Swing part 1

1
Swing part 1

• Basic widgets and features

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How to learn new a widget

• How does it looks?
• The Swing tutorial has a visual index
• What does it do?
• What properties does it have?
• What meaningful events does if fire?
• What data does is supplies?
• Data properties, like value or text
• Alternatively, get the model itself by getModel()

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The JComponent class

• Root class for Swings widgets (except top-level)
• Based on AWT container
• A simple Swing widget is a composite AWT widget
• But it doesnt mean you should exploit this!
• Supports basic properties and events
• Widgets state
• Basic appearance
• Layout properties
• Mouse and key events

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Widgets state properties

• Visible or not
• Enabled / disabled
• Disabled widgets do not respond and are grayed
  out
• Tool tip text
• Important for helping the user
• Name
• Each widget has a name which identifies it within
  the containing widget

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Basic appearance properties

• Foreground and background color
• Mostly handled by the look feel
• Opaque or transparent
• Font (for text)
• Border
• Borders can surrounds all widgets
• Useful mainly for panels
• To be mentioned in details later

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Basic layout properties

• Usually the laying out will be done by the layout
  manager of the containing widget
• The programmer can supply tips for the layout
• Minimum and maximum size
• Preferred size
• X and Y alignment
• Most widgets (buttons, labels, toolbars...) will
  handle size automatically

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Common events

• All widgets support mouse and key events
• Mouse event occurs in respond to mouse pointer
  actions
• Motions
• Clicks
• But usually the widget will handle those
• Key events respond to key pressing
• Used for key accelerators

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JLabel class

• A very basic doesnt do anything but show
• Can include the properties
• Text
• Icon
• The text can be set relatively to the icon
• Horizontal text position
• LEFT, CENTER, RIGHT, LEADING or TRAILING
• Vertical text position
• TOP, CENTER or BUTTOM

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The Icon interface

• A common interface for icons in labels, buttons,
  and so on
• Most commonly used implementation is the
  ImageIcon, which reads the icon from GIF or JPEG
  files
• Need to be available to the application!
• Other implementations are usually specific to
  special look feels

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Swing buttons

• Swing has a family of buttons which derives from
  the AbstractButton class
• Simple buttons (JButton)
• Toggle buttons
• Simple toggle buttons (on/off)
• Check boxes
• Radio buttons
• Menu buttons like regular buttons but in a menu
• All of these share similar behavior and props.

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Making buttons

• Buttons are constructed similarly to labels, with
  text and/or an icon
• To make them work, it is needed also to add and
  action listener
• Alternatively you can use the action interface
  and the DefaultAction implementation
• Default action is also made with text and icon
• And also has abstract actionPerformed() method
• Useful to make a single action to different
  buttons
• Can be enabled or disabled for all usage

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The JButton class

• Simplest button class
• Used for buttons on panels or toolbars
• Uses ActionEvent and takes ActionEventListener as
  listener
• Resized by icon and text
• Very simple

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Selectable buttons

• Toggle buttons, check boxes and radio buttons can
  be selected or not
• All buttons has a selected property with
  isSelected() and setSelected() methods
• Makes sense only for selectable buttons, though
• When selected, they fire
• actionEvent
• ItemEvent
• ChangeEvent
• You can always get the state from the source
  itself

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Handling buttons events

• Need to
• Identify what to do which button was pressed
• Get the value selected or not
• You can
• Set a different listener to each button
• Can be an anonymous one
• Use the same listener to several buttons and
• Get the source from the event
• Use the action command string of action event

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On event objects

• Event objects are the parameters to the event
  listener methods, which are called when the event
  is fired
• Usually contains
• A pointer to the firing widget (the source)
• Information on the event (indexes, keys pressed)
• A marker isValueAdjusting()
• The combination of the event info and the sources
  properties allows getting all the needed
  information on the users actions

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Radio buttons

• Radio buttons are selectable buttons which comes
  in groups
• In a group only one radio button can be selected
• To force this, add the buttons to a ButtonGroup
  which forces this policy
• The button group works with the abstract button
  class, so actually it can work with any sort of
  button

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Menu buttons

• Menus are contained in a menu bar (JMenuBar)
• The frame has a JMenuBar property
• The menu bar contains menus
• The menus can contain
• Menu buttons, like menu items, menu check boxes
  and menu radio buttons
• Sub-menus, recursively (which extends menu item)
• Menu buttons acts just like normal buttons

