java-Unit4 chap2- awt controls and layout managers of applet

UNIT 4- CHAPTER 2
USING AWT CONTROLS,
LAYOUT MANAGERS AND
MENUS

contents
 Control Fundamentals
 Labels, Buttons, CheckBoxes, CheckboxGroup,
Choice Controls, Lists, Scroll Bars, TextField,
TextArea.
 Layout Managers:
 FlowLayout, BorderLayout, GridLayout, Menu Bars
and Menus

labels
 A label is an object of type Label, and it
contains a string, which it displays.
 Labels are passive controls that do not support
any interaction with the user.
 Label( )
 Label(String str)
 Label(String str, int how)

 The first version creates a blank label.
 The second version creates a label that contains
the string specified by str. This string is left-
justified.
 The third version creates a label that contains
the string specified by str using the alignment
specified by how.
 The value of how must be one of these three
constants: Label.LEFT, Label.RIGHT, or
 Label.CENTER.

 void setText(String str)
 String getText( )
 For setText( ), str specifies the new label.
 For getText( ), the current label is returned
 For alignment of the text
 void setAlignment(int how)
 int getAlignment( )
 Ex: labeldemo.java

buttons
 A push button is a component that contains a
label and that generates an event when it is
pressed
 Button( )
 Button(String str)
 The first version creates an empty button. The
second creates a button that contains str as a
label.

 After a button has been created, you can set its
label by calling setLabel( ). You can retrieve its
label by calling getLabel( )
 void setLabel(String str)
 String getLabel( )

Handling Buttons
 Each time a button is pressed, an action event is
generated.
 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 the
argument to this method.
 It contains both a reference to the button that
generated the event and a reference to the string
that is the label of the button.
 Ex: ButtonDemo.java

Applying Check Boxes
 A check box is a control that is used to turn an
option on or off.
 It consists of a small box that can either contain
a check mark or not.
 There is a label associated with each checkbox
that describes what option the box represents.
 You change the state of a check box by clicking
on it.
 Check boxes can be used individually or as part
of a group.

 Checkbox( )
 creates a check box whose label is initially blank
 Checkbox(String str)
 creates a check box whose label is specified by str.
 Checkbox(String str, boolean on)
 If on is true, the check box is initially checked;
otherwise, it is cleared.
 Checkbox(String str, boolean on,
CheckboxGroup cbGroup)
 Checkbox(String str, CheckboxGroup cbGroup,
boolean on)
 The fourth and fifth forms create a check box whose
label is specified by str and whosegroup is specified
by cbGroup.
 If this check box is not part of a group, then cbGroup
must be null.

 To retrieve the current state of a check box, call
getState( ).
 To set its state, call setState(
 You can obtain the current label associated with a
check box by calling getLabel( ).
 To set the label, call setLabel( ).
 boolean getState( )
 void setState(boolean on)
 String getLabel( )
 void setLabel(String str)

Handling Check Boxes
 Each time a check box is selected or
deselected, an item event is generated.
 Each listener implements the ItemListener
interface.
 That interface defines the itemStateChanged( )
method.
 An ItemEvent object is supplied as the
argument to this method
 Ex: checkbocdemo.java

Checkbox group
 It is possible to create a set of mutually exclusive
check boxes in which one and only one check box
in the group can be checked at any one time.
 To create a set of mutually exclusive check boxes,
you must first define the group to which they will
belong and then specify that group when you
construct the check boxes.
 Check box groups are objects of type
CheckboxGroup.
 You can determine which check box in a group is
currently selected by calling
getSelectedCheckbox( ).
 You can set a check box by calling
setSelectedCheckbox( ).

 Checkbox getSelectedCheckbox( )
 void setSelectedCheckbox(Checkbox which)
 Here, which is the check box that you want to be
selected.
 Eg: Cbgroup.java

Choice Controls
 The Choice class is used to create a pop-up list
of items from which the user may choose.
 When inactive, a Choice component takes up
only enough space to show the currently
selected item
 When the user clicks on it, the whole list of
choices pops up, and a new selection can be
made.
 To add a selection to the list, call addItem( ) or
add( ).
 void addItem(String name)
 void add(String name)

 To determine which item is currently selected,
you may call either getSelectedItem( ) or
getSelectedIndex( ).
 String getSelectedItem( )
 int getSelectedIndex( )
 To obtain the number of items in the list, call
getItemCount( )
 You can set the currently selected item using the
select( ) method with either a zero-based
integer index or a string that will match a name
in the list

 int getItemCount( )
 void select(int index)
 void select(String name)
 Given an index, you can obtain the name
associated with the item at that index by calling
getItem( )
 String getItem(int index)

Handling Choice Lists
 Here is an example that creates two Choice
menus.
 One selects the operating system.
 Eg: ChoiceDemo.java

Using Lists
 The List class provides a compact, multiple-choice, scrolling
selection list.
 a List object can be constructed to show any number of
choices in the visible window.
 List( )
 creates a List control that allows only one item to be selected at any
one time.
 List(int numRows)
 the value of numRows specifies the number of entries in the list that
will always be visible
 List(int numRows, boolean multipleSelect)
 In the third form, if
 multipleSelect is true, then the user may select two or more items at
a time. If it is false, then only one item may be selected.

