GUIs and Events

Rick Mercer 1

Event-Driven Programming with Graphical user Interfaces

 Most applications have graphical user interfaces to respond to user desires 2

A Few Graphical Components

 A Graphical User Interface (GUI) presents a graphical view of an application to users.

 To build a GUI application, you must: — Have a well-tested model that is independent of the view — Make graphical components visible to the user — Ensure the correct things happen for each event • user clicks button, moves mouse, presses enter key, ...

 Let's first consider some of Java's GUI components: — windows, buttons, and text fields 3

Classes in the swing package

 The

javax.swing

package has components that show in a graphical manner

JFrame

: window with title, border, menu, buttons

JButton

: A component that can "clicked"

JLabel

: A display area for a small amount of text

JTextField

: Allows editing of a single line of text 4

Get a window to show itself

import javax.swing.JFrame; public class FirstGUI extends JFrame { public static void // Construct an object that has all the methods of JFrame JFrame aWindow = main(String[] args) { new FirstGUI(); aWindow.setVisible( true ); } } // Set up the GUI public this FirstGUI() { // Make sure the program terminates when window closes .setDefaultCloseOperation(JFrame.

EXIT_ON_CLOSE );

// Set the title of this instance of FirstGUI this .setTitle( "Graffiti" ); // … more to come … }

5

Some JFrame messages

 Set the size of the window with // this not necessary if message is in // a FirstGUI method like this constructor setSize(220, 100); — The first int is the width of the window in pixels — the second int is the height of the window in pixels 6

Building components

 So far we have an empty window  Let us add a button, a label, and an editable line  First construct three graphical components

JButton clickMeButton = new JButton( "Nobody is listening to me" ); JLabel aLabel = new JLabel( "Button above, text field below" ); JTextField textEditor = new JTextField( "You can edit this text " );

 Next, add these objects to a JFrame 7

Add components to a window

 Could use the default BorderLayout and add components to one of the five areas of a

JFrame add(clickMeButton, BorderLayout.

NORTH

); add(aLabel, BorderLayout.

CENTER

); add(textEditor, BorderLayout.

SOUTH

);

8

The 5 areas of BorderLayout

 By default,

JFrame

objects have only five places where you can add components —

a 2nd add wipes out the 1 st

 There are many layout managers  We will use null for layout: — Must set the size and location of each component 9

Null layout manager

easier to layout components  Explicitly state where each component goes

// Add three graphical components using a null layout setLayout( null ); // Controversial ... clickMeButton .setSize(200, 25); clickMeButton .setLocation(0, 0); add( clickMeButton ); // Add a label not needed by other methods in this class JLabel aLabel= new JLabel( "Button above, text field below" ); aLabel.setSize(200, 25); aLabel.setLocation(0, 25); add(aLabel); textEditor .setSize(200, 25); textEditor .setLocation(0, 50); add( textEditor );

10

So what happens next?

 You can layout a real pretty GUI  You can click on buttons, enter text into a text field, move the mouse, press a key — And NOTHING happens  So let ’ s make something happen… 11

Java's Event Model

 Java lets the operating system notify graphical components of user interaction — JButton objects know when a user clicks it — JTextField objects with focus know when the user presses the enter (return) key — Event driven programs respond to many things • • • mouse clicks, mouse movements clicks on hyperlinks or menu items Users pressing any key, selecting a list item, moving a slider bar, checking a radio button, … 12

Example: Action Events

 The buttons and text fields do not execute code — Instead

JButton

and

JTextField

send

actionPerformed

objects messages to other objects that have been registered to “listen”  We write the code we want to execute in

actionPerformed

methods — This requires a class that implements an interface, for example — Registering an instance of that class to listen 13

Event Driven Programs with GUIs

 Key elements of an event-driven GUI —

Graphical components

• The screen elements that a user manipulates with the mouse and keyboard

JFrame JLabel JButton JScrollbar JMenuItem JTextField JTextArea JList ...

—

Layout managers

• • Govern how the components appear on the screen Examples

FlowLayout GridLayout

null layout —

Events

• • Signal that a user interacted with the GUI Examples: mouse clicks, keys pressed, hyperlink selected, time expires on a timer, … 14

Java's Event Model

1

Layout Graphical Components JFrame JButton JFrame JButton JTextField JMenuItem

4

Users interact with these graphical components

3

You register objects that waits for messages from graphical components addActionListener Listener Listener Listener

2

You write classes that implement the correct interface ActionListener 15

A Java GUI: Rick's model

 We’ve begin by setting up the GUI 1.

