Event-Driven Programming

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Chapter 7
Event-Driven Programming Using the AWT

• Event-Driven Programming
• GUIs and the AWT
• Simple Window Interfaces
• Components, Containers, and Layout Managers
• Panels and Text Components
• Adding Menus
• Inner Classes

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Event-Driven Programming

• A different approach to program design
• Programs react to events, e.g.
• mouse click
• key press
• menu selection
• button selection
• message sent from printer
• etc.
• GUI graphical user interface
• windows, scroll bars, menus, radio buttons, etc.
• a special case of event-driven programming
• the only type of event we will be concerned with
  in this chapter
• An event is an object

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AWT

• Abstract Window Toolkit
• Implements GUI
• A standard part of Java
• Not fancy, but adequate
• Makes extensive use of inheritance
• Based on events and event handlers
• Firing an event when an object generates an
  event
• Listener object
• every object that can fire an event, such as a
  button, can have one or more listener objects
• Listener objects have methods (called event
  handlers) that perform actions depending on the
  event
• You the programmer write these event handlers

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WARNING!

• Programs that use the AWT can hang the computer
• Save your work before running
• a program using the AWT!

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Example include the AWT library extend Frame

• The 1st two lines are needed when you use the AWT
  library
• 3rd line starts the class definition
• FirstWindow is derived from AWT Frame class
• Frame is derived from the more basic class AWT
  Window class
• Frame has a window, border, title bar, and close
  button
• Frame will be modified by adding properties to
  meet the program requirements

The first few lines of FirstWindow (Display
7.1/page 329)
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Chapter 6
Java an Introduction to Computer Science
Programming - Walter Savitch
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A word about displays pixels
(0, 0)

• Displays are organized as a 2-dimensional grid of
  pixels
• pixel picture element
• the smallest "addressable" unit of a screen
• "Address" coordinates
• (width, height)
• The origin has coordinates (0,0) and is in upper
  left
• increasing width is to right
• increasing height is down

Increasing width (x direction)
(75, 100)
Increasing height (y direction)
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Example define paint method Graphics class and
drawString
Additional method paint (Display 7.1/page 329)

• FirstWindow inherits properties of Frame and adds
  paint method
• paint is called by AWT, not by FirstWindow
• One argument, g, of type Graphics, is passed
• think of it as a portion of the screen in which
  the object is to be painted
• Every Graphics object has a drawString method
• 75 and 100 are the coordinates of the start of
  the window
• they depend on the programming environment
• you may have to adjust the numbers
• some things are done by trial and error try it
  and see

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Chapter 6
Java an Introduction to Computer Science
Programming - Walter Savitch
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Exampledefine system constants
Conventional style of declaring systemconstants
(Display 7.1/page 329)

• The window width and height are system constants
  that specify the window size in pixels
• may adjust for different systems
• public allows other classes to read the values
• static final prevents any other class from
  changing the values
• Using variables instead of "hard coded" numbers
  simplifies maintenance
• just change these two lines and recompile to
  change the window size
• avoids looking through the entire file for every
  occurrence of the numbers

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Chapter 6
Java an Introduction to Computer Science
Programming - Walter Savitch
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Example display windowdefine and register
listeners

• 1) setSize a Frame method to set the size of the
  window
• 2) listener an object of the windowDestroyer
  class
• an object that receives events from an object
• should close window if "close window" button is
  clicked
• must be defined (written)
• 3) Registering the listener
• associates the listener object with an
  event-firing object
• the listener will listen for events fired by the
  associated object
• 4) setVisible a Frame method to make the window
  visible

The main method (Display 7.1/page 329)
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Chapter 6
Java an Introduction to Computer Science
Programming - Walter Savitch
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Example windowAdapter abstract class and
System.exit

