Java Swing 4 th April 2008 CS 180 Department of Computer Science, - - PowerPoint PPT Presentation

Java Swing 4th April 2008

CS 180 Department of Computer Science, Purdue University

GUIs

• Windowing systems that interact with

users often are called GUIs.

• A GUI accepts information from a user

and makes it available to the program for processing.

• Most of the interaction is graphical in

nature.

Controls on GUI

• WINDOW
• FRAME
• MENU
• BUTTON
• TEXT BOX
• COMBO BOX

Many more….

Event Driven Programming

• Most GUI programs involve events and

event handlers.

• A GUI event is an object that represents

some action such as clicking the mouse, dragging the mouse, pressing a keyboard key, clicking the close-window button on a window, etc.

• When an object generates an event, it is

said to fire the event.

Programming Example: A Simple Window

• This simple program produces a window and displays

some text.

– We will explore the following

• Jframe : to create a window
• JLabel : to create a label
• getContentPane().add() : add a component such as a label to the

content pane of the window

• setTitle() : set the title of the window
• setSize() : set the size of the window
• setVisible() : Method setVisible permits the programmer to specify

when GUI objects should be displayed and when they should not

import javax.swing.*; import java.awt.*; public class makeWindow { public static void main(String args[]) { int width=300; int height=200; JFrame myWindow=new JFrame(); //create the window myWindow.setSize(width,height); //set the title of the window myWindow.setTitle("this is a window"); //create the label JLabel myLabel=new JLabel("this is a label"); //add the label to the content pane of the window myWindow.getContentPane().add(myLabel); //set color of the content pane myWindow.getContentPane().setBackground(Color.CYAN); //make the window visible myWindow.setVisible(true); } }

A Frame This colored area is the content pane

Example

Exploring JFrame : Some methods

• A window class normally is derived from class JFrame.
• A derived window class inherits all the methods from class JFrame.

Window Listeners

• A window listener listens to events from a

window, such as a click on the close- window button.

• A window listener is registered when it

becomes associated with the object(s) to which it listens.

Clicking the Close-Window Button

An Example

• Component : Window
• Event: Window Closing
• Listener: performs the following tasks

– Display a dialog box – telling the user that you are exiting the system – Exit the window

• Note: If the window-close button is not

programmed, a click causes the window to disappear, but does not cause the program to end.

Example Code

import javax.swing.*; import java.awt.*; public class makeWindow { public static void main(String args[]) { int width=300; int height=200; JFrame myWindow=new JFrame(); ……… ………….. //same code as explained in slide 6 …………………… //set listener makeWindowListner myListener= new makeWindowListner(); myWindow.addWindowListener(myListener); }

• }

import java.awt.*; import java.awt.event.WindowAdapter; import java.awt.event.WindowEvent; import javax.swing.JPopupMenu; import javax. swing.*; public class makeWindowListner extends WindowAdapter{ public void windowClosing(WindowEvent e) { // Modal dialog with OK button String message = "You are exiting this window"; JOptionPane.showMessageDialog(null, message); System.exit(0); } }

Listener Class

Example: Output

Step 1: user clicks this close button Step 2: user will see this dialog box Step 3: window is closed

Methods of Class WindowAdapter

Some examples:

Methods of Class WindowAdapter, cont.

• When you define a derived class of

abstract class WindowAdapter, you override and redefine only those methods that you need.

• Because class WindowAdapter is abstract,

this class can be used only as a base class for defining other classes.

What to Import

• It may be simpler to use

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

rather than trying to determine which import statements are needed for a particular window interface.

– event.* represents a package within java.awt.

Frame-Content Pane Layout

• A frame is a component container that displays

its contents in a top-level window with a title bar and buttons to resize, iconify, maximize, and close the frame.

• Unlike most Swing containers, adding a

component to a frame is not done with the JFrame.add() method. This is because the frame holds several panes and it is necessary to specify a particular pane to which to add the component.

• The pane that holds child components is called

the content pane.

• By default the content pane has a border

layout

Border Layout, cont.

• A BorderLayout manager can place a component into

any of five regions.

• Regions which are unused give up their space to

BorderLayout.CENTER.

• This layout limits the GUI to five objects, these are

almost always five (or fewer) JPanels.

• equivalent forms:

content.add(label3, BorderLayout.CENTER);

and

content.add(label3, “Center”);

and (for center ONLY)

content.add(label3);

Some Layout Managers

Buttons

• A button is a GUI component that looks

like a button and does something when it is clicked using a mouse.

• Like a label, a button is created and added

to a container.

• Unlike a label, a button can fire an event

and the event can cause a GUI to perform some action.

Adding Buttons

• A button is created using

JButton Button_Name = new JButton(“Button_Label”);

• A button is added to a container using

Container_Name.add(Button_Name);

Adding Buttons, cont.

Adding Buttons, cont.

Close-Window Buttons and JButtons

• A button added to a GUI is an object of

class JButton.

• A close-window button is not an object of

class JButton. Instead, it is part of a JFrame

• bject.

Action Listeners and Action Events

• For each button, the GUI needs to

– register (specify) the listener object(s). – define the methods to be invoked when an event is fired.

• For a statement such as

stopButton.addActionListener(this);

the class ButtonDemo is itself the listener class.

• Buttons fire action events which are handled by

action listeners.

• An action listener is an object of type

ActionListener, and ActionListener is an interface

Action Listeners and Action Events, cont.

