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Java GUI programming and Java Threads

GUI example taken from “Computing Concepts with Java 2” byCay Horstmann

Thread examples taken from “The Java Programming Language”By Arnold and Gosling and from Cay Horstmann’s “Core Java 2 Advanced”

GUI Programming with threads

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Frame Windows

A Frame window has a border and a title bar

A Frame window has an addWindowListener method. We canuse this method to add listeners to our frame window.

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An example

import javax.swing.*;import java.awt.event.*;

public class InternetFrameExample {

public static void main(String[] args) {

JFrame f = new InternetFrame("Example"); f.setTitle("Internet browser"); f.show(); }}

javax means Java standardextension

Tell the window manager todisplay the frame.

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class InternetFrame extends JFrame {

public InternetFrame(String s){

setSize(300,300); WindowCloser listener = new WindowCloser(); addWindowListener(listener); }

private class WindowCloser extends WindowAdapter {

public void windowClosing(WindowEvent e) { System.exit(0); } } }

windowOpened()widowClosed()windowClosing() : :

Add the handler

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Adding User Interface Components to a Frame

•Do not draw directly on the surface of a frame.•Frames have been designed to arrange user interface components.•User interface components are such things as buttons, menus, scroll bars, and so on.•If you want to draw on a frame, draw on a separate component and then add that component to the frame. The Swing UI toolkit provides the Jpanel class for this purpose.

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Drawing on a JPanel

To draw on a Jpanel you override the paintComponent method.

Make sure that from within your paintComponent method youcall super.paintComponent(…) so that the superclass method paintComponent has a chance to erase the existing contents,redraw the borders and decorations, etc.

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Adding the Panel to the JFrame

• The surface of a Swing frame is covered with four panes.

• Each of these four has a purpose.• We are interested in getting access to the JFrame’s content pane.• So, call the getContentPane on the JFrame.• This call returns a Container object.• Add your Panel object to the content pane

Container.

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Adding a Panel to a JFrame

x = getContentPane (a contentPane holdscomponents for display).

x refers to a Container (may contain other components)

c = a Panel or some other component.

Add c to x using x’s layoutmanager (a content pane usesborder layout)

JFrame

getContentPane()

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Adding a JTextfield to the JFrame

JFrame

getContentPane()

class MyFrame extends JFRAME { private JTextField textField; public MyFrame() { Container cp = getContentPane();

textField = new JTextField();

cp.add(textField, “South”);

cp

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We may want to add a listener to the TextField

JFrame

getContentPane()

Class MyFrame extends JFRAME { private JTextField textField; public myFrame() { Container cp = getContentPane();

textField = new JTextField();

cp.add(textField, “South”);

textField.addActionListener( new TextFieldListener());

cp

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Output first! -- user enters number of eggs and we draw them

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Strategy

Think about • What do we want on the screen?• What events should we listen for?• What should we do when those events occur?• What processing will we do when user input arrives?• What object has responsibilities for what activities?

Think about

• The ‘has-a’ relationship,e.g., the Jframe’s ContentPane “has-a” Panel and a TextField.• The ‘is-a’ relationship,e.g., The TextFieldListener ‘is-an’ actionListener.

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// Example// Eggs.java

import java.awt.Container;import java.awt.Graphics;import java.awt.Graphics2D;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.awt.event.WindowAdapter;import java.awt.event.WindowEvent;import java.awt.geom.Ellipse2D;import java.util.Random;import javax.swing.JFrame;import javax.swing.JPanel;import javax.swing.JTextField;

We need classes fromthe awt, util, and swingpackages.

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public class Eggs{ public static void main(String[] args) { EggFrame frame = new EggFrame(); frame.setTitle("Enter number of eggs"); frame.show(); }}

This thread is doneafter creating a frame andstarting up the frame thread.

A frame now exists and is running.

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The EggFrame Constructor

• Set the size of the frame• Add a listener to listen for the stop event• Create a JPanel object to draw on and a

JTextField object to interact with the user via the keyboard

• Add a listener for the JTextField• Add the Jpanel and the JTextField to the contentPane container.

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class EggFrame extends JFrame{ private JTextField textField; private EggPanel panel;

public EggFrame() { final int DEFAULT_FRAME_WIDTH = 300; final int DEFAULT_FRAME_HEIGHT = 300; setSize(DEFAULT_FRAME_WIDTH, DEFAULT_FRAME_HEIGHT);

addWindowListener(new WindowCloser()); panel = new EggPanel(); textField = new JTextField(); textField.addActionListener(new TextFieldListener()); Container contentPane = getContentPane(); contentPane.add(panel, "Center"); contentPane.add(textField, "South"); }

A listener for the jframewindow

A textField listener

As before

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The constructor will be called from our main thread.

The other thread operates asynchronously.

What do we mean by asynchronous execution?

Who is running the show?

Don’t programs run sequentially?

We have to think differently.

Event driven programming

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The TextField

The TextField object will call us when it detectsan event.

We don’t ‘read the input’. We set up a babysitter torespond.

The TextField object sends us an event object.

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// Use an inner class to listen on the text field

private class TextFieldListener implements ActionListener { public void actionPerformed(ActionEvent event) { String input = textField.getText(); panel.setEggCount(Integer.parseInt(input)); textField.setText(""); } } We do two things when we

have a textfield event.1) Get the data2) Tell the panel the number of eggs to display

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// Use an inner class to listen on the text field

private class TextFieldListener implements ActionListener { public void actionPerformed(ActionEvent event) { String input = textField.getText(); panel.setEggCount(Integer.parseInt(input)); textField.setText(""); } } Note how handy the inner class is. Both panel and

textField are available. What if the listener were notan object of an inner class. how would it get accessto these variables? We can’t just pass the variables onthe call because we don’t make the call.

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private class WindowCloser extends WindowAdapter { public void windowClosing(WindowEvent event) { System.exit(0); } }}

This is how we respond when theclose signal is received.system.exit(0) stops the java virtual machine. 0 means normal termination.

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How about the panel object

What are the panel object’s responsibilities?

get input? _____ repaint itself? _____ keep track of the egg count? _____ hold the data it needs to repaint itself? _____

Do we want our panel to inherit properties and methods from anyexisting classes? _____

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How about the panel object

What are the panel object’s responsibilities?

get input? No, that’s the TextField’s job. repaint itself? Sure, that’s its main job. keep track of the egg count? Yes, better here where it’s needed hold the data it needs to repaint itself? Yes

Do we want our panel to inherit properties from any existing classes? Sure, we want to re-use existing code whenever possible.

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How about the panel object

When should the panel object repaint itself?

What will the panel need to repaint itself?

Who actually calls the paintComponent method?

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How about the panel object

When should the panel object repaint itself? When a new input arrives from the user. When the egg count changes. What will the panel need to repaint itself? A graphics object to draw on.

Who actually calls the paintComponent method? While we have to provide a paintComponent method we don’t call it directly. It’s called by the Java run-time environment after we make a call on repaint.

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class EggPanel extends JPanel{

public void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2 = (Graphics2D)g; // draw eggCount ellipses with random centers Random generator = new Random(); for (int i = 0; i < eggCount; i++) { double x = getWidth() * generator.nextDouble(); double y = getHeight() * generator.nextDouble(); Ellipse2D.Double egg = new Ellipse2D.Double(x, y, EGG_WIDTH, EGG_HEIGHT); g2.draw(egg); } }

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public void setEggCount(int count) { eggCount = count; repaint(); }

private int eggCount; private static final double EGG_WIDTH = 30; private static final double EGG_HEIGHT = 50;}