• windowAdapter
• Often the parent for windows (descendents of
  Frame class)
• an abstract class it has methods but they are
  undefined, so you need to define whichever
  methods you will use
• See Display 7.3/page 334 for windowAdapter
  methods
• "close window" in the only event in this case
• so windowClosing method must be defined
• System.exit(0) ends the Java program
• "0" is the conventional value used to indicate
  normal termination

windowDestroyer definition (Display 7.2/page 332)
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Chapter 6
Java an Introduction to Computer Science
Programming - Walter Savitch
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Useful methods in Frame class

• setTitle inserts a string in the title bar of the
  window
• setBackground(Color.blue) specifies the
  background color of the window
• Display 7.6/page 343 lists other predefined color
  values
• addWindowListener registers a listener for events
  
• setSize and paint have already been described
• etc. - see Display 7.7/page 344

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Two principle ways tocreate new classes from old
ones

• One method Inheritance
• Use extend to create a custom window based on an
  existing class
• For example
• FirstWindow extends Frame

• Another Container class
• Use a container class
• Use the add method to place items in it
• Items placed in a container class are called
  components
• Three kinds of objects to deal with
• 1. The container class itself (usually a window)
• 2. The components to add to it
• 3. The layout manager

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AWT container class

• AWT container classes Window, Panel, Frame
• All three are descendents of the class Container
• Custom windows are built by adding components to
  a container class, usually in the constructor
• Use the add method inherited from Container to
  place items in the new (container) class
• Components items added to a container
• for windows buttons, menus, text fields, etc.
• Component is also a class
• The AWT class hierarchy with Container and
  Component classes is shown on the next slide
  (Display 7.9/page 350)

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Hierarchy of AWT classes
Object
Display 7.9/page 350
MenuComponent
Component
AWT
MenuItem
Container
Button
Menubar
Label
Menu
Window
TextComponent
Panel
Frame
TextArea
TextField
Key
Class
If there is a line between two classes, then the
lower class is a derived class of the higher
class. Not all AWT components are shown.
Abstract Class
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Example creating a window using a container
class and components
ButtonDemo constructor

• ButtonDemo(Display 7.8/page 347)
• The constructor is shown in box at right
• Note the code to create and add buttons (shown in
  blue)

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ExampledrawString and repaint
Using a String instance variable in drawString

• The String instance variable theText is used as
  an argument in drawString so the text inside the
  window can be changed easily
• actionPerformed method determines which event
  occurred (which button was clicked) and changes
  the value of theText accordingly
• repaint()is called to update the window with the
  new value of theText

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Example action listeners

• Different kinds of components require different
  kinds of listeners
• Buttons fire events of type ActionEvent which
  require action listeners
• ActionListener is not a class, it is a property
  (or interface) of a class
• To give the ActionListener property to a class
• 1. Add implements ActionListener to the
  beginning of the class definition, e.g.
• public class ButtonDemo extends Frame implements
  ActionListener
• ltstatements to define classgt
• 2. Define a method named actionPerformed
• Use addActionListener to add an action listener
  for a button, e.g.
• Button stopButton new Button("Red")
• stopButton.addActionListener(this)
• See definition of ButtonDemo, Display 7.8/page
  347

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actionPerformed method
actionPerformed method for ButtonDemo

• Usually a selection (branching) statement that
  determines which action event fired and performs
  the appropriate action
• getActionCommand reads the text value assigned to
  the whichever button was clicked
• the text value was assigned when the Button
  object was created

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Layout managers

• Three basic layout managers provided with AWT1.
  FlowLayout2. BorderLayout3. GridLayout
• There are others but they are not covered in this
  text

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FlowLayout

• To create a FlowLayout objectsetLayout(new
  FlowLayout())
• Arranges components in the order they are added
• starting in upper left of window (or other
  object) and proceeding to the right
• when the top line is full it goes to the next
  line, etc.
• Example ButtonDemo (Display 7.8/page 347) uses a
  flow layout

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BorderLayout

• To create a BorderLayout objectsetLayout(new
  BorderLayout())
• Components can be placed in any one of five
  regions
• "North"
• "South"
• "East"
• "West"
• "Center"

