Java Swing4th April 2008

CS 180Department 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 BOXMany 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 windowmyWindow.setSize(width,height);

//set the title of the windowmyWindow.setTitle("this is a window");

//create the labelJLabel myLabel=new JLabel("this is a label");

//add the label to the content pane of the windowmyWindow.getContentPane().add(myLabel);

//set color of the content panemyWindow.getContentPane().setBackground(Color.CYAN);

//make the window visiblemyWindow.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 Codeimport 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 buttonString message = "You are exiting this window";JOptionPane.showMessageDialog(null, message);

System.exit(0);}

}

Listener Class

Example: OutputStep 1: user clicks this close button

Step 2: user will see this dialog box

Step 3: window is closed

Methods of Class WindowAdapterSome 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 useimport 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);andcontent.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 usingJButton 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 JFrameobject.

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 one listener.)

Buttons and an Action Listener

The actionPerformed Method

• An actionListener class must have a method named actionPerformed that has one parameter of type ActionEvent.

• syntaxpublic void actionPerformed(ActionEvente)

{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.

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

• This permits the same string to be written on 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.

• exampleContainer 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 objects.

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