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Selection widgets

• A common role for a widget is selecting one (or
  several) items out of a group
• Radio buttons and check boxes can serve somehow
  to do that
• More specialized widgets for that purpose
• Lists
• Combo boxes
• The items are objects, and can be anything,
  although rendered as strings by default

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JList class

• Can be initialized with an array or vector of
  items, or with a given list model (interface)
• Selection models single selection, single
  interval or multiple intervals
• Fires ListSelectionEvent which holds the first
  and last index selected
• can get selected items from list itself
• Can use DefaultListModel as model to add and
  remove items dynamically

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JComboBox class

• Very much like a single-selection list
• Can be edited as a text field if set as editable
• Therefore has a slightly different model
• Fires an ActionEvent when
• Selection changes
• Edited and user presses enter
• Can use actions

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Text widgets

• Swing has a set of widgets for handling text,
  from very simple to elaborated editors
• Simple text field and Password text field
• Plain text area
• Styled text area editor pane and text pane
• All can display and edit text conveniently
• The styled widgets ca display rich text format,
  pictures and even HTML
• Based on a document model

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JTextField class

• Has a text property which can be used to display
  text and read the entered text
• Size can be set by number of columns
• Events
• ActionEvent fired on enter
• Document model fires events on every character
• Better handling can be to get the text when
  needed (OK pressed, dialog closed)
• The JTextArea is very similar, but with rows also

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Code example

• A Swing calculator!

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Class declaration

• public class Calculator extends JFrame
• private JTextField _arg1
• private JTextField _arg2
• private JTextField _result
• private JComboBox _operands
• private JButton _equals

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Making the fields

• public Calculator()
• super("Swing calculator")
• setSize(500, 100)
• _arg1 new JTextField(10)
• _arg2 new JTextField(10)
• _result new JTextField(10)
• _operands new JComboBox( new String "Add",
  "Subtract", "Multiply", "Divide")

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Making a button using an action listener

• _equals new JButton("")
• _equals.addActionListener(
• new ActionListener()
• public void actionPerformed(ActionEvent e)
• calculate()
• )

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Putting the widgets together

• Container tPane getContentPane()
• tPane.setLayout(new FlowLayout())
• tPane.add(_arg1)
• tPane.add(_operands)
• tPane.add(_arg2)
• tPane.add(_equals)
• tPane.add(_result)
• setJMenuBar(makeMenuBar())

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Making an enabling action

• private Action makeEnabler(final JComponent
  comp, String text)
• return new AbstractAction(text, icon)
• public void actionPerformed( ActionEvent e)
  
• AbstractButton tBtn (AbstractButton)e
  .getSource()
• comp.setEnabled(tBtn.isSelected())

Actually, could also be done with an action
listener, or just a button
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Making the menu bar (1)

• private JMenuBar makeMenuBar()
• JMenu tMenu new JMenu("widgets")
• JMenuItem tArg1Stat
• new JCheckBoxMenuItem(
• makeEnabler(_arg1, "enable arg 1"))
• JMenuItem tArg2Stat
• new JCheckBoxMenuItem(
• makeEnabler(_arg2, "enable arg 2"))
• JMenuItem tOpsStat
• new JCheckBoxMenuItem(
• makeEnabler(_operands, "enable operands"))

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Making the menu bar (2)

• tArg1Stat.setSelected(true)
• tArg2Stat.setSelected(true)
• tOpsStat.setSelected(true)
• tMenu.add(tArg1Stat)
• tMenu.add(tOpsStat)
• tMenu.add(tArg2Stat)
• JMenuBar tRet new JMenuBar()
• tRet.add(tMenu)
• return tRet

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Calculating

• private void calculate()
• int tArg1 Integer.parseInt(_arg1.getText())
• int tArg2 Integer.parseInt(_arg2.getText())
• int tRes 0
• String tOp (String)_operands.getSelectedItem()
  
• if (tOp.equals("Add"))
• tRes tArg1 tArg2
• else if // and so on
• _result.setText(""tRes)

There are better ways to do this
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Running the calculator

• public static void main(String args)
• new Calculator().setVisible(true)

• That wasnt too bad But notice that a more
  complex GUI might need to
• Split into several classes (by panes or roles)
• Separate event handlers (if cant handle by
  anonymous ones)
• Separate the handled data (the model) from the
  display
• GUI is not a reason to write oversized classes!

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Using basic widgets - summary

• Did not covered (yet)
• Tables, trees and other heavily-modeled widgets
• Spinners and sliders (widgets with int value)
• Rich text editors
• Dialogs and choosers
• But covered enough to
• Learn a new widget from documentation
• Write simple event handlers
• Try it out!