• A GUI (graphical user interface) is a windowing system that interacts with the user

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Chapter 17

Swing I

Introduction to Swing Introduction to Swing

A GUI ( hi l i t f ) i i d i t th t

• A GUI (graphical user interface) is a windowing system that interacts with the user

• The Java AWT (Abstract Window Toolkit) package is the ( ) p g original Java package for creating GUIs

• The Swing package is an improved version of the AWT

– However it does not completely replace the AWT – However, it does not completely replace the AWT

– Some AWT classes are replaced by Swing classes, but other AWT classes are needed when using Swing

• Swing GUIs are designed using a form of object oriented

• Swing GUIs are designed using a form of object‐oriented programming known as event‐driven programming

Events Events

E d i i i i l

• Event‐driven programming is a programming style that uses a signal‐and‐response approach to programming

programming

• An event is an object that acts as a signal to another object know as a listener

object know as a listener

• The sending of an event is called firing the event

Th bj t th t fi th t i ft GUI t

– The object that fires the event is often a GUI component, such as a button that has been clicked

Listeners Listeners

• A listener object performs some action in response to the event p

– A given component may have any number of listeners

listeners

– Each listener may respond to a different kind of event or multiple listeners might may respond to event, or multiple listeners might may respond to the same events

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Exception Objects Exception Objects

• An exception object is an event

– The throwing of an exception is an example of g p p firing an event

• The listener for an exception object is the

• The listener for an exception object is the catch block that catches the event

Event Handlers Event Handlers

• A listener object has methods that specify what will happen when events of various pp kinds are received by it

– These methods are called event handlers – These methods are called event handlers

• The programmer using the listener object will define or redefine these event‐handler

methods

Event Firing and an Event Listener

Event Firing and an Event Listener Event Driven Programming Event‐Driven Programming

• Event‐driven programming is very different from most programming seen up until now

– So far, programs have consisted of a list of statements executed in order

– When that order changed, whether or not to perform

certain actions (such as repeat statements in a loop,

branch to another statement or invoke a method) was

branch to another statement, or invoke a method) was

controlled by the logic of the program

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Event Driven Programming Event‐Driven Programming

d i i bj

• In event‐driven programming, objects are created that can fire events, and listener objects are created that can react to the events

• The program itself no longer determines the order in which things can happen g pp

– Instead, the events determine the order

Event Driven Programming Event‐Driven Programming

• In an event‐driven program, the next thing that happens depends on the event that occurs

• In particular, methods are defined that will never be explicitly invoked in any program

– Instead, methods are invoked automatically when an event signals that the method needs to be called

A Simple Window A Simple Window

A i l i d i t f bj t f th JF l

• A simple window can consist of an object of the JFrame class

– A JFrame object includes a border and the usual three buttons for minimizing, changing the size of, and closing the window

Th l i f d i h j i k

– The JFrame class is found in the javax.swing package JFrame firstWindow = new JFrame();

• A JFrame can have components added to it, such as buttons, p , , menus, and text labels

– These components can be programmed for action firstWindow.add(endButton); ( );

– It can be made visible using the setVisible method firstWindow.setVisible(true);

A First Swing Demonstration (Part 1 of 4) A First Swing Demonstration (Part 1 of 4)

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A First Swing Demonstration (Part 2 of 4) A First Swing Demonstration (Part 2 of 4)

A First Swing Demonstration (Part 3 of 4) A First Swing Demonstration (Part 3 of 4)

A First Swing Demonstration (Part 4 of 4)

A First Swing Demonstration (Part 4 of 4) Some Methods in the Class JFrame (Part 1 of

3)

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Some Methods in the Class JFrame (Part 2 of 3)

Some Methods in the Class JFrame (Part 3 of 3)

Pixels and the Relationship between Resolution and Size

• A pixel is the smallest unit of space on a screen

– Both the size and position of Swing objects are measured in pixels

– The more pixels on a screen, the greater the screen resolution

• A high‐resolution screen of fixed size has many pixels

– Therefore each one is very small Therefore, each one is very small

• A low‐resolution screen of fixed size has fewer pixels

– Therefore, each one is much larger

• Therefore, a two‐pixel figure on a low‐resolution screen will look larger than a two‐pixel figure on a high‐

resolution screen

Pitfall: Forgetting to Program the Close‐Window Button

• The following lines from the FirstSwingDemo program ensure that when the user clicks the close‐

window button, nothing happens g pp

firstWindow.setDefaultCloseOperation(

JFrame.DO_NOTHING_ON_CLOSE);

