Section 6.4
Mouse Events
Events are central to programming for a graphical user interface. A GUI program doesn't have a main() routine that outlines what will happen when the program is run, in a step-by-step process from beginning to end. Instead, the program must be prepared to respond to various kinds of events that can happen at unpredictable times and in an order that the program doesn't control. The most basic kinds of events are generated by the mouse and keyboard. The user can press any key on the keyboard, move the mouse, or press a button on the mouse. The user can do any of these things at any time, and the computer has to respond appropriately.
In Java, events are represented by objects. When an event occurs, the system collects all the information relevant to the event and constructs an object to contain that information. Different types of events are represented by objects belonging to different classes. For example, when the user presses one of the buttons on a mouse, an object belonging to a class called MouseEvent is constructed. The object contains information such as the source of the event (that is, the component on which the user clicked), the (x,y) coordinates of the point in the component where the click occurred, the exact time of the click, and which button on the mouse was pressed. When the user presses a key on the keyboard, a KeyEvent is created. After the event object is constructed, it is passed as a parameter to a designated method. By writing that method, the programmer says what should happen when the event occurs.
As a Java programmer, you get a fairly high-level view of events. There is a lot of processing that goes on between the time that the user presses a key or moves the mouse and the time that a subroutine in your program is called to respond to the event. Fortunately, you don't need to know much about that processing. But you should understand this much: Even though your GUI program doesn't have a main() routine, there is a sort of main routine running somewhere that executes a loop of the form
while the program is still running:
Wait for the next event to occur
Call a subroutine to handle the event
This loop is called an event loop. Every GUI program has an event loop. In Java, you don't have to write the loop. It's part of "the system." If you write a GUI program in some other language, you might have to provide a main routine that runs the event loop.
In this section, we'll look at handling mouse events in Java, and we'll cover the framework for handling events in general. The next section will cover keyboard-related events and timer events. Java also has other types of events, which are produced by GUI components. These will be introduced in Section 6.6.
6.4.1 Event Handling
For an event to have any effect, a program must detect the event and react to it. In order to detect an event, the program must "listen" for it. Listening for events is something that is done by an object called an event listener. An event listener object must contain instance methods for handling the events for which it listens. For example, if an object is to serve as a listener for events of type MouseEvent, then it must contain the following method (among several others):
public void mousePressed(MouseEvent evt) { . . . }
The body of the method defines how the object responds when it is notified that a mouse button has been pressed. The parameter, evt, contains information about the event. This information can be used by the listener object to determine its response.
The methods that are required in a mouse event listener are specified in an interface named MouseListener. To be used as a listener for mouse events, an object must implement this MouseListener interface. Java interfaces were covered in Subsection 5.7.1. (To review briefly: An interface in Java is just a list of instance methods. A class can "implement" an interface by doing two things. First, the class must be declared to implement the interface, as in "class MouseHandler implements MouseListener" or "class MyApplet extends JApplet implements MouseListener". Second, the class must include a definition for each instance method specified in the interface. An interface can be used as the type for a variable or formal parameter. We say that an object implements the MouseListener interface if it belongs to a class that implements the MouseListener interface. Note that it is not enough for the object to include the specified methods. It must also belong to a class that is specifically declared to implement the interface.)
Many events in Java are associated with GUI components. For example, when the user presses a button on the mouse, the associated component is the one that the user clicked on. Before a listener object can "hear" events associated with a given component, the listener object must be registered with the component. If a MouseListener object, mListener, needs to hear mouse events associated with a Component object, comp, the listener must be registered with the component by calling
comp.addMouseListener(mListener);
The addMouseListener() method is an instance method in class Component, and so can be used with any GUI component object. In our first few examples, we will listen for events on a JPanel that is being used as a drawing surface.
The event classes, such as MouseEvent, and the listener interfaces, such as MouseListener, are defined in the package java.awt.event. This means that if you want to work with events, you should either include the line "import java.awt.event.*;" at the beginning of your source code file or import the individual classes and interfaces.
Admittedly, there is a large number of details to tend to when you want to use events. To summarize, you must
- Put the import specification "import java.awt.event.*;" (or individual imports) at the beginning of your source code;
- Declare that some class implements the appropriate listener interface, such as MouseListener;
- Provide definitions in that class for the methods specified by the interface;
- Register the listener object with the component that will generate the events by calling a method such as addMouseListener() in the component.
