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Lecture 6Lecture 6

Using AWT controls, Layout Managers, and

Menus

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IntroductionIntroduction

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

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Control fundamentalsControl 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

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Adding and removing controlsAdding 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

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LabelsLabels 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)

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ButtonsButtons 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

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Buttons (contd.)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

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Check BoxesCheck 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

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Check Boxes (contd.)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

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Check Box GroupCheck 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

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Choice ControlsChoice 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 objectvoid 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()

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Choice Controls (contd.)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

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ListsLists List class provides a compact, multiple-choice,

scrolling selection list List object can be constructed to show any number

of choices in the visible windowList() //allow one item to be selected

List(int numRows) //specifies number of rowsList(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

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Lists (contd.)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

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Lists (contd.)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

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Scroll BarsScroll 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)

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Scroll Bars (contd.)Scroll Bars (contd.) Scroll bars are encapsulated by the class Scrollbar

Scrollbar() //create vertical scroll barScrollbar(int style) //style is Scrollbar.VERTICAL or

Scrollbar.HORIZONTALScrollbar(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)

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Scroll Bars (contd.)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)

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Scroll Bars (contd.)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

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TextField/Edit controlTextField/Edit control TextField is a single-line text-entry and a subclass of

TextComponent Textfield defines the following constructors:

TextField() //default text fieldTextField(int numChars) //wide specifiedTextField(String str) //default string in the text fieldTextField(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)

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TextField/Edit control (contd.)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

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TextAreaTextArea

It is a multiline editorTextArea()TextArea(int numLines, int numChars) //height specifiedTextArea(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

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TextArea (contd.)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

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Understanding Layout ManagersUnderstanding 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

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Layout Managers (contd.)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

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Layout Managers (contd.)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

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FlowLayoutFlowLayout

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

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Border LayoutBorder 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 centerBorderLayout() //default order layout

BorderLayout(int horz,int vert) //allows to specify horizontal and vertical space between components

Components are added byvoid add(Component compObj, Object region);

Example: BorderLayoutDemo

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Using InsetsUsing 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

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Grid LayoutGrid Layout

It lays out components in a two-dimensional gridGridLayout() //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

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Menu Bars and MenusMenu 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 objectsMenuBar()

Menu object can be created by Menu()Menu(String optionName)

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Menu Bars and Menus (contd.)Menu Bars and Menus (contd.) Each Menu object contains a list of MenuItem objects,

that can be selected by user. Its constructors areMenuItem()

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()

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Menu Bars and Menus (contd.)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

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Dialog BoxesDialog 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

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File dialogFile dialog

It is used to specify a fileFileDialog(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