FirstGUI extends Jframe

It IS-A JFrame so we inherit many methods 2.

main

starts up the GUI (could be separate file) 3. Added instance variables – graphical components that will be needed by two or more methods (a JTextField, JButton, or JList will be needed by listeners later 4. Lay out a GUI and initialize instance variables in the constructor 16

But no one is "Listening"

 Okay, now we have a GUI — but when run, nothing happens  Wanted: An object to listen to the button that understands a specific message such as —

actionPerformed

 Also need to tell the button who it can send the

actionPerfomed

message to — Register the listener with this method

addActionListener(ActionListener al)

17

Handling Events

5. Add a private inner class that can listen to the event that the graphical component will generate Your class must implement a listener interface to guarantee that it has the expected methods: First up:

ActionListener

6. Register the listener object to the GUI component (JButton JTextField) so it can later send

actionPerformed

messages to that listener when the use clicks the button or presses enter.

events occur anytime in the future--the listener is listening (waiting for user generated events such as clicking a button or entering text into a text field)

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ActionEvent / ActionListener

 When a

JButton

object is clicked, it constructs an

ActionEvent

object and sends it to the

actionPerformed

method of its listeners — We can usually ignore that parameter, other times we can't  To register a listener to a

JButton

, send an

addActionListener

message to button

public void addActionListener(ActionListener al)

— You need an

ActionListener

• • There is no

ActionListener

What can we do?????

class!

object 19

Implement an

interface

Then your object can be treated as if it were an

ActionListener

Polymorphism in action: We can have any number of

actionPerformed

methods that do whatever they are supposed to. The Button does not care what happened 20

Inner class

 Add an inner class — inner classes have access to the enclosing classes' instance variables  Make it private since no one else needs to know about it  Java added inner classes for the very purpose: to have listener objects respond to user interactions with GUI components 21

And register the listener with

addActionListener // 6. Register the instance of the listener so the // component can later send messages to that object ButtonListener aListener = new ButtonListener(); clickMeButton .addActionListener(aListener); } // End constructor Caution: this is easy to forget. It is an error no one will tell you about // 5. inner class to listen to events private class ButtonListener implements ActionListener { // No constructor needed here.

// Must have this method to implement ActionListener public void actionPerformed(ActionEvent anActionEvent) { System.

out

.println( "Button was clicked." ); } }

22

Another benefit of interfaces

 Can have many ActionListener objects — Any class that implements ActionListener — may need a different class for every button and text field in the GUI • • But they all can be treated as ActionListener objects They can be passed as arguments to this method

public void addActionListener(ActionListener aL)

Adds an action listener to receive action events from JButtons, TextFields, ...

aL - an instance of a class that implements the ActionListener interface 23

Assignment Compatible

 Can pass instances of classes implementing an interface to the interface type parameter

addActionListener(ActionListener anyListener) addActionListener(new ButtonListener()); addActionListener(new TextFieldListener());



ButtonListener

and

TextFieldListener

must implement

interface ActionListener

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Listen to JTextField

 Add to the current GUI — Have the button click toggle the JTextField form upper to lower case • Need methods

getText

and

setText

25

Jlist and DefaultListModel

 Code demo: — Show a JList GUI component with a DefaultListModel set as the model • will contain a list of strings  Respond to selections by printing the selected string 26

public class ShowJListListModel extends JFrame { public static void main(String[] args) { JFrame window = new ShowJListListModel(); window.setVisible(true); } private JList guiList; private DefaultListModel allStrings; public ShowJListListModel() { setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); setSize(250, 300); setLocation(100, 100); setLayout(null); initializeAllStrings(); guiList = new JList(allStrings); guiList.setSelectedIndex(allStrings.getSize() - 1); guiList.setSize(150, 250); guiList.setLocation(20, 10); guiList.setFont(new Font("Arial", Font.BOLD + Font.ITALIC, 14)); add(guiList); } registerListeners(); private void registerListeners() { ListSelectionListener listener = new GuiListListListener(); guiList.addListSelectionListener(listener); } private class GuiListListListener implements ListSelectionListener { public void valueChanged(ListSelectionEvent arg0) { String str = allStrings.get(guiList.getSelectedIndex()).toString(); System.out.println(str + " selected"); } } private void initializeAllStrings() { allStrings = new DefaultListModel(); allStrings.addElement("Riley"); allStrings.addElement("Dakota"); allStrings.addElement("Casey"); allStrings.addElement("Angel"); allStrings.addElement("Reese");

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}