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Example BorderLayout

• In the constructor for some container
  objectsetLayout(new BorderLayout())add(new
  Button("Up"),"North")add(new Button("Down"),"Sou
  th")add(new Button("Right"),"East")add(new
  Button("Left"),"West")add(new Button("Straight
  ahead"),"Center")

The result
Up
Left
Right
Straight ahead
Down
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GridLayout

• Arranges components in a two-dimensional grid
  (rows and columns)
• Each entry is the same size
• For a grid with two rows and three
  columnssetLayout(new GridLayout(2,3))
• the first argument is the number of rows
• the second argument is the number of columns
• creates a layout like this

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Summary for creating simple window interface

• 1. Derive a window object from Frame and add
  components.
• 2. You must register a window listener for the
  close-window button
• e.g., by adding the following to the GUI class
  definition within a constructor
  addWindowListener(new WindowDestroyer())
• See Display 7.2/page 332 for a definition of
  WindowDestroyer.
• 3. Some components, e.g. buttons, generate
  action events, so the GUI (or some other class)
  must be made an action listener.
• Every component that generates an action event
  should have an action listener registered with
  it.
• Use addActionListener to register an action
  listener.
• 4. Use implements ActionListener at the
  beginning of the class definition to make your
  GUI (or other class) an action listener.
• You also need to add a definition of the method
  actionPerformed to the class.
• 5. There are other ways, but this one is simple
  and commonly used.

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Programming tips

• 1. Don't reinvent the wheel copy a similar
  program and modify it.
• Obviously there is a lot of detail to deal with,
  so start with a program similar to what you need
  - the sample programs in each chapter are often
  good starting points for doing the Programming
  Exercises at the end of the chapter.
• However, it is NOT ok to copy another student's
  work!
• 2. See Display 7.11/page 360 for an outline of
  the code to create a simple GUI class. You may
  want to create this file and use it as a
  template copy it and modify the copy to create a
  new GUI.

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WindowListener options

• 1. Make the window itself the button listener by
  implementing the ActionListener interface
• we did this when we placed buttons in a window
• 2. Make the window listener a separate class
• we did this for the window-closing button by
  defining WindowDestroyer
• 3. Make the window itself the window listener by
  implementing the WindowListener interface

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More about interfaces

• Interfaces are just a collection of undefined
  methods
• A Java class can be derived from only one base
  class but can implement more than one interface
• All methods in an interface must be defined
• unused methods can be defined with empty bodies,
  e.g.public void windowDeiconified(WindowEvent
  e)
• ActionListener has only one method
  (ActionPerformed), but WindowListener has seven
  methods
• if you do not need the other methods, using
  ActionListener instead of WindowListener avoids
  having to write the empty method definitions

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The Panel class

• A container class to group objects
• Used to subdivide a Frame into different areas
• For example, if you want two buttons in the south
  area of a border layout, put the two objects in a
  panel and place the panel in the south position
• Typical program organization
• Create new panel
• set panel attributes, e.g. background color and
  panel layout
• create and add objects such as buttons
• Establish the overall layout and place the panel
  in the layout
• Example code Display 7.13/page 366

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Example Panel
PanelDemo constructor from Display 7.13/page 366
create panel
place panel in GUI
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Chapter 7
Java an Introduction to Computer Science
Programming - Walter Savitch
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Resulting GUI for PanelDemo
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TextArea and TextField classes

• TextArea defines an area to display text
• specifies how many lines and how many characters
  per line to display
• TextField also defines an area to display text
• but only the number of characters in the line is
  specified
• the field has only one line
• For both classes
• The user can enter text in the field
• getText reads text entered by the user
• setText writes text to the field
• more text can be written to the field than the
  specified size, but only an amount equal to the
  size will be displayed (the scroll bar must be
  used to see the remaining text)
• initial text can be specified when the object is
  created with new

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Example TextArea

• The constructor for TextAreaDemo (Display
  7.14/page 369) creates a panel with a text area
  10 lines by 40 characters
• The panel color is set to blue and the background
  of the text area to white
• The panel is placed in the center of the window

