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15-GUIDynamic - GUI Statics and GUI Dynamics CS/ENGRD 2110...

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6/15/2011 1 CS/ENGRD 2110 Object-Oriented Programming and Data Structures Spring 2011 Thorsten Joachims Lecture 14: Graphical User Interfaces (Dynamic) GUI Statics and GUI Dynamics Statics: what’s drawn on the screen Components E.g. buttons, labels, lists, sliders, menus, ... Containers components that contain other components E.g. frames, panels, dialog boxes, ... Layout managers control placement and sizing of components Dynamics: user interactions Events E.g. button-press, mouse- click, key-press, ... Listeners an object that responds to an event Helper classes E.g. Graphics, Color, Font, FontMetrics, Dimension, ... 2 Dynamics Overview Dynamics = causing and responding to actions What actions? Called events: mouse clicks, mouse motion, dragging, keystrokes We would like to write code (a handler) that is invoked when an event occurs so that the program can respond appropriately In Java, you can intercept events by providing an object that “hears” the event – a listener What objects do we need to know about? Events Event listeners 3 Brief Example Revisited import javax.swing.*; import java.awt.*; import java.awt.event.*; public class Intro extends JFrame { private int count = 0; private JButton myButton = new JButton("Push Me!"); private JLabel label = new JLabel("Count: " + count); public Intro() { setDefaultCloseOperation( EXIT_ON_CLOSE ); setLayout(new FlowLayout(FlowLayout. LEFT )); //set layout manager add(myButton); //add components add(label); label.setPreferredSize(new Dimension(60, 10)); myButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) { count++; label.setText("Count: " + count); } }); pack(); setVisible(true); } public static void main(String[] args) { new Intro(); } } 4 The Java Event Model Timeline User or program does something to a component clicks on a button, resizes a window, ... Java issues an event object describing the event A special type of object (a listener) “hears” the event The listener has a method that “handles” the event The handler does whatever the programmer programmed What you need to understand Events: How components issue events Listeners: How to make an object that listens for events Handlers: How to write a method that responds to an event 5 Events An Event is a Java object Events are normally created by the Java runtime system You can create your own, but
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