Any object can act as an event listener, provided that it implements the appropriate interface. A component can listen for the events that it itself generates. A panel can listen for events from components that are contained in the panel. A special class can be created just for the purpose of defining a listening object. Many people consider it to be good form to use anonymous inner classes to define listening objects (see Subsection 5.7.3). You will see all of these patterns in examples in this textbook.
6.4.2 MouseEvent and MouseListener
The MouseListener interface specifies five different instance methods:
public void mousePressed(MouseEvent evt); public void mouseReleased(MouseEvent evt); public void mouseClicked(MouseEvent evt); public void mouseEntered(MouseEvent evt); public void mouseExited(MouseEvent evt);
The mousePressed method is called as soon as the user presses down on one of the mouse buttons, and mouseReleased is called when the user releases a button. These are the two methods that are most commonly used, but any mouse listener object must define all five methods; you can leave the body of a method empty if you don't want to define a response. The mouseClicked method is called if the user presses a mouse button and then releases it, without moving the mouse. (When the user does this, all three routines -- mousePressed, mouseReleased, and mouseClicked -- will be called in that order.) In most cases, you should define mousePressed instead of mouseClicked. The mouseEntered and mouseExited methods are called when the mouse cursor enters or leaves the component. For example, if you want the component to change appearance whenever the user moves the mouse over the component, you could define these two methods.
As a first example, we will look at a small addition to the RandomStringsPanel example from the previous section. In the new version, the panel will repaint itself when the user clicks on it. In order for this to happen, a mouse listener should listen for mouse events on the panel, and when the listener detects a mousePressed event, it should respond by calling the repaint() method of the panel. Here is an applet version of the ClickableRandomStrings program for you to try; when you click the applet, a new set of random strings is displayed:
For the new version of the program, we need an object that implements the MouseListener interface. One way to create the object is to define a separate class, such as:
import java.awt.Component;
import java.awt.event.*;
/**
* An object of type RepaintOnClick is a MouseListener that
* will respond to a mousePressed event by calling the repaint()
* method of the source of the event. That is, a RepaintOnClick
* object can be added as a mouse listener to any Component;
* when the user clicks that component, the component will be
* repainted.
*/
public class RepaintOnClick implements MouseListener {
public void mousePressed(MouseEvent evt) {
Component source = (Component)evt.getSource();
source.repaint(); // Call repaint() on the Component that was clicked.
}
public void mouseClicked(MouseEvent evt) { }
public void mouseReleased(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }
}
This class does three of the four things that we need to do in order to handle mouse events: First, it imports java.awt.event.* for easy access to event-related classes. Second, it is declared that the class "implements MouseListener". And third, it provides definitions for the five methods that are specified in the MouseListener interface. (Note that four of the five event-handling methods have empty definitions. We really only want to define a response to mousePressed events, but in order to implement the MouseListener interface, a class must define all five methods.)
We must do one more thing to set up the event handling for this example: We must register an event-handling object as a listener with the component that will generate the events. In this case, the mouse events that we are interested in will be generated by an object of type RandomStringsPanel. If panel is a variable that refers to the panel object, we can create a mouse listener object and register it with the panel with the statements:
RepaintOnClick listener = new RepaintOnClick(); // Create MouseListener object. panel.addMouseListener(listener); // Register MouseListener with the panel.
Once this is done, the listener object will be notified of mouse events on the panel. When a mousePressed event occurs, the mousePressed() method in the listener will be called. The code in this method calls the repaint() method in the component that is the source of the event, that is, in the panel. The result is that the RandomStringsPanel is repainted with its strings in new random colors, fonts, and positions.
Although we have written the RepaintOnClick class for use with our RandomStringsPanel example, the event-handling class contains no reference at all to the RandomStringsPanel class. How can this be? The mousePressed() method in class RepaintOnClick looks at the source of the event, and calls its repaint() method. If we have registered the RepaintOnClick object as a listener on a RandomStringsPanel, then it is that panel that is repainted. But the listener object could be used with any type of component, and it would work in the same way.