 To add a selection to the list, call add( ). It has
the following two forms:
 void add(String name)
 void add(String name, int index)
 adds the item at the index specified by index.
 Indexing begins at zero.
 You can specify –1 to add the item to the end of the
list.
 String getSelectedItem( )
 int getSelectedIndex( )

 int getItemCount( )
 To obtain the number of items in the list, call
getItemCount( ).
 void select(int index)
 You can set the currently selected item by using
the select( ) method with a zero-based integer
index.

Handling Lists
 To process list events, you will need to implement
the ActionListener interface.
 Each time a List item is double-clicked, an
ActionEvent object is generated.
 Its getActionCommand( ) method can be used to
retrieve the name of the newly selected item.
 Each time an item is selected or deselected with a
single click, an ItemEvent object is generated.
 Its getStateChange( ) method can be used to
determine whether a selection or deselection
triggered this event.
 getItemSelectable( ) returns a reference to the
object that triggered this event.
 Ex: ListDemo.java

Managing Scroll Bars
 Scroll bars are used to select continuous values
between a specified minimum and maximum.
 Scroll bars may be oriented horizontally or
vertically.
 Each end has an arrow that you can click to
move the current value of the scroll bar one unit
in the direction of the arrow.
 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

 Scrollbar( )
 creates a vertical scroll bar.
 Scrollbar(int style)
 If style is Scrollbar.VERTICAL, a vertical scroll bar is
created.
 If style is Scrollbar.HORIZONTAL, the scroll bar is
horizontal
 Scrollbar(int style, int initialValue, int thumbSize, int
min, int max)
 the initial value of the scroll bar is passed in initialValue.
 The number of units represented by the height of the
thumb is passed in thumbSize.
 The minimum and maximum values for the scroll bar are
specified by min and max.

 void setValues(int initialValue, int thumbSize, int
min, int max)
 int getValue( )
 void setValue(int newValue)
 int getMinimum( )
 int getMaximum( )
 void setUnitIncrement(int newIncr)
 By default, 1 is the increment added to or subtracted from
the scroll bar each time it is scrolled up or down one line
 void setBlockIncrement(int newIncr)
 By default, page-up and page-down increments are 10

Handling Scroll Bars
BLOCK_DECREM
ENT
A page-down event has been
generated.
BLOCK_INCREM
ENT
A page-up event has been
generated
TRACK An absolute tracking event has
been generated.
UNIT_DECREMEN
T
The line-down button in a scroll bar
has been pressed.
UNIT_INCREMEN
T
The line-up button in a scroll bar
has been pressed.

Using a TextField
 The TextField class implements a single-line text-
entry area, usually called an edit control.
 TextField is a subclass of TextComponent.
 TextField( )
 creates a default text field
 TextField(int numChars)
 creates a text field that is numChars characters wide.
 TextField(String str)
 initializes the text field with the string contained in str.
 TextField(String str, int numChars)
 initializes a text field and sets its width.

 String getText( )
 void setText(String str)
 String getSelectedText( )
 void select(int startIndex, int endIndex)
 the contents of a text field may be modified by
the user by calling setEditable( ).
 one can determine editability by calling
isEditable( ).
 boolean isEditable( )
 void setEditable(boolean canEdit)

 There may be times when you will want the user to
enter text that is not displayed, such as a password.
 You can disable the echoing of the characters as they
are typed by calling setEchoChar( ).
 You can retrieve the echo character by calling the
getEchoChar( ) method.
 void setEchoChar(char ch)
 boolean echoCharIsSet( ) // to check the mode of
echochar
 char getEchoChar( )
 Eg : textfielddemo.java

Using a TextArea
 When single line of data is not enough, we have
multiline editor called TextArea.
 TextArea( )
 TextArea(int numLines, int numChars)
 TextArea(String str)
 TextArea(String str, int numLines, int numChars)
 TextArea(String str, int numLines, int numChars,
int sBars)
 numLines specifies the height,
 in lines, of the text area
 numChars specifies its width, in characters.
 Initial text can be specified by str.

 In fifth form, we can have scroll bars also
 sBars must be one of these values:
 SCROLLBARS_BOTH
 SCROLLBARS_NONE
 SCROLLBARS_HORIZONTAL_ONLY
 SCROLLBARS_VERTICAL_ONLY
 void append(String str)
 void insert(String str, int index)
 void replaceRange(String str, int startIndex, int
endIndex)
 Eg: textareaDemo.java

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java-Unit4 chap2- awt controls and layout managers of applet