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					      Lecture 6



Using AWT controls,
Layout Managers, and
       Menus
                       1
Introduction
   Controls are components that allow a user
    to interact with your application in various
    ways
   A layout manager automatically positions
    components within a container
   Frame window can include a standard-style
    menu bar. Each entry in a menu bar
    activates a drop-down menu of options
    from which user can choose

                                                   2
Control fundamentals
   The Awt support the following types of
    controls:
       Labels
       Push buttons
       Check boxes
       Lists
       Scroll bars
       Text editing
   These controls are subclass of components

                                                3
    Adding and removing controls
   To include a control in a window, we have to
          First create an instance of the desired control
          Then add it to a window by calling add() (defined by
           Container)
         i.e. Component add (Component compObj)
   Once a control has been added, it will automatically
    be visible whenever its parent window is displayed
   We can remove control from a window by calling
       void remove(Component obj)
   We can remove all controls by calling removeAll()
   Except labels (passive control), all controls generate
    events when they are accessed by the user
                                                                  4
Labels
   A label is an object of type Label
   It contains a String which it displays
   Label defines the following constructors:
       Label() //create blank label
       Label (String str) //create label with the string in left-justified
       Label (String str, int how) //create label with the string using
        alignment specified by how
   The value of how must be one of the three constants:
    Label.LEFT, Label.Right or Label.CENTER
   We can set or change the text by setText(String str) method
   Current label can be obtained by String getText() method
   Methods are also available like setAlignment() or
    getAlignment (See LabelDemo for example)
                                                                              5
Buttons
   Push buttons are objects of type Button
    Button()
    Button(String str) // str will label
   Label can be set by setLabel(String str) and label
    can be retrieved by String getLabel()
   Each time a button is pressed, an action event is
    generated, which is sent to registered listeners
   Each listener implements the ActionListener
    interface
   That interface defines the actionPerformed()
    method, which is called when an event occurs
   An ActionEvent object is supplied as an argument
                                                         6
7
Buttons (contd.)
   Which button has been pushed can be identified
    by a reference to the button that generated the
    event or by a reference to the label of the button
   The reference to the button can be obtained by
    getSource() method of ActionEvent object
   The label can be obtained by calling
    getActionCommand() method of ActionEvent
    object
   Button references can be stored into an array of
    buttons
   Example: ButtonDemo and ButtonList
                                                         8
Check Boxes
   A control that is used to turn an option on or off
   Check box can be used individually or as part of a
    group
   Check boxes are objects of the Checkbox class
    Checkbox() //unchecked checkbox with blank label
    Checkbox(String str) // unchecked but have a label
    Checkbox(String str, boolean on) //can be checked and have a label
    Checkbox(String str, boolean on, CheckboxGroup cbGroup)
    Checkbox(String str, CheckboxGroup cbGroup, boolean on) //can be
       included in a group if cbGroup is not null



                                                                         9
Check Boxes (contd.)
   Current state of a check box can be set by
    setState(boolean on) or retreived by boolean
    getState()
   Each time a check box is selected or deselected,
    an item event is generated
   Each listener implements the ItemListener
    interface that defines itemStateChanged() method
   An ItemEvent object is supplied as the argument
    to this method, which contain information about
    the event
   Example: CheckboxDemo
                                                       10
    Check Box Group
   Mutually exclusive check boxes in which one and only
    one check box in the group can be checked at a time
   Also called radio buttons
   First define a check box group by creating an object of
    type CheckboxGroup
   Then mention the group name in the check box
    constructor
   Which check box is selected can be determined by
    Checkbox getSelectedCheckbox()
   Any chekbox can be set by
    setSelectedCheckbox(Checkbox which)
   Example: CBGroup
                                                              11
    Choice Controls
   Choice class is used to create a pop-up list of items
    from which user may choose
   Each item in the list is a string that appears as a left-
    justified label in the order it is added to the Choice
    object
    void addItem (String name)
    void add(String name)
   Which item is selected can be can be determined by
    String getSelectedItem() or int getSelectedIndex()
   Number of items can be determined by int
    getItemCount()