• If this were not set the default action would be

• If this were not set, the default action would be JFrame.HIDE_ON_CLOSE

– This would make the window invisible and inaccessible, but would not end the program

would not end the program

– Therefore, given this scenario, there would be no way to click the "Click to end program" button

• Note that the close window and other two

• Note that the close‐window and other two

accompanying buttons are part of the JFrame object, and not separate buttons

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Buttons Buttons

• A button object is created from the class JButton and can be added to a JFrame

– The argument to the JButton constructor is the string that appears on the button when it is displayed

d

JButton endButton = new

JButton("Click to end program.");

firstWindow add(endButton);

firstWindow.add(endButton);

Action Listeners and Action Events Action Listeners and Action Events

Cli ki b tt fi t

• Clicking a button fires an event

• The event object is "sent" to another object called a listener

– This means that a method in the listener object is invoked j automatically

– Furthermore, it is invoked with the event object as its argument

• In order to set up this relationship, a GUI program must do In order to set up this relationship, a GUI program must do two things

1. It must specify, for each button, what objects are its listeners, i.e., it must register the listeners

must register the listeners

2. It must define the methods that will be invoked automatically when the event is sent to the listener

Action Listeners and Action Events Action Listeners and Action Events

EndingListener buttonEar = new

EndingListener());

endButton.addActionListener(buttonEar);

• Above, a listener object named buttonEar is created and registered as a listener for the button named endButton

– Note that a button fires events known as action events, which are handled by listeners known as action listeners

Action Listeners and Action Events Action Listeners and Action Events

• Different kinds of components require different kinds of listener classes to handle the events they fire

• An action listener is an object whose class implements the ActionListener interface

– The ActionListener interface has one method heading that must be implemented

public void actionPerformed(ActionEvent e)

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Action Listeners and Action Events Action Listeners and Action Events

public void actionPerformed(ActionEvent e) {

System.exit(0);

}

• The EndingListener class defines its actionPerformed method as above actionPerformed method as above

– When the user clicks the endButton, an action event is sent to the action listener for that button

– The The EndingListener EndingListener object object buttonEar buttonEar is the action is the action listener for endButton

– The action listener buttonEar receives the action event as the parameter e p to its actionPerformed method, which is , automatically invoked

– Note that e must be received, even if it is not used

Pitfall: Changing the Heading for actionPerformed

Wh th ti P f d th d i i l t d i

• When the actionPerformed method is implemented in an action listener, its header must be the one specified in the ActionListener interface

– It is already determined, and may not be changed – Not even a throws clause may be added

bli id ti P f d(A ti E t ) public void actionPerformed(ActionEvent e)

• The only thing that can be changed is the name of the parameter, since it is just a placeholder

parameter, since it is just a placeholder

– Whether it is called e or something else does not matter, as long as it is used consistently within the body of the method

Tip: Ending a Swing Program Tip: Ending a Swing Program

GUI f b d ki d f i fi i l

• GUI programs are often based on a kind of infinite loop

– The windowing system normally stays on the screen until the user indicates that it should go away

• If the user never asks the windowing system to go away, it will never go away

d d b d

• In order to end a GUI program, System.exit must be used when the user asks to end the program

– It must be explicitly invoked or included in some library code that is It must be explicitly invoked, or included in some library code that is executed

– Otherwise, a Swing program will not end after it has executed all the code in the program

code in the program

A Better Version of Our First Swing GUI A Better Version of Our First Swing GUI

• A better version of FirstWindow makes it a derived class of the class JFrame

– This is the normal way to define a windowing interface y g

• The constructor in the new FirstWindow class starts by calling the constructor for the parent class using super();

super();

– This ensures that any initialization that is normally done for all objects of type JFrame will be done

• Almost all initialization for the window FirstWindow

• Almost all initialization for the window FirstWindow is placed in the constructor for the class

• Note that this time, an anonymous object is used as the action listener for the endButton

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The Normal Way to Define a JFrame (Part 1 of 4)

The Normal Way to Define a JFrame (Part 2 of 4)

The Normal Way to Define a JFrame (Part 3 of 4)

The Normal Way to Define a JFrame (Part 4 of

4)

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Labels Labels

• A label is an object of the class JLabel

– Text can be added to a JFrame using a label

– The text for the label is given as an argument when the JLabel is created

– The label can then be added to a JFrame

JLabel greeting = new JLabel("Hello");

dd( ti ) add(greeting);