Similarly, the RandomStringsPanel class contains no reference to the RepaintOnClick class -- in fact, RandomStringsPanel was written before we even knew anything about mouse events! The panel will send mouse events to any object that has registered with it as a mouse listener. It does not need to know anything about that object except that it is capable of receiving mouse events.
The relationship between an object that generates an event and an object that responds to that event is rather loose. The relationship is set up by registering one object to listen for events from the other object. This is something that can potentially be done from outside both objects. Each object can be developed independently, with no knowledge of the internal operation of the other object. This is the essence of modular design: Build a complex system out of modules that interact only in straightforward, easy to understand ways. Then each module is a separate design problem that can be tackled independently. Java's event-handling framework is designed to offer strong support for modular design.
To make this clearer, consider the application version of the ClickableRandomStrings program. I have included RepaintOnClick as a nested class, although it could just as easily be a separate class. The main point is that this program uses the same RandomStringsPanel class that was used in the original program, which did not respond to mouse clicks. The mouse handling has been "bolted on" to an existing class, without having to make any changes at all to that class:
import java.awt.Component;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.JFrame;
/**
* Displays a window that shows 25 copies of the string "Java!" in
* random colors, fonts, and positions. The content of the window
* is an object of type RandomStringsPanel. When the user clicks
* the window, the content of the window is repainted, with the
* strings in newly selected random colors, fonts, and positions.
*/
public class ClickableRandomStringsApp {
public static void main(String[] args) {
JFrame window = new JFrame("Random Strings");
RandomStringsPanel content = new RandomStringsPanel();
content.addMouseListener( new RepaintOnClick() ); // Register mouse listener.
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(100,75);
window.setSize(300,240);
window.setVisible(true);
}
private static class RepaintOnClick implements MouseListener {
public void mousePressed(MouseEvent evt) {
Component source = (Component)evt.getSource();
source.repaint();
}
public void mouseClicked(MouseEvent evt) { }
public void mouseReleased(MouseEvent evt) { }
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }
}
}
6.4.3 Mouse Coordinates
Often, when a mouse event occurs, you want to know the location of the mouse cursor. This information is available from the MouseEvent parameter to the event-handling method, which contains instance methods that return information about the event. If evt is the parameter, then you can find out the coordinates of the mouse cursor by calling evt.getX() and evt.getY(). These methods return integers which give the x and y coordinates where the mouse cursor was positioned at the time when the event occurred. The coordinates are expressed in the coordinate system of the component that generated the event, where the top left corner of the component is (0,0).
The user can hold down certain modifier keys while using the mouse. The possible modifier keys include: the Shift key, the Control key, the Alt key (called the Option key on the Mac), and the Meta key (called the Command or Apple key on the Mac). You might want to respond to a mouse event differently when the user is holding down a modifier key. The boolean-valued instance methods evt.isShiftDown(), evt.isControlDown(), evt.isAltDown(), and evt.isMetaDown() can be called to test whether the modifier keys are pressed.
You might also want to have different responses depending on whether the user presses the left mouse button, the middle mouse button, or the right mouse button. Now, not every mouse has a middle button and a right button, so Java handles the information in a peculiar way. It treats pressing the right button as equivalent to holding down the Meta key while pressing the left mouse button. That is, if the right button is pressed, then the instance method evt.isMetaDown() will return true (even if the Meta key is not pressed). Similarly, pressing the middle mouse button is equivalent to holding down the Alt key. In practice, what this really means is that pressing the right mouse button under Windows or Linux is equivalent to holding down the Command key while pressing the mouse button on the Mac. A program tests for either of these by calling evt.isMetaDown().
As an example, consider a JPanel that does the following: Clicking on the panel with the left mouse button will place a red rectangle on the panel at the point where the mouse was clicked. Clicking with the right mouse button (or holding down the Command key while clicking on a Mac) will place a blue oval on the applet. Holding down the Shift key while clicking will clear the panel by removing all the shapes that have been placed. Here is an applet version of the program:
There are several ways to write this example. There could be a separate class to handle mouse events, as in the previous example. However, in this case, I decided to let the panel itself respond to mouse events. Any object can be a mouse listener, as long as it implements the MouseListener interface. In this case, the panel class implements the MouseListener interface, so the object that represents the main panel of the program can be the mouse listener for the program. The constructor for the panel class contains the statement
addMouseListener(this);
which is equivalent to saying this.addMouseListener(this). Now, the ordinary way to register a mouse listener is to say X.addMouseListener(Y) where Y is the listener and X is the component that will generate the mouse events. In the statement addMouseListener(this), both roles are played by this; that is, "this object" (the panel) is generating mouse events and is also listening for those events. Although this might seem a little strange, you should get used to seeing things like this. In a large program, however, it's usually a better idea to write a separate class to do the listening in order to have a more organized division of responsibilities.
