be dealt with in the handleEvent() method of the Container or a subclass of the Scrollbar. You can determine which particular event occurred by checking the id variable of the event, and find out the new position of the thumb by looking at the arg variable or calling getValue() on the scrollbar. See also the description of the AdjustmentListener interface later in this chapter.
The scrollbar peers deliver the SCROLL_LINE_UP event when the user presses the arrow pointing up for the vertical scrollbar or the arrow pointing left for the horizontal scrollbar. This decreases the scrollbar setting by one back toward the minimum value. If you are using the 1.1 event model, the interface method AdjustmentListener.adjustmentValueChanged() handles this event.
The peers deliver the SCROLL_LINE_DOWN event when the user presses the arrow pointing down for the vertical scrollbar or the arrow pointing right for the horizontal scrollbar. This increases the scrollbar setting by one toward the maximum value. If you are using the 1.1 event model, the interface method AdjustmentListener.adjustmentValueChanged() handles this event.
The peers deliver the SCROLL_PAGE_UP event when the user presses the mouse with the cursor in the area between the slider and the decrease arrow. This decreases the scrollbar setting by the paging increment, which defaults to 10, back toward the minimum value. If you are using the 1.1 event model, the interface method AdjustmentListener.adjustmentValueChanged() handles this event.
The peers deliver the SCROLL_PAGE_DOWN event when the user presses the mouse with the cursor in the area between the slider and the increase arrow. This increases the scrollbar setting by the paging increment, which defaults to 10, toward the maximum value. If you are using the 1.1 event model, the interface method AdjustmentListener.adjustmentValueChanged() handles this event.
The peers deliver the SCROLL_ABSOLUTE event when the user drags the slider part of the scrollbar. There is no set time period or distance between multiple SCROLL_ABSOLUTE events. If you are using the Java version 1.1 event model, the AdjustmentListener.adjustmentValueChanged() interface method handles this event.
The SCROLL_BEGIN event is not delivered by peers, but you may wish to use it to signify when a user drags the slider at the beginning of a series of SCROLL_ABSOLUTE events. SCROLL_END, described next, would then be used to signify the end of the series.
The SCROLL_END event is not delivered by peers, but you may wish to use it to signify when a user drags the slider at the end of a series of SCROLL_ABSOLUTE events. SCROLL_BEGIN, described previously, would have been used to signify the beginning of the series.
Two events specific to the List class are passed along by the peers. They signify when the user has selected or deselected a specific choice in the List. It is not ordinarily necessary to capture these events, because the peers deliver the ACTION_EVENT when the user double-clicks on a specific item in the List and it is this ACTION_EVENT that triggers something to happen. However, if there is reason to do something when the user has just single-clicked on a choice, these events may be useful. An example of how they would prove useful is if you are displaying a list of filenames with the ability to preview files before loading. Single selection would preview, double-click would load, and deselect would stop previewing.
No default methods are called for any of the list events. They must be dealt with in the handleEvent() method of the Container of the List or a subclass of the List. You can determine which particular event occurred by checking the id variable of the event.
The peers deliver the LIST_SELECT event when the user selects an item in a List. If you are using the 1.1 event model, the interface method ItemListener.itemStateChanged() handles this event.
The peers deliver the LIST_DESELECT event when an item in a List has been deselected. This is generated only if the List permits multiple selections. If you are using the 1.1 event model, the ItemListener.itemStateChanged() interface method handles this event.
The peers deliver focus events when a component gains (GOT_FOCUS) or loses (LOST_FOCUS) the input focus. No default methods are called for the focus events. They must be dealt with in the handleEvent() method of the Container of the component or a subclass of the component. You can determine which particular event occurred by checking the id variable of the event.
NOTE:
Early releases of Java (1.0.2 and before) did not propagate focus events on all platforms. This is fixed in release 1.1 of Java. Still, you should avoid capturing focus events if you want to write portable 1.0 code.
The peers deliver the GOT_FOCUS event when a component gets the input focus. If you are using the 1.1 event model, the FocusListener.focusGained() interface method handles this event.
The peers deliver the LOST_FOCUS event when a component loses the input focus. If you are using the 1.1 event model, the FocusListener.focusLost() interface method handles this event.
The FileDialog events are another set of nonportable events. Ordinarily, the FileDialog events are completely dealt with by the system, and you never see them. Refer to Chapter 6, Containers for exactly how to work with the FileDialog object. If you decide to create a generic FileDialog object, you can use these events to indicate file loading and saving. These constants would be used in the id variable of the specific event instance:
public static final int LOAD_FILE
public static final int SAVE_FILE
ACTION_EVENT is probably the event you deal with most frequently. It is generated when the user performs the desired action for a specific component type (e.g., when a user selects a button or toggles a checkbox). This constant would be found in the id variable of the specific event instance.
The circumstances that lead to the peers delivering the ACTION_EVENT event depend upon the component that is the target of the event and the user's platform. Although the event can be passed along differently on different platforms, users will be accustomed to how the peers work on their specific platforms and will not care that it is different on the other platforms. For example, a Java 1.0 List component on a Microsoft Windows platform allows the user to select an item by pressing the first letter of the choice, whereupon the List tries to find an item that starts with the letter. The X Window System List component does not provide this capability. It works like a normal X List, where the user must scroll to locate the item and then select it.