Color Color

I J l i bj t f th l C l

• In Java, a color is an object of the class Color

– The class Color is found in the java.awt package

– There are constants in the Color class that represent a number of

b i l

basic colors

• A JFrame can not be colored directly

– Instead, a program must color something called the content pane of , p g g p the JFrame

– Since the content pane is the "inside" of a JFrame, coloring the content pane has the effect of coloring the inside of the JFrame – Therefore, the background color of a JFrame can be set using the

following code:

getContentPane().setBackground(Color);

The Color Constants The Color Constants

A JFrame with Color (Part 1 of 4) A JFrame with Color (Part 1 of 4)

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A JFrame with Color (Part 2 of 4) A JFrame with Color (Part 2 of 4)

A JFrame with Color (Part 3 of 4) A JFrame with Color (Part 3 of 4)

A JFrame with Color (Part 4 of 4)

A JFrame with Color (Part 4 of 4) Containers and Layout Managers Containers and Layout Managers

• Multiple components can be added to the content pane of a JFrame using the add method

method

– However, the add method does not specify how these components are to be arranged

• To describe how multiple components are to be arranged, a layout manager is used

Th b f l l h

– There are a number of layout manager classes such as BorderLayout, FlowLayout, and

GridLayout

– If a layout manager is not specified, a default layout

manager is used

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Border Layout Managers Border Layout Managers

• A BorderLayout manager places the

components that are added to a JFrame object into five regions

into five regions

– These regions are: BorderLayout.NORTH, BorderLayout.SOUTH, BorderLayout.EAST,

d BorderLayout.WEST, and BorderLayout.Center

• A A BorderLayout BorderLayout manager is added to a manager is added to a JFrame using the setLayout method

– For example:

setLayout(new BorderLayout());

The BorderLayout Manager (Part 1 of 4) y g ( )

The BorderLayout Manager (Part 2 of 4) y g ( )

The BorderLayout Manager (Part 3 of 4) y g ( )

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The BorderLayout Manager (Part 4 of 4) y g ( )

BorderLayout Regions BorderLayout Regions

Border Layout Managers Border Layout Managers

Th i di h h f h fi

• The previous diagram shows the arrangement of the five border layout regions

– Note: None of the lines in the diagram are normally visible Note: None of the lines in the diagram are normally visible

• When using a BorderLayout manager, the location of the component being added is given as a second argument to the

dd h d add method

add(label1, BorderLayout.NORTH);

– Components can be added in any order since their location is specified Components can be added in any order since their location is specified

Flow Layout Managers Flow Layout Managers

Th Fl L t i th i l t l t

• The FlowLayout manager is the simplest layout manager

setLayout(new FlowLayout());

– It arranges components one after the other, going from left to right

– Components are arranged in the order in which they are added

• Since a location is not specified the Since a location is not specified, the add add method has method has only one argument when using the

FlowLayoutManager

add.(label1);

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Grid Layout Managers

A G idL t t i t

Grid Layout Managers

• A GridLayout manager arranges components in a two‐

dimensional grid with some number of rows and columns

setLayout(new GridLayout(rows, columns)); y ( y ( , ));

– Each entry is the same size

– The two numbers given as arguments specify the number of rows and columns

columns

– Each component is stretched so that it completely fills its grid position

– Note: None of the lines in the diagram are normally visible

Grid Layout Managers Grid Layout Managers

Wh i th G idL t l th th d

• When using the GridLayout class, the method add has only one argument

add(label1);

– Items are placed in the grid from left to right

– The top row is filled first, then the second, and so forth – Grid positions may not be skipped

• Note the use of a main method in the GUI class it lf i th f ll i l

itself in the following example

– This is often a convenient way of demonstrating a class

The GridLayout Manager (Part 1 of 4) The GridLayout Manager (Part 1 of 4)

The GridLayout Manager (Part 2 of 4) The GridLayout Manager (Part 2 of 4)

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The GridLayout Manager (Part 3 of 4) The GridLayout Manager (Part 3 of 4)

The GridLayout Manager (Part 4 of 4) The GridLayout Manager (Part 4 of 4)

Some Layout Managers

Some Layout Managers Panels Panels

• A GUI is often organized in a hierarchical fashion, with containers called panels inside other containers

• A panel is an object of the JPanel class that serves as a simple container

– It is used to group smaller objects into a larger component (the panel)