The source code for the panel class is shown below. You should check how the instance methods in the MouseEvent object are used. You can also check for the Four Steps of Event Handling ("import java.awt.event.*", "implements MouseListener", definitions for the event-handling methods, and "addMouseListener"):
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
/**
* A simple demonstration of MouseEvents. Shapes are drawn
* on a black background when the user clicks the panel. If
* the user Shift-clicks, the applet is cleared. If the user
* right-clicks the applet, a blue oval is drawn. Otherwise,
* when the user clicks, a red rectangle is drawn. The contents of
* the panel are not persistent. For example, they might disappear
* if the panel is resized or is covered and uncovered.
*/
public class SimpleStamperPanel extends JPanel implements MouseListener {
/**
* This constructor simply sets the background color of the panel to be black
* and sets the panel to listen for mouse events on itself.
*/
public SimpleStamperPanel() {
setBackground(Color.BLACK);
addMouseListener(this);
}
/**
* Since this panel has been set to listen for mouse events on itself,
* this method will be called when the user clicks the mouse on the panel.
* This method is part of the MouseListener interface.
*/
public void mousePressed(MouseEvent evt) {
if ( evt.isShiftDown() ) {
// The user was holding down the Shift key. Just repaint the panel.
// Since this class does not define a paintComponent() method, the
// method from the superclass, JPanel, is called. That method simply
// fills the panel with its background color, which is black. The
// effect is to clear the panel.
repaint();
return;
}
int x = evt.getX(); // x-coordinate where user clicked.
int y = evt.getY(); // y-coordinate where user clicked.
Graphics g = getGraphics(); // Graphics context for drawing directly.
// NOTE: This is considered to be bad style!
if ( evt.isMetaDown() ) {
// User right-clicked at the point (x,y). Draw a blue oval centered
// at the point (x,y). (A black outline around the oval will make it
// more distinct when shapes overlap.)
g.setColor(Color.BLUE); // Blue interior.
g.fillOval( x - 30, y - 15, 60, 30 );
g.setColor(Color.BLACK); // Black outline.
g.drawOval( x - 30, y - 15, 60, 30 );
}
else {
// User left-clicked (or middle-clicked) at (x,y).
// Draw a red rectangle centered at (x,y).
g.setColor(Color.RED); // Red interior.
g.fillRect( x - 30, y - 15, 60, 30 );
g.setColor(Color.BLACK); // Black outline.
g.drawRect( x - 30, y - 15, 60, 30 );
}
g.dispose(); // We are finished with the graphics context, so dispose of it.
} // end mousePressed();
// The next four empty routines are required by the MouseListener interface.
// They don't do anything in this class, so their definitions are empty.
public void mouseEntered(MouseEvent evt) { }
public void mouseExited(MouseEvent evt) { }
public void mouseClicked(MouseEvent evt) { }
public void mouseReleased(MouseEvent evt) { }
} // end class SimpleStamperPanel
Note, by the way, that this class violates the rule that all drawing should be done in a paintComponent() method. The rectangles and ovals are drawn directly in the mousePressed() routine. To make this possible, I need to obtain a graphics context by saying "g = getGraphics()". After using g for drawing, I call g.dispose() to inform the operating system that I will no longer be using g for drawing. It is a good idea to do this to free the system resources that are used by the graphics context. I do not advise doing this type of direct drawing if it can be avoided, but you can see that it does work in this case, and at this point we really have no other way to write this example.
6.4.4 MouseMotionListeners and Dragging
Whenever the mouse is moved, it generates events. The operating system of the computer detects these events and uses them to move the mouse cursor on the screen. It is also possible for a program to listen for these "mouse motion" events and respond to them. The most common reason to do so is to implement dragging. Dragging occurs when the user moves the mouse while holding down a mouse button.