                                                                12
Choice Controls (contd.)
   Any item can be set by select (int index) or
    select (String name)
   Any item name can be found by index by
    calling String getItem(int index)
   Choice control creates ItemEvent object
   Listeners implement ItemListener
    interface, that defines itemStateChanged()
    method
   Example: ChoiceDemo
                                                   13
Lists
   List class provides a compact, multiple-choice,
    scrolling selection list
   List object can be constructed to show any
    number of choices in the visible window
    List() //allow one item to be selected
    List(int numRows) //specifies number of rows
    List(int numRows, boolean multipleSelect) //multiple item
                           can be selected
   To add a slection to the list, call add(String name)
    or add(String name, int index)
   Indexing begin at 0and –1 indicates end of the
    list
                                                                14
Lists (contd.)
   Which item is/are currently selected can be
    determined by
    String getSelectedItem(),
    int getSelectedIndex()
Or String[ ] getSelectedItems(),
    int[ ] getSelectedIndexes
   Similar methods available like select(), getItem(),
    getItemCount() etc
   For double clicking an item, list control produce
         ActionEvent type object
         Implement ActionListener interface, which has
          actionPerformed() method
         Object’s getActionCommand() is used to track which
          item is double-clicked                             15
Lists (contd.)
   For single clicking an item, list control
    produce
       ItemEvent type object
       Implement ItemListener interface, which has
        itemStateChanged() method
       Object’s getStateChanged() is used to track
        whether any item is selected or deselected
       Object’s getItemSelectable() is used to track
        which item triggered this event
   Example: Listdemo
                                                        16
Scroll Bars
   This control is used to select continuous values
    between a specified minimum and maximum
   It may be oriented horizontally or vertically
   Each end has an arrow, click on it move current
    value of the scroll bar one unit in the direction
   The current value of the scroll bar relative to its
    minimum and maximum values is indicated by
    the slider box (or thumb) for the scroll bar
   Slider can be dragged to a new position
   By mouse click, slider can jump by more than 1
    (like page up and page down)
                                                          17
Scroll Bars (contd.)
   Scroll bars are encapsulated by the class Scrollbar
    Scrollbar() //create vertical scroll bar
    Scrollbar(int style) //style is Scrollbar.VERTICAL or
                         Scrollbar.HORIZONTAL
    Scrollbar(int style, int initialValue, int thumbSize, int min,
    int max) //specifies initial position, height of thumb,
                minimum and maximum position
   If Scroll bar is created by one of the first two
    constructors, then its other parameters must be set
    by
    setValues(int initialValue, int thumbSize, int min, int max)

                                                                     18
Scroll Bars (contd.)
   getValue() is used to obtain the current value of
    the scroll bar. setValue(int newValue) is used to
    set current value
   Minumim and maximum values can be obtained
    by getMinimum() and getMaximum()
   By default increment or decrement is 1. It can be
    changed by setUnitIncrement(int newIncr)
   By default, page-up and page-down increments
    are 10. It can be changed by
    setBlockIncrement(int newIncr)

                                                        19
Scroll Bars (contd.)
   Interaction with a scroll bar creates
    AdjustmentEvent object
   Its getAdjustmentType() is used to
    determine type of adjustment (i.e.
    BLOCK_DECREMENT, TRACK,
    UNIT_INCREMENT etc)
   To handle this event, we need to implement
    AdjustmentListener interface
   Example: SBDemo

                                                 20
      TextField/Edit control
   TextField is a single-line text-entry and a subclass of
    TextComponent
   Textfield defines the following constructors:
     TextField() //default text field
     TextField(int numChars) //wide specified
     TextField(String str) //default string in the text field
     TextField(String str, int numChars)
   Method to obtain the current string is
        String getText()
   To set a text, call setText(String str)
   A portion of the text under selection can be obtained
    by String getSelectedText()
   To select a portion of a text, call select(int startIndex,int
    endIndex)                                                       21
TextField/Edit control (contd.)
   Editibility of a text field can be set by
    setEditable(boolean canEdit)
   Text can be modified or not is checked by
    boolean isEditable()
   We can disable the echoing of the characters and
    display another character by setEchoChar(char
    ch)
   Similar methods are boolean echoCharIsSet() and
    char getEchoChar()
   TextField generates ActionEvent object
   Example: TextFieldDemo
                                                       22
TextArea
   It is a multiline editor
    TextArea()
    TextArea(int numLines, int numChars) //height specified
    TextArea(String str)
    TextArea(String str,int numLines, int numChars)
    TextArea(String str,int numLines,int numChars,int
       sBars) //specify scroll bar to have with it
   TextArea is a subclass of TextComponent, so it
    supports the getText(), setText(),
    getSelectedText(), select(), isEditable(),
    setEditable() etc. methods

                                                              23
TextArea (contd.)
   TextArea adds the following methods
    append(String str) //appends str at the end of current text
    insert(String str, int index) //insert str at specified index
    replaceRange(String str, int startIndex, int endIndex)
   TextArea generates got-focus and lost-
    focus events
   Example: TextAreaDemo


                                                                    24
Understanding Layout Managers
   Layout manager automatically arranges controls
    within a window by using some types of
    algorithm
   Until now, we have used he default layout
    manager
   Placing controls in a window by hand is not
    feasible, because:
       It is very tedious
       Sometimes the width and height information is not
        yet available