– One of the main functions of a JPanel object is to

subdivide a JFrame or other container

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Panels Panels

• Both a JFrame and each panel in a JFrame can use different layout managers

– Additional panels can be added to each panel, and each panel p p , p can have its own layout manager

– This enables almost any kind of overall layout to be used in a GUI setLayout(new BorderLayout());

JPanel somePanel = new JPanel();

somePanel.setLayout(new FlowLayout());

• Note in the following example that panel and button g p p objects are given color using the setBackground method without invoking getContentPane

– The The getContentPane getContentPane method is only used when adding color method is only used when adding color to a JFrame

Using Panels (Part 1 of 8) Using Panels (Part 1 of 8)

Using Panels (Part 2 of 8) Using Panels (Part 2 of 8)

Using Panels (Part 3 of 8) Using Panels (Part 3 of 8)

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Using Panels (Part 4 of 8) Using Panels (Part 4 of 8)

Using Panels (Part 5 of 8) Using Panels (Part 5 of 8)

Using Panels (Part 6 of 8)

Using Panels (Part 6 of 8) Using Panels (Part 7 of 8) Using Panels (Part 7 of 8)

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Using Panels (Part 8 of 8) Using Panels (Part 8 of 8)

The Container Class The Container Class

A l h i d d l f h l C i i

• Any class that is a descendent class of the class Container is considered to be a container class

– The Container e Co ta e class is found in the java.awt c ass s ou d t e ja a.a t package, not in the pac age, ot t e Swing library

• Any object that belongs to a class derived from the

Container class (or its descendents) can have components Container class (or its descendents) can have components added to it

• The classes The classes JFrame JFrame and and JPanel JPanel are descendent classes of are descendent classes of the class Container

– Therefore they and any of their descendents can serve as a container

The JComponent Class The JComponent Class

d d l f h l

• Any descendent class of the class

JComponent is called a component class

– Any JComponent object or component can be added to any container class object

– Because it is derived from the class Container, a JComponent can also be added to another JComponent

Objects in a Typical GUI Objects in a Typical GUI

l b il i i i

• Almost every GUI built using Swing container classes will be made up of three kinds of objects:

1. The container itself, probably a panel or window‐like object

2. The components added to the container such as labels, buttons, and panels

3. A layout manager to position the components inside the container

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Hierarchy of Swing and AWT Classes

Tip: Code a GUI's Look and Actions Separately Tip: Code a GUI s Look and Actions Separately

Th t k f d i i S i GUI b di id d i t t

• The task of designing a Swing GUI can be divided into two main subtasks:

1. Designing and coding the appearance of the GUI on the screen 2. Designing and coding the actions performed in response to user

actions

• In particular, it is useful to implement the p p

actionPerformed method as a stub, until the GUI looks the way it should

public void actionPerformed(ActionEvent e)

p ( )

{}

• This philosophy is at the heart of the technique used by the Model View Controller pattern

Model‐View‐Controller pattern

The Model View Controller Pattern

The Model‐View‐Controller Pattern Menu Bars Menus and Menu Items Menu Bars, Menus, and Menu Items

A i bj f h l

• A menu is an object of the class JMenu

• A choice on a menu is called a menu item, and is an

bj f h l

object of the class JMenuItem

– A menu can contain any number of menu items

A i i id ifi d b h i h l b l i d i

– A menu item is identified by the string that labels it, and is displayed in the order to which it was added to the menu

• The add method is used to add a menu item to a

• The add method is used to add a menu item to a menu in the same way that a component is added to a container object

a container object

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Menu Bars Menus and Menu Items Menu Bars, Menus, and Menu Items

• The following creates a new menu, and then adds a menu item to it

JMenu diner = new

JMenu("Daily Specials");

t l h

JMenuItem lunch = new

JMenuItem("Lunch Specials");

lunch addActionListener(this);

lunch.addActionListener(this);

diner.add(lunch);

– Note that the this parameter has been registered as – Note that the this parameter has been registered as

an action listener for the menu item

Nested Menus Nested Menus

• The class JMenu is a descendent of the JMenuItem class

– Every JMenu can be a menu item in another menu

– Therefore, menus can be nested

b dd d h h

• Menus can be added to other menus in the same way as menu items

Menu Bars and JFrame Menu Bars and JFrame

A b i t i f t i ll l d

• A menu bar is a container for menus, typically placed near the top of a windowing interface

• The add method is used to add a menu to a menu bar in the same way that menu items are added to a menu

JMenuBar bar = new JMenuBar();

bar.add(diner);