The methods for responding to mouse motion events are defined in an interface named MouseMotionListener. This interface specifies two event-handling methods:
public void mouseDragged(MouseEvent evt); public void mouseMoved(MouseEvent evt);
The mouseDragged method is called if the mouse is moved while a button on the mouse is pressed. If the mouse is moved while no mouse button is down, then mouseMoved is called instead. The parameter, evt, is an object of type MouseEvent. It contains the x and y coordinates of the mouse's location. As long as the user continues to move the mouse, one of these methods will be called over and over. (So many events are generated that it would be inefficient for a program to hear them all, if it doesn't want to do anything in response. This is why the mouse motion event-handlers are defined in a separate interface from the other mouse events: You can listen for the mouse events defined in MouseListener without automatically hearing all mouse motion events as well.)
If you want your program to respond to mouse motion events, you must create an object that implements the MouseMotionListener interface, and you must register that object to listen for events. The registration is done by calling a component's addMouseMotionListener() method. The object will then listen for mouseDragged and mouseMoved events associated with that component. In most cases, the listener object will also implement the MouseListener interface so that it can respond to the other mouse events as well.
Here is a small sample applet that displays information about mouse events. It is programmed to respond to any of the seven different kinds of mouse events by displaying the coordinates of the mouse, the type of event, and a list of the modifier keys that are down (Shift, Control, Meta, and Alt). Experiment to see what happens when you use the mouse on the applet.
The source code for the program can be found in SimpleTrackMousePanel.java, which defines the panel that is used as the content pane, and in SimpleTrackMouseApplet.java, which defines the applet class. The panel class includes a nested class, MouseHandler, that defines the mouse-handling object. I encourage you to read the source code. You should now be familiar with all the techniques that it uses.
It is interesting to look at what a program needs to do in order to respond to dragging operations. In general, the response involves three methods: mousePressed(), mouseDragged(), and mouseReleased(). The dragging gesture starts when the user presses a mouse button, it continues while the mouse is dragged, and it ends when the user releases the button. This means that the programming for the response to one dragging gesture must be spread out over the three methods! Furthermore, the mouseDragged() method can be called many times as the mouse moves. To keep track of what is going on between one method call and the next, you need to set up some instance variables. In many applications, for example, in order to process a mouseDragged event, you need to remember the previous coordinates of the mouse. You can store this information in two instance variables prevX and prevY of type int. It can also be useful to save the starting coordinates, where the original mousePressed event occurred, in instance variables. I also suggest having a boolean variable, dragging, which is set to true while a dragging gesture is being processed. This is necessary because in many applications, not every mousePressed event starts a dragging operation to which you want to respond. The mouseDragged and mouseReleased methods can use the value of dragging to check whether a drag operation is actually in progress. You might need other instance variables as well, but in general outline, a class that handles mouse dragging looks like this:
import java.awt.event.*;
public class MouseDragHandler implements MouseListener, MouseMotionListener {
private int startX, startY; // Point where the original mousePress occurred.
private int prevX, prevY; // Most recently processed mouse coords.
private boolean dragging; // Set to true when dragging is in process.
. . . // other instance variables for use in dragging
public void mousePressed(MouseEvent evt) {
if ( we-want-to-start-dragging ) {
dragging = true;
startX = evt.getX(); // Remember starting position.
startY = evt.getY();
prevX = startX; // Remember most recent coords.
prevY = startY;
.
. // Other processing.
.
}
}
public void mouseDragged(MouseEvent evt) {
if ( dragging == false ) // First, check if we are
return; // processing a dragging gesture.
int x = evt.getX(); // Current position of Mouse.
int y = evt.getY();
.
. // Process a mouse movement from (prevX, prevY) to (x,y).
.
prevX = x; // Remember the current position for the next call.
prevY = y;
}
public void mouseReleased(MouseEvent evt) {
if ( dragging == false ) // First, check if we are
return; // processing a dragging gesture.
dragging = false; // We are done dragging.
.
. // Other processing and clean-up.
.
