Events

1. Events

2. Objectives • Understand the difference between applets and applications • Identify meaningful applet resources for academic subjects • Create and demonstrate a basic Java applet • Write a basic HTML page that uses a Java applet

3. Vocabulary • Here are some terms that you’ll encounter in your lesson on Events: • Interface • Listener • Overrides • Implements • Buffer • Empty methods

4. An Event • As mentioned in the introduction, an “event” is just something that happens. • When you clicked your mouse to get to this slide, we call this a “mouse event”. • Tricky, eh?

5. An Event • What other “events” does a computer deal with? • Keyboard events • Software events • Sound events • Basically, any interaction with your software.

6. An Event • This lesson will focus on the basic two events, mouse and keyboard. • But, a riddle: if a tree falls in the forest with no one to hear it, does it make a sound? • Hmm…

7. Listeners • What the tree thing means: • Something can happen, but does that mean it was heard? • For that, Java also created “listeners”. We have one for each type of event.

8. Listeners • The next slides will present what we mean by “mouse events” and “mouse listeners”. • I bet you never thought you’d listen to a mouse!

9. Interaction • Review your program from the Applets lesson, because we’ll be using that to start. • We left off with it printing during the four methods of init(), start(), stop() and destroy()

10. Interaction • Our goal: make the program listen to the mouse and print out “squeak” when the mouse is clicked. • First, we need to make our program “listen”.

11. Add Listener • Programming used to be “procedural”, and you would have to write ALL of the code to make the program listen. • Because Java is object-oriented, we can just “import” the Listener code.

12. Add Listener • So, first step is to add an import statement: • import java.awt.event.MouseListener; • The MouseListener class is an “interface”. It allows us to use certain methods to “interface” or “interact” with other things well. • We won’t go into much more detail on interfaces here, it’s one of the toughest concepts in programming Java.

13. Add Listener • After importing MouseListener, we use it by “implementing” it. • import java.awt.event.MouseListener; • public class SimpleClick extends Applet implements MouseListener { • As you can see, we still “extend” Applet (or JApplet) because it’s an object, but we use the word “implement” to use the MouseListener interface.

14. Add Listener • Next, we have to tell the program when to start “listening”. Remember our init() method? • public void init() { • addMouseListener(this); • buffer = new StringBuffer(); • addItem("initializing... "); • } • We add the listener at the initialization, or beginning, of our program.

15. Add Listener • public void init() { • addMouseListener(this); • buffer = new StringBuffer(); • addItem("initializing... "); • } These other lines in init() do two simple tasks: buffer = new StringBuffer() creates an object to hold the words that we’re going to print, like a sentence that gets words added to it. addItem(“initializing…”) adds the word “initializing” to the buffer for printing.

16. Add Events • Okay, so the program is listening! • Since it’s listening to the mouse, what can the mouse do? • A mouse can: • Click • Press • Release • Enter the applet • Exit the applet • (It can also do moving things, but that’s MouseMotionListener)

17. Add Events • Just like the Listener, we need to import the code for the MouseEvent: • import java.awt.event.MouseEvent; • MouseEvent is an object, so we don’t need to “implement” it, like the MouseListener interface. • Also, when we added the “addMouseListener()” to init(), that is our connection to mouse events.

18. Add Events • We ARE required to write methods for the 5 major mouse events, even if they do nothing! • This is a requirement when using an interface, and the MouseListener is listening for the 5 we listed before: • Click • Press • Release • Enter the applet • Exit the applet

19. Add Events • Your code for the methods will look like this: • public void mouseEntered(MouseEvent event) { } • public void mouseExited(MouseEvent event) { } • public void mousePressed(MouseEvent event) { } • public void mouseReleased(MouseEvent event) { } • public void mouseClicked(MouseEvent event) { } • The methods are “empty”, there is no code in the curly braces. That’s okay! Interfaces only require that you write the methods, not to fill them! • But, we do want our mouseClicked method to do something.

20. Add Events • Notice that the methods have one parameter, a MouseEvent: • public void mouseEntered(MouseEvent event) { } • They call the MouseEvent “event”, but you could call it anything. “event” just seems to make sense.

21. Add Events • When we click the mouse, we want it to say “ouch” and add it to the buffer string. • It’s just like the other methods you have: • public void mouseClicked(MouseEvent event) { • addItem(“ouch!... "); • } • This calls the addItem method that you already have, and it adds “ouch!...” to the buffer.

22. Java - Assignments Now try the “Mouse Lab”. Most of the code is written for you (Java is so cool like that!), you will have to modify it.