• The menu bar can be added to a JFrame in two different ways

1 Using the setJMen Bar method 1. Using the setJMenuBar method

setJMenuBar(bar);

2. Using the add method – which can be used to add a menu bar to a JFrame or any other container

JFrame or any other container

A GUI with a Menu (Part 1 of 8) A GUI with a Menu (Part 1 of 8)

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A GUI with a Menu (Part 2 of 8) A GUI with a Menu (Part 2 of 8)

A GUI with a Menu (Part 3 of 8) A GUI with a Menu (Part 3 of 8)

A GUI with a Menu (Part 4 of 8)

A GUI with a Menu (Part 4 of 8) A GUI with a Menu (Part 5 of 8) A GUI with a Menu (Part 5 of 8)

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A GUI with a Menu (Part 6 of 8) A GUI with a Menu (Part 6 of 8)

A GUI with a Menu (Part 7 of 8) A GUI with a Menu (Part 7 of 8)

A GUI with a Menu (Part 8 of 8) A GUI with a Menu (Part 8 of 8)

The AbstractButton and Dimension Classes

• The classes JButton and JMenuItem are derived classes of the abstract class named AbstractButton

AbstractButton

– All of their basic properties and methods are inherited from the class AbstractButton

• Objects of the Dimension class are used with buttons, menu items, and other objects to specify a size

specify a size

– The Dimension class is in the package java.awt

Dimension(int width, int height) ( , g )

– Note: width and height parameters are in pixels

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The setActionCommand Method The setActionCommand Method

Wh li k b i i fi d

• When a user clicks a button or menu item, an event is fired that normally goes to one or more action listeners

– The action event becomes an argument to an actionPerformed e act o e e t beco es a a gu e t to a act o e o ed method

– This action event includes a String instance variable called the action command for the button or menu item

action command for the button or menu item

– The default value for this string is the string written on the button or the menu item

This string can be retrieved with the getActionCommand method – This string can be retrieved with the getActionCommand method

e.getActionCommand()

The setActionCommand Method The setActionCommand Method

Th i d h d b d

• The setActionCommand method can be used to change the action command for a component

Thi i i ll f l h t b tt

– This is especially useful when two or more buttons or menu items have the same default action command strings g

JButton nextButton = new JButton("Next");

nextButton.setActionCommand("Next Button");

JMenuItem choose = new JMenuItem("Next");

choose.setActionCommand("Next Menu Item");

Some Methods in the Class AbstractButton (P 1 f 3)

(Part 1 of 3)

Some Methods in the Class AbstractButton (P 2 f 3)

(Part 2 of 3)

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Some Methods in the Class AbstractButton (P 3 f 3)

(Part 3 of 3)

Listeners as Inner Classes Listeners as Inner Classes

Of i d f h i i li bj

• Often, instead of having one action listener object deal with all the action events in a GUI, a separate ActionListener class is created for each button ActionListener class is created for each button or menu item

– Each button or menu item has its own unique action – Each button or menu item has its own unique action

listener

– There is then no need for a multiway if‐else statement y

• When this approach is used, each class is usually made a private inner class

Listeners as Inner Classes (Part 1 of 6) Listeners as Inner Classes (Part 1 of 6)

Listeners as Inner Classes (Part 2 of 6) Listeners as Inner Classes (Part 2 of 6)

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Listeners as Inner Classes (Part 3 of 6) Listeners as Inner Classes (Part 3 of 6)

Listeners as Inner Classes (Part 4 of 6) Listeners as Inner Classes (Part 4 of 6)

Listeners as Inner Classes (Part 5 of 6)

Listeners as Inner Classes (Part 5 of 6) Listeners as Inner Classes (Part 6 of 6) Listeners as Inner Classes (Part 6 of 6)

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Text Fields Text Fields

• A text field is an object of the class JTextField

– It is displayed as a field that allows the user to enter a single line of text

private JTextField name;

. . .

name = new JTextField(NUMBER_OF_CHAR);

– In the text field above at least In the text field above, at least NUMBER_OF_CHAR NUMBER OF CHAR characters can be visible

Text Fields Text Fields

Th i l t t ith dditi l St i

• There is also a constructor with one additional String parameter for displaying an initial String in the text field

JTextField name = new JTextField( (

"Enter name here.", 30);

• A Swing GUI can read the text in a text field using the h d

getText method

String inputString = name.getText();

• The method setText can be used to display a new text

• The method setText can be used to display a new text string in a text field

name.setText("This is some output");