}
}
As an example, let's look at a typical use of dragging: allowing the user to sketch a curve by dragging the mouse. This example also shows many other features of graphics and mouse processing. In the program, you can draw a curve by dragging the mouse on a large white drawing area, and you can select a color for drawing by clicking on one of several colored rectangles to the right of the drawing area. The complete source code can be found in SimplePaint.java, which can be run as a stand-alone application. Here is an applet version of the program for you to try:
I will discuss a few aspects of the source code here, but I encourage you to read it carefully in its entirety. There are lots of informative comments in the source code. (The source code uses one unusual technique: It defines a subclass of JApplet, but it also includes a main() routine. The main() routine has nothing to do with the class's use as an applet, but it makes it possible to run the class as a stand-alone application. When this is done, the application opens a window that shows the same panel that would be shown in the applet version. This example thus shows how to write a single file that can be used either as a stand-alone application or as an applet.)
The panel class for this example is designed to work for any reasonable size, that is, unless the panel is too small. This means that coordinates are computed in terms of the actual width and height of the panel. (The width and height are obtained by calling getWidth() and getHeight().) This makes things quite a bit harder than they would be if we assumed some particular fixed size for the panel. Let's look at some of these computations in detail. For example, the large white drawing area extends from y = 3 to y = height - 3 vertically and from x = 3 to x = width - 56 horizontally. These numbers are needed in order to interpret the meaning of a mouse click. They take into account a gray border around the panel and the color palette along the right edge of the panel. The gray border is 3 pixels wide. The colored rectangles are 50 pixels wide. Together with the 3-pixel border around the panel and a 3-pixel divider between the drawing area and the colored rectangles, this adds up to put the right edge of the drawing area 56 pixels from the right edge of the panel.
A white square labeled "CLEAR" occupies a 50-by-50 pixel region beneath the colored rectangles on the right edge of the panel. Allowing for this square, we can figure out how much vertical space is available for the seven colored rectangles, and then divide that space by 7 to get the vertical space available for each rectangle. This quantity is represented by a variable, colorSpace. Out of this space, 3 pixels are used as spacing between the rectangles, so the height of each rectangle is colorSpace - 3. The top of the N-th rectangle is located (N*colorSpace + 3) pixels down from the top of the panel, assuming that we count the rectangles starting with zero. This is because there are N rectangles above the N-th rectangle, each of which uses colorSpace pixels. The extra 3 is for the border at the top of the panel. After all that, we can write down the command for drawing the N-th rectangle:
g.fillRect(width - 53, N*colorSpace + 3, 50, colorSpace - 3);
That was not easy! But it shows the kind of careful thinking and precision graphics that are sometimes necessary to get good results.
The mouse in this program is used to do three different things: Select a color, clear the drawing, and draw a curve. Only the third of these involves dragging, so not every mouse click will start a dragging operation. The mousePressed() method has to look at the (x,y) coordinates where the mouse was clicked and decide how to respond. If the user clicked on the CLEAR rectangle, the drawing area is cleared by calling repaint(). If the user clicked somewhere in the strip of colored rectangles, the corresponding color is selected for drawing. This involves computing which color the user clicked on, which is done by dividing the y coordinate by colorSpace. Finally, if the user clicked on the drawing area, a drag operation is initiated. In this case, a boolean variable, dragging, is set to true so that the mouseDragged and mouseReleased methods will know that a curve is being drawn. The code for this follows the general form given above. The actual drawing of the curve is done in the mouseDragged() method, which draws a line from the previous location of the mouse to its current location. Some effort is required to make sure that the line does not extend beyond the white drawing area of the panel. This is not automatic, since as far as the computer is concerned, the border and the color bar are part of the drawing surface. If the user drags the mouse outside the drawing area while drawing a line, the mouseDragged() routine changes the x and y coordinates to make them lie within the drawing area.
