Event-Driven Programming

You have seen Buttons and ActionListeners; TextFields and ActionListeners; Component and java.awt.event.MouseListener .

Remember MVC? In Java, much of programming is event driven. Thus the user has components (views) which are associated with particular actions (interfaces - which constitute the controllers) when events happen (eg., pushing the button).

Different components generate different events and expect different interfaces to be implemented.

To implement an interface means to actually have code for the methods defined in a given interface. The API can be used to see which interfaces certain components expect to be implemented.

Follow some Listeners on the API to see the kinds of methods that are expected to be implemented. Look at Lists, CheckBoxes, etc. Also note that the methods in different Listener's have different event parameters.

Here is an example with a scrollbar:

import java.awt.*; import java.applet.Applet; import java.awt.event.*; public class FirstEvent extends Applet implements AdjustmentListener { private Scrollbar slider; private int sliderValue = 0; public void init() { slider=new Scrollbar(Scrollbar.HORIZONTAL ,0, 1, 0, 100); add(slider); slider.addAdjustmentListener(this); } public void paint(Graphics g) { g.drawString("Current value is " + sliderValue, 100,100); } public void adjustmentValueChanged(AdjustmentEvent e) { sliderValue = slider.getValue(); repaint(); } }

Remember how each component needs the three statements in the init():

1. instantiate the component
2. add the component to the applet
3. add the listener to the component

A bunch of examples using Scrollbar: (look at the applets and then the code to see how different values change)

• WindowBlind and code here

• LabelDemo and code here

• InchesToCm and code here

• ScrollbarValues and code here

• FunnySteps and code here

• ExtendLine and code here

Also, SWING might interest you

Remember the code that you were supposed to make Object Models for in Lab 5: DrawTest and its applet and TicTacToe and its applet . Let's look and see if we understand why certain methods are there now.