mouseDown(), mouseUp() These methods inform the applet that the mouse button has been in Java

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mouseDown(), mouseUp() These methods inform the applet that the mouse button has been
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mouseEnter(), mouseExit() These methods inform the applet that the pointer has entered or left
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mouseDrag(), mouseMove() These methods inform the applet when the mouse is moved with
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either the button pressed (mouseDrag) or the button released (mouseMove)
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keyDown() This method notifies the applet that the user has pressed a key
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In general this set of seven methods will form the core of the events that you are interested in Be careful when typing screen updates to user events to ensure that you make the most efficient use of the paint and update methods The flow of events, particularly with those related to mouse motion, can be so fast that the screen updates will cause the applet to flicker While it is possible to override the handleEvent class you should consider if it is essential before doing so 1035 Adding event handling to HelloWorld To demonstrate the event methods, we ll extend the HelloWorld class to have two extra features When the mouse moves over the area of the applet, we ll have the string move with the cursor, and when the user presses a key, we ll append the key character to the string The code to do this is: import javaawtGraphics; import javaawtColor; import javaawtEvent; public class HelloWorld extends javaappletApplet { int sx,sy; // The coordinates of the string StringBuffer hw;// The string to display public void init() { sx = 5; sy = 20;
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hw = new StringBuffer("Hello World!"); Systemoutprintln("Applet: system has called init"); resize(100, 100); } public void paint(Graphics g) { gsetColor(Colorred); gfill3DRect(5,5,95,95,true); gsetColor(Colorblack); gdrawString(hwtoString(),sx,sy); Systemoutprintln("Applet: system has called paint"); } public void update(Graphics g) { superupdate(g); Systemoutprintln("Applet: system has called update"); } public void start() { Systemoutprintln("Applet: system has called start"); } public void stop() { Systemoutprintln("Applet: system has called stop"); } public void destroy() { Systemoutprintln("Applet: system has called destroy"); } public boolean mouseMove(Event evt, int x, int y) { sx = x; sy = y; repaint(); return true; } public boolean keyDown(Event evt, int ch) { hwappend((char)ch); repaint(); return true; } } As you might expect by now we have had to import the javaawtEvent class so that we could override the event-based methods We ve used two ints to hold the coordinates of the string (the initial values are defined in the init method) Because we are going to be appending characters to the "Hello World!" string, we ve declared a StringBuffer, which is also initialized in the init() method The paint() method then converts the StringBuffer to a String and plots it at the coordinates held by the two integers (sx and sy) The method
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toString converts a StringBuffer to a String which is expected by drawString The mouseMove method simply takes two coordinates of the pointer as supplied by the system and sets the int values The repaint() method is called to have the applet redrawn As an aside, this simple applet would benefit from overriding the update() method Currently, update() calls paint() and the whole of the applet is drawn A more effective way to do this would be just to redraw the area that the string has left and then call drawString() to place the text in the new position Clearly, if such a simple applet can benefit from this technique, larger and more complex drawing operations should certainly be optimized The last remaining method we ve added takes the keyDown event and appends the character to the StringBuffer Because the keyDown method receives the character in the form of an int, we have made an explicit cast to a char and then appended to the StringBuffer If we had not cast in this manner, StringBuffer would append the integer value directly It is worth pointing out that the call to repaint() in mouseMove and keyDown causes the system to call the update() method which, by default, clears the graphics context and then calls the paint method In this simple applet, there is no support for handling special case characters, such as the delete key We can extend our keyDown method quite simply to demonstrate one possible technique: public boolean keyDown(Event evt, int ch) { switch (ch) { case(8)://This is the delete key if (hwlength() > 0) { hwsetLength(hwlength() - 1); } break; default:hwappend((char)ch); } repaint(); return true; } In this example, if there are any elements in the StringBuffer when the delete key is pressed, then the StringBuffer is made smaller by one character When any other key is pressed, the character matching the integer representation is appended to the StringBuffer as before
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