When the ACTION_EVENT is generated, the arg variable of the specific Event instance is set based upon the component type. In Chapters 5-11, which describe Java's GUI components, the description of each component contains an "Events" subsection that describes the value of the event's arg field. If you are using the 1.1 event model, the ActionListener.actionPerformed() and ItemListener.itemStateChanged() interface methods handle this event, depending upon the component type.
Ordinarily, the peers deliver all your events for you. However, if you are creating your own components or want to communicate across threads, it may be necessary to create your own events. You can also create your own events to notify your component's container of application-specific occurrences. For example, if you were implementing your own tab sequencing for text fields, you could create a "next text field" event to tell your container to move to the next text field. Once you create the event, you send it through the system using the Component.postEvent() method.
The first version of the constructor is the most complete and is what the other two call. It initializes all the fields of the Event to the parameters passed and sets clickCount to 0. See the descriptions of the instance variables Variables for the meanings of the arguments.
The second constructor version calls the first with arg set to null.
The final version calls the first constructor with the when, x, y, key, and modifiers parameters set to 0.
The modifier methods check to see if the different modifier mask values are set. They report the state of each modifier key at the moment an event occurred. It is possible for multiple masks to be set if multiple modifiers are pressed when the event occurs.
There is no altDown() method; to check whether the Alt key is pressed you must directly compare the event's modifiers against the Event.ALT_MASK constant. The metaDown() method is helpful when dealing with mouse events to see if the user pressed the right mouse button.
The shiftDown() method returns true if the Shift key was pressed and false otherwise. There is no way to differentiate left and right shift keys.
The controlDown() method returns true if the Control key was pressed and false otherwise.
The metaDown() method returns true if the Meta key was pressed and false otherwise.
The translate() method translates the x and y coordinates of the Event instance by x and y. The system does this so that the coordinates of the event are relative to the component receiving the event, rather than the container of the component. The system takes care of all this for you when passing the event through the containment hierarchy (not the object hierarchy), so you do not have to bother with translating them yourself. Figure 4.3 shows how this method would change the location of an event from a container down to an internal component.
When you call the toString() method of Event, the paramString() method is called in turn to build the string to display. In the event you subclass Event to add additional information, instead of having to provide a whole new toString() method, you need only add the new information to the string already generated by paramString(). Assuming the new information is foo, this would result in the following method declaration:
![[Graphic: Figure 4-3]](figs/jawt0404.gif)
protected String paramString() {
return super.paramString() + ",foo=" + foo;
}
The toString() method of Event returns a string with numerous components. The only variables that will always be in the output will be the event ID and the x and y coordinates. The others will be present if necessary (i.e., non-null): key (as the integer corresponding to a keyboard event), shift when shiftDown() is true; control, when controlDown() is true; meta, when metaDown() is true; target (if it was a Component); and arg (the value depends on the target and ID). toString() does not display all pieces of the Event information. An event when moving a Scrollbar might result in the following:
java.awt.Event[id=602,x=374,y=110,target=java.awt.Scrollbar[374, 110,15x50,val=1,vis=true,min=0,max=255,vert],arg=1]
As stated earlier, the modifiers component of Event can be used to differentiate the different mouse buttons. If the user has a multibutton mouse, the modifiers field is set automatically to indicate which button was pressed. If the user does not own a multibutton mouse, he or she can press the mouse button in combination with the Alt or Meta keys to simulate a three-button mouse. Example 4.2 is a sample program called mouseEvent that displays the mouse button selected.
import java.awt.*;
import java.applet.*;
public class mouseEvent extends Applet {
String theString = "Press a Mouse Key";
public synchronized void setString (String s) {
theString = s;
}
public synchronized String getString () {
return theString;
}
public synchronized void paint (Graphics g) {
g.drawString (theString, 20, 20);
}
public boolean mouseDown (Event e, int x, int y) {
if (e.modifiers == Event.META_MASK) {
setString ("Right Button Pressed");
} else if (e.modifiers == Event.ALT_MASK) {
setString ("Middle Button Pressed");
} else {
setString ("Left Button Pressed");
}
repaint ();
return true;
}
public boolean mouseUp (Event e, int x, int y) {
setString ("Press a Mouse Key");
repaint ();
return true;
}
}
Unfortunately, this technique does not always work. With certain components on some platforms, the peer captures the mouse event and does not pass it along; for example, on Windows, the display-edit menu of a TextField appears when you select the right mouse button. Be cautious about relying on multiple mouse buttons; better yet, if you want to ensure absolute portability, stick to a single button.
Unfortunately, there are many platform-specific differences in the way event handling works. It's not clear whether these differences are bugs or whether vendors think they are somehow improving their product by introducing portability problems. We hope that as Java matures, different platforms will gradually come into synch. Until that happens, you might want your programs to assume the lowest common denominator. If you are willing to take the risk, you can program for a specific browser or platform, but should be aware of the possibility of changes.
Appendix C, Platform-Specific Event Handling, includes a table that shows which components pass along which events by default in the most popular environments. This table was developed using an interactive program called compList, which generates a list of supported events for each component. You can find compList on this book's Web site, http://www.ora.com/catalog/javawt. If you want to check the behavior of some new platform, or a newer version of one of the platforms in Appendix C, Platform-Specific Event Handling, feel free to use compList. It does require a little bit of work on your part. You have to click, toggle, type, and mouse over every object. Hopefully, as Java matures, this program will become unnecessary.
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The Java 1.1 Event Model |
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