A Text Field (Part 1 of 7) A Text Field (Part 1 of 7)

A Text Field (Part 2 of 7) A Text Field (Part 2 of 7)

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A Text Field (Part 3 of 7) A Text Field (Part 3 of 7)

A Text Field (Part 4 of 7) A Text Field (Part 4 of 7)

A Text Field (Part 5 of 7)

A Text Field (Part 5 of 7) A Text Field (Part 6 of 7) A Text Field (Part 6 of 7)

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A Text Field (Part 7 of 7) A Text Field (Part 7 of 7)

Text Areas Text Areas

A t t i bj t f th l JT tA

• A text area is an object of the class JTextArea

– It is the same as a text field, except that it allows multiple lines – Two parameters to the Two parameters to the JTextArea JTextArea constructor specify the minimum constructor specify the minimum

number of lines, and the minimum number of characters per line that are guaranteed to be visible

JTextArea theText = new JTextArea(5,20);

JTextArea theText new JTextArea(5,20);

– Another constructor has one addition String parameter for the string initially displayed in the text area

JTe tArea theTe t ne JTe tArea(

JTextArea theText = new JTextArea(

"Enter\ntext here." 5, 20);

Text Areas Text Areas

• The line‐wrapping policy for a JTextArea can be set using the method setLineWrap

– The method takes one boolean type argument

– If the argument is true, then any additional characters at the end of a line will appear on the following line of the text area

If th t i f l th t h t ill i

– If the argument is false, the extra characters will remain on the same line and not be visible

theText setLineWrap(true);

theText.setLineWrap(true);

Text Fields and Text Areas Text Fields and Text Areas

• A JTextField or JTextArea can be set so that it can not be changed by the user

theText.setEditable(false);

– This will set theText so that it can only be edited by the

GUI t th

GUI program, not the user

– To reverse this, use true instead (this is the default)

theText setEditable(true);

theText.setEditable(true);

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Tip: Labeling a Text Field Tip: Labeling a Text Field

• In order to label one or more text fields:

– Use an object of the class j JLabel

– Place the text field(s) and label(s) in a JPanel Treat the JPanel as a single component – Treat the JPanel as a single component

Numbers of Characters Per Line Numbers of Characters Per Line

• The number of characters per line for a

JTextField or JTextArea object is the number of em spaces

• An em space is the space needed to hold one uppercase letter M

– The letter M is the widest letter in the alphabet

– A line specified to hold 20 M 's will almost always be able to hold more than 20 characters

Tip: Inputting and Outputting Numbers Tip: Inputting and Outputting Numbers

Wh i i b f S i

• When attempting to input numbers from any Swing GUI, input text must be converted to numbers

If th t th b 42 i JT tFi ld th

– If the user enters the number 42 in a JTextField, the program receives the string "42" and must convert it to the integer 42 g

• The same thing is true when attempting to output a number

– In order to output the number 42, it must first be converted to the string "42"

The Class JTextComponent The Class JTextComponent

• Both JTextField and JTextArea are derived classes of the abstract class JTextComponent

• Most of their methods are inherited from

JTextComponent and have the same meanings

– Except for some minor redefinitions to account for having

just one line or multiple lines

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Some Methods in the Class JTextComponent (Part 1 of 2)

Some Methods in the Class JTextComponent (Part 2 of 2)

A Swing Calculator A Swing Calculator

A GUI f i l l l t k i t t l f

• A GUI for a simple calculator keeps a running total of numbers

– The user enters a number in the text field and then clicks The user enters a number in the text field, and then clicks either + or –

– The number in the text field is then added to or subtracted from the r nning total and displa ed in the te t field from the running total, and displayed in the text field – This value is kept in the instance variable result – When the GUI is first run, or when the user clicks the When the GUI is first run, or when the user clicks the

Reset button, the value of result is set to zero

A Swing Calculator A Swing Calculator

f h b i i

• If the user enters a number in an incorrect format, then one of the methods throws a NumberFormatException

– The exception is caught in the catch block inside the actionPerformed method

– Note that when this exception is thrown, the value of the instance variable result is not changed

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A Simple Calculator (Part 1 of 11) A Simple Calculator (Part 1 of 11)

A Simple Calculator (Part 2 of 11) A Simple Calculator (Part 2 of 11)

A Simple Calculator (Part 3 of 11)

A Simple Calculator (Part 3 of 11) A Simple Calculator (Part 4 of 11) A Simple Calculator (Part 4 of 11)