• To make a class into an ActionListener

– add implements ActionListener to the heading of the class definition – define a method named ActionPerformed. – register the ActionListener object with the component that will fire the event using the method addActionListener – (A component may register with more than

• ne listener.)

Buttons and an Action Listener

The actionPerformed Method

• An actionListener class must have a

method named actionPerformed that has

• ne parameter of type ActionEvent.
• syntax

public void actionPerformed(ActionEvent e) { Code_for_Actions_Performed }

Code a GUIs Appearance and Actions Separately

• Code for a Swing GUI is simpler if it is

divided into two parts:

– the GUI’s appearance on the screen – the GUI’s actions.

• In a complicated Swing GUI, either of

these tasks by itself can be formidable.

The Model-View-Controller Pattern

What is MVC?

• Model-view-controller (MVC) is an architectural pattern.
• Pattern isolates business logic from user interface.
• Results in an application where it is easier to modify

either the visual appearance or the underlying business rules without affecting the other.

• Model represents the information (the data) of the

application and the business rules used to manipulate the data

• View corresponds to elements of the user interface such

as text, checkbox items, and so forth

• Controller manages details involving the communication

to the model of user actions such as keystrokes and mouse movements.

Method setActionCommand

• Every object that fires an action event has an

associated string known as the action command for that component.

• e.getActionCommand() returns the action

command for the component that fired e.

• The default action command for a button is

the string written on it.

• Method setActionCommand can be used to

change the action command for the object.

Method setActionCommand, cont.

• example

JButton stopButton = new JButton(“Red”); stopButton.setActionCommand(“Stop”);

• This permits the same string to be written
• n two different buttons, but with the two

buttons distinguished from one another by the program.

Container Classes

• The JPanel Class
• The Container Class
• Class JPanel
• java.lang.Object

– java.awt.Component

• java.awt.Container

– javax.swing.JComponent » javax.swing.JPanel

Building GUI Classes

• New classes for a GUI often are built using

existing classes.

• Two principal (and compatible) approaches:

– use inheritance (for example, using a JFrame to make a derived class) – use a Swing class as a container, placing components in the container (for example, adding a button to a content pane)

The JPanel Class

• A GUI can be organized hierarchically, with window-like

containers inside other window-like containers.

• Class JPanel is a simple container that does little more

than hold components.

• Components can be placed in a JPanel which can be

placed in another JPanel, … which can be placed in a

JFrame.

• To place two components in BorderLayout.SOUTH for

example, simply place the two components in a panel and place the panel in the BorderLayout.SOUTH position.

• The panel has its own layout manager.

The JPanel Class, cont.

The JPanel Class, cont.

The Container Class

• Class Container is a predefined class.
• An object of a class which descends from class

Container is called a container class and can

have components added to it.

• Class JFrame is a descendent of class Container,

permitting any JFrame object to hold labels, buttons, panels, and other components.

• Class JPanel is a descendent of class Container,

permitting any JPanel object to hold labels, buttons, other panels, and other components.

The JComponent Class

• Class JComponent is similar to class Container, and

plays a similar role for components.

• Any class that descends from JComponent is called

a JComponent class.

• Any JComponent object can be added to any

Container object.

• Since class JComponent descends from class

Container, a JComponent object can be added to

another JComponent object.

Adding Components

• To add a component to a JFrame, use method

getContentPane to obtain the content pane, and the use

method add with the content pane as the calling object.

• example

Container contentPane = getContentPane(); Jlabel label = new Jlabel(“Click Here”); contentPane.add(label);

• Add a button to a Panel:

JPanel buttonPanel = new JPanel(); JButton stopButton = new JButton(“Stop”); buttonPanel.add(stopButton);

Objects in Swing Containers

• Swing containers use three kinds of objects:

– the container class itself (such as a panel) – the components (labels, buttons, panels, etc.) – the layout manager.

• Typically, a GUI interface, and many subparts of

the GUI, will consist of these three kinds of

• bjects.

Text I/O using jTextField

• Create a text field with some initial text

JTextField textfield = new JTextField("Initial Text");

• Create a text field with some initial text and a default number of

columns. // The number of columns controls the preferred width of the component; // each column is roughly the size of an M in the current font. int cols = 30; textfield = new JTextField("Initial Text", cols);

Text I/O using JTextArea

• Create a text area with some initial text

JTextArea textarea = new JTextArea("Initial Text");

• Create a text area with some initial text and a default number of

rows and columns. // This number of rows and columns controls the preferred width and height of the component; // each row and column is rougly the size of an M in the current font. int rows = 20; int cols = 30; textarea = new JTextArea("Initial Text", rows, cols);

Text box / Text area: Methods used

• Method getText returns the text written in an object of class

JTextArea.

• Method setText of class JTextArea changes the text in the text area

into whatever is provided as the argument to method setText.

• Limit the # of characters entered:

JTextField name = new JTextField(10);

• The number of characters (per line) is not absolute, but represents

the space needed for one ‘m’ character.

– Note: the lower case "m" is the widest character in most character sets

• For ReadOnly data: for a textbox named “Name”

– Name.setEditable(false);

• To Retrieve text

– Name.getText().trim();

• Note: check for NumberFormatException to check for unwanted

special characters (comma, - ,””) in “Name”

Quiz

• Create a text field object named “Name”

– Set the default value of the text as “put name” – Set the length of this text field to 10