6.4.5 Anonymous Event Handlers and Adapter Classes
As I mentioned above, it is a fairly common practice to use anonymous inner classes to define listener objects. As discussed in Subsection 5.7.3, a special form of the new operator is used to create an object that belongs to an anonymous class. For example, a mouse listener object can be created with an expression of the form:
new MouseListener() {
public void mousePressed(MouseEvent evt) { . . . }
public void mouseReleased(MouseEvent evt) { . . . }
public void mouseClicked(MouseEvent evt) { . . . }
public void mouseEntered(MouseEvent evt) { . . . }
public void mouseExited(MouseEvent evt) { . . . }
}
This is all just one long expression that both defines an unnamed class and creates an object that belongs to that class. To use the object as a mouse listener, it can be passed as the parameter to some component's addMouseListener() method in a command of the form:
component.addMouseListener( new MouseListener() { public void mousePressed(MouseEvent evt) { . . . } public void mouseReleased(MouseEvent evt) { . . . } public void mouseClicked(MouseEvent evt) { . . . } public void mouseEntered(MouseEvent evt) { . . . } public void mouseExited(MouseEvent evt) { . . . } } );
Now, in a typical application, most of the method definitions in this class will be empty. A class that implements an interface must provide definitions for all the methods in that interface, even if the definitions are empty. To avoid the tedium of writing empty method definitions in cases like this, Java provides adapter classes. An adapter class implements a listener interface by providing empty definitions for all the methods in the interface. An adapter class is useful only as a basis for making subclasses. In the subclass, you can define just those methods that you actually want to use. For the remaining methods, the empty definitions that are provided by the adapter class will be used. The adapter class for the MouseListener interface is named MouseAdapter. For example, if you want a mouse listener that only responds to mouse-pressed events, you can use a command of the form:
component.addMouseListener( new MouseAdapter() {
public void mousePressed(MouseEvent evt) { . . . }
} );
To see how this works in a real example, let's write another version of the ClickableRandomStringsApp application from Subsection 6.4.2. This version uses an anonymous class based on MouseAdapter to handle mouse events:
import java.awt.Component;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.JFrame;
public class ClickableRandomStringsApp {
public static void main(String[] args) {
JFrame window = new JFrame("Random Strings");
RandomStringsPanel content = new RandomStringsPanel();
content.addMouseListener( new MouseAdapter() {
// Register a mouse listener that is defined by an anonymous subclass
// of MouseAdapter. This replaces the RepaintOnClick class that was
// used in the original version.
public void mousePressed(MouseEvent evt) {
Component source = (Component)evt.getSource();
source.repaint();
}
} );
window.setContentPane(content);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLocation(100,75);
window.setSize(300,240);
window.setVisible(true);
}
}
There is also an adapter class for mouse motion listeners, MouseMotionAdapter, which implements MouseMotionListener and defines empty versions of mouseDragged() and mouseMoved(). In Java 6 and later, the MouseAdapter class actually implements MouseMotionListener as well as MouseListener, so there is less use for MouseMotionAdapter.
Anonymous inner classes can be used for other purposes besides event handling. For example, suppose that you want to define a subclass of JPanel to represent a drawing surface. The subclass will only be used once. It will redefine the paintComponent() method, but will make no other changes to JPanel. It might make sense to define the subclass as an anonymous inner class. As an example, I present HelloWorldGUI4.java. This version is a variation of HelloWorldGUI2.java that uses anonymous inner classes where the original program uses ordinary, named nested classes:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
/**
* A simple GUI program that creates and opens a JFrame containing
* the message "Hello World" and an "OK" button. When the user clicks
* the OK button, the program ends. This version uses anonymous
* classes to define the message display panel and the action listener
* object. Compare to HelloWorldGUI2, which uses nested classes.
*/
public class HelloWorldGUI4 {
/**
* The main program creates a window containing a display panel
* and a button that will end the program when the user clicks it.
*/
public static void main(String[] args) {
JPanel displayPanel = new JPanel() {
// An anonymous subclass of JPanel that displays "Hello World!".
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawString( "Hello World!", 20, 30 );
}
};
JButton okButton = new JButton("OK");
okButton.addActionListener( new ActionListener() {
// An anonymous class that defines the listener object.
public void actionPerformed(ActionEvent e) {
System.exit(0);
}
} );
JPanel content = new JPanel();
content.setLayout(new BorderLayout());
content.add(displayPanel, BorderLayout.CENTER);
content.add(okButton, BorderLayout.SOUTH);
JFrame window = new JFrame("GUI Test");
window.setContentPane(content);
window.setSize(250,100);
window.setLocation(100,100);
window.setVisible(true);
}
}