                                                            25
Layout Managers (contd.)
   Each container object has a layout manager
    associated with it.
   A layout manager is an instance of any class that
    implements the LayoutManager interface
   The layout manager is set by
    setLayout(LayoutManager layoutObj)
   layoutObj is a reference to the desired layout
    manager
   If layoutObj is set null, we have to position
    components manually

                                                        26
Layout Managers (contd.)
   The layout manager is notified each time a
    component is added to a container
   Whenever a container needs to be resized, the
    layout manager is consulted via its
    minimumLayoutSize() and preferredLayoutSize()
    methods
   Each component that is being managed by a
    layout manager contains the getPreferredSize()
    and getMinimumSize() methods
   The layout manager will honor these requests if at
    all possible, maintaining the integrity of the
    layout policy
                                                         27
FlowLayout
   It is the default layout manager, similar to
    how words flow in a text
    FlowLayout() //centers components and leaves five pixels
                      between each component
    FlowLayout(int how) //how can be FlowLayout.LEFT..etc
    FlowLayout(int how, int horz, int vert) //specifies
       horizontal and vertical spacing between components
   Example: FlowLayoutDemo


                                                               28
    Border Layout
   It has 4 narrow, fixed-width components at
    the edges and one large area in the center
   4 sides are called north, south, east and
    west, middle are is called center
    BorderLayout() //default order layout
    BorderLayout(int horz,int vert) //allows to specify
       horizontal and vertical space between components
   Components are added by
       void add(Component compObj, Object region);
   Example: BorderLayoutDemo
                                                          29
Using Insets
   Allows to leave a small amount of space
    between the container that holds our
    components and the window that contains it
    Insets(int top,int left,int bottom,int right)
   Insets is created in the overridden
    getInsets() method defined by Container
   Example: InsetsDemo


                                                    30
    Grid Layout
   It lays out components in a two-dimensional
    grid
    GridLayout() //single column grid layout
    GridLayout(int numRows, int numColumns)
    GridLayout(int numRows,int numColumns,int horz,int vert)
   If numRows=0, the column length is
    unlimited. Also if numColumns=0, the row
    length is unlimited
   Example: GridLayoutDemo

                                                               31
Menu Bars and Menus
   A menu bar displays a list of top-level menu
    choices
   Each choice is associated with a drop-down menu
   Menu is implemented by the classes MenuBar,
    Menu and MenuItem.
   First create a Menu bar, that contains one or more
    Menu objects
    MenuBar()
   Menu object can be created by
    Menu()
    Menu(String optionName)


                                                         32
    Menu Bars and Menus (contd.)
   Each Menu object contains a list of MenuItem
    objects, that can be selected by user. Its constructors
    are
    MenuItem()
    MenuItem (String itemName)
    MenuItem (String itemName, MenuShortcut keyAcccel)
   Since Menu is a subclass of MenuItem, hierarchy of
    nested submenus can be created
   A menu item can be enabled or disable by
    setEnable() method
   An items status can be determined by isEnabled()
   The name of a menu item can be changed by
    setLabel(); name can be retrieved by getLabel()
                                                          33
    Menu Bars and Menus (contd.)
   A checkable menu item can be created by
    CheckboxMenuItem, a subclass of MenuItem
   MenuItem can be added to a Menu by add()
   Also menu is added to a ManuBar by add()
   Menus only generate only ActionEvent when an
    item of type MenuItem is selected (ItemEvent is
    generated by CheckboxMenuItem)
   getItem() method of ItemEvent returns a reference
    to the item that generated this event
   Eample: MenuDemo
                                                        34
Dialog Boxes
   It holds a set of related controls and primarily
    used to obtain user input
   They are always a child window and don’t have
    any menu bars
   Dialog boxes can be modal (all input is directed
    to it until it is closed, other part of the program
    remains inactive) or modeless (input focus can be
    directed to another window)
    Dialog (Frame parentWindow, boolean mode)
    Dialog (Frame parentWindow, String title, boolean mode)
   When the dialog box is closed, dispose() is called,
    which is defined by Window
   Example: DialogDemo                                 35
File dialog
   It is used to specify a file
    FileDialog(Frame parent, String boxName) //parent is the
        honour of the dialog box, boxName is the title of the box
    FileDialog(Frame parent, String boxName,int how) //        how =
    FileDialog.LOAD for reading and       FileDialog.SAVE for writing
    FileDialog(Frame parent)
   getDirectory() and getFile() is used to get
    directory and file name
   Example: FileDialogDemo


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