Creating a text area (excerpt from Display
7.14/page 369)
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Example setText and getText

• Using the same example, TextAreaDemo (Display
  7.14/page 369)
• Clear the text areatheText.setText("")
• Read the text entered by the user into String
  instance variable memo1memo1
  theText.getText()

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Label class

• Used to label text fields or other components
• Typical program organization to attach a label to
  a text field
• Create new panel
• set panel attributes, e.g. background color and
  panel layout
• add text field and label
• Place the panel in the layout

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Example Label

• Excerpt from LabelDemo (Display 7.15/page 374)
• name new TextField(20)
• namePanel.add(name, "South")
• Label nameLabel new Label("Enter your name
  here")
• namePanel.add(nameLabel, "Center")
• Resulting GUI

Name Tester
X

Enter your name here
A very good name!
A label
Test
Clear
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Inputting and outputting numbers in the AWT

• Problem the numbers read from or written to a
  text area are String type but we want numeric
  types
• all AWT input and output is String
• Conversion methods are needed
• to convert from numeric types to String, and
• to convert String to numeric types

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Inputting numbers methods, methods, methods, ...

• Numeric wrapper classes have a valueOf method
  that converts text numbers to the wrapper numeric
  type (Integer, Double, etc.)
• but we often want the corresponding primitive
  type, so we also need to use the appropriate
  "Value" method (intValue, doubleValue, etc.)
• and we want to ignore white space before and
  after the input, so the trim method is also used

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Examplereading an integer from a text area

• Combining all these results in a rather long
  statement, but it should make sense on careful
  analysis
• Input exampleRead an integer from text field
  inputOutputField
• int n Integer.valueOf(inputOutputField.getText()
  .trim()).intValue()

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Outputting numbers

• Not so many methods need to be strung together
• Just use toString to convert a numeric value to
  String, and setText to write it to the text
  area.
• Output exampleWrite an integer to text field
  inputOutputField
• int n 31
• inputOutputField.setText(Integer.toString(n))

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Adding menus

• In the AWT menus are not a component but are
  added in ways similar to adding components
• Three AWT classes are used
• Menu
• Menu
• MenuItem
• The basic approach is to use an add method to
• put MenuItems in a Menu and
• put Menus in a Menu.

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Examplecreating a menu
Excerpt from MenuDemo constructor (Display
7.18/page 386)
The resulting GUI before and after clicking Memos
is shown on page 388
Note the method to add a menu bar is setMenuBar,
not add
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Chapter 7
Java an Introduction to Computer Science
Programming - Walter Savitch
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Inner classes

• An inner class is a class that is defined inside
  another class
• A full description is beyond the scope of this
  text
• However, one situation has already been
  described
• using the AWT to create GUIs
• Another simple situation is common and is also
  described
• helping classes
• Moving the definition of a helping class inside
  the class definition has two advantages
• it allows the helping class to access all
  instance variables, including private ones
• it improves the level of information hiding
• For these reasons inner classes are often used as
  listeners to handle events fired within the outer
  class

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Summary ...

• GUIs are written using event-driven programming
• In event-driven programming, a user action, like
  a mouse click, generates an event and the that
  event is automatically passed to an
  event-handling program to perform the appropriate
  action.
• Two main ways to build a GUI with the AWT
• 1. Use inheritance to create a derived class
  from a preexisting one
• 2. Add components to a container
• A window is defined as a derived class of Frame
• A button is an object of the class button
• The add method is used to add components to a
  container
• Components in a container are arranged by a
  layout manager object

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summary, continued

• A panel is a container to organize objects inside
  a larger container.
• Text fields and text areas are used by GUIs for
  input and output of text, including numbers in
  text format
• methods exist to convert numeric types to and
  from text
• MenuBars are created by adding Menus and
  MenuItems in a manner similar to putting
  components in containers, but they are not
  members of the classes Container or Components
• Both buttons and menu items fire action events
  and so should have an ActionListener registered
  with them to respond to the event