1. Miscellaneous Topics #6: Polygons GUI Components and Event Handlers

2. Polygons and Polylines
Arrays can be helpful in graphics processing
For example, they can be used to store a list of coordinates
A polygon is a multisided, closed shape
A polyline is similar to a polygon except that its endpoints do not meet, and it cannot be filled
See PolygonShape.java
See RocketApp.java

3. The Polygon Class
The Polygon class can also be used to define and draw a polygon
It is part of the java.awt package
Versions of the overloaded drawPolygon() and fillPolygon() methods take a single Polygon object as a parameter instead of arrays of coordinates
A Polygon class has the intersects() methods which returns true if the Polygon intersects with a given rectangle. This method takes four argument, the (x,y) top- left coordinate of the rectangle and its width, and height.

4. Events
An event is an object that represents some activity to which we may want to respond
For example, we may want our program to perform some action when the following occurs:
the mouse is moved
the mouse is dragged
a mouse button is clicked
a graphical button is clicked
a keyboard key is pressed
a timer expires
Events often correspond to user actions, but not always

5. Events and Listeners
The Java standard class library contains several classes that represent typical events
Components, such as a graphical button, generate (or fire) an event when it occurs
A listener object "waits" for an event to occur and responds accordingly
We can design listener objects to take whatever actions are appropriate when an event occurs

6. A corresponding listener
Events and Listeners
Event
Component
A component object
may generate an event
Listener
A corresponding listener
object is designed to
respond to the event
When the event occurs, the component calls
the appropriate method of the listener,
passing an object that describes the event

7. GUI Development
Generally we use components and events that are predefined by classes in the Java class library
Therefore, to create a Java program that uses a Graphical User Interface (GUI) we must:
instantiate and set up the necessary components (like JFrames)
implement listener classes for any events we care about
establish the relationship between listeners and components that generate the corresponding events
Let's now look at one such listener that works for the JFrame components and see how this all comes together

8. KeyListener Example
Listener classes are written by implementing a listener interface
The KeyAdapter class implements the KeyListener interface
An interface is a list of methods that the implementing class must define
One method in the KeyListener interface that is implemented in the KeyAdapter class is the KeyTyped method, and this method is sent the event when a keyboard character is typed
The Java class library contains interfaces for many types of events

9. KeyListener Example
Here is an example of how to add a handler for certain typed keyboard characters to a GraphicsWindow object (named “graphicsWin”) by creating a KeyListener for it’s JFrame:
graphicsWin.getFrame.addKeyListener(new KeyAdapter() {
public void keyTyped(KeyEvent e) {
char ch = e.getKeyChar();
if (ch == 'q' || ch == 'Q')
System.exit(0); }
});

10. KeyListener Example
The keyTyped method only handles alpha-numeric keys that are typed. You need to use the keyPressed() method to handle special keys:
graphicsWin.getFrame.addKeyListener(new KeyAdapter() {
public void keyPressed(KeyEvent e) {
char ch = e.getKeyChar();
int code = e.getKeyCode();
if (ch == 'q' || ch == 'Q')
System.exit(0);
if (ch == 'u' || ch == 'U' || code == KeyEvent.VK_UP)
paddle.moveUp();
if (ch == 'd' || ch == 'D' || code == KeyEvent.VK_DOWN)
paddle.moveDown(); }
});

11. KeyListener Example
Another approach is to send the characters typed to a separate method like handleKey() to take the appropriate action:
graphicsWin.getFrame.addKeyListener(new KeyAdapter() {
public void keyTyped(KeyEvent e) {
handleKey(e.getKeyChar()); }
});
public static void handleKey(char ch)
if (ch == 'p' || ch == 'P')
pauseSimulation = (pauseSimulation) ? false : true;
if (ch == 'q' || ch == 'Q')
System.exit(0);

12. KeyListener Example
Another approach is to send the characters typed to a separate SubClass like KeyListener below, which has a keyPressed() method to take the appropriate action:
graphicsWin.getFrame.addKeyListener(new KeyListener());
public static class KeyListener extends KeyAdapter
{ public void keyPressed(KeyEvent e) {
char ch = e.getKeyChar();
int code = e.getKeyCode();
if (ch == 'u' || ch == 'U' || code == KeyEvent.VK_UP)
paddle.moveUp();
if (ch == 'd' || ch == 'D' || code == KeyEvent.VK_DOWN)
paddle.moveDown(); }
});