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Universidad Nacional de Universidad Nacional de ColombiaColombia

Facultad de IngenieríaFacultad de Ingeniería

Departamento de SistemasDepartamento de Sistemas

ertificación enertificación en

AVAAVA

11. 11. COMPONENTSCOMPONENTS

Components in GeneralComponents in General

The Visual ComponentsThe Visual Components

The Container Components The Container Components

The Menu ComponentsThe Menu Components

ObjectivesObjectives

• Components are Java's building blocks for creating graphical user interfaces. Some component types, such as buttons and scroll bars, are used directly for GUI control. Other kinds of components (those that inherit from the abstract ContainerContainer class) providespatial organization • GUIs are an important part of any program. Java's Toolkit (AWT) provides extensive functionality. This chapter reviews components

Components in General Components in General

Java's components are implemented by the many subclasses of the java.awt.Component and java.awt.MenuComponent superclasses

There are 19 non-superclass components in all, and you should know the basics of all the component classes

One way to organize this fairly large number of classes is to divide them into categories:

• Visual components• Container components• Menu components

There are several methods that are implemented by all the visual and container components, by virtue of inheritance from java.awt.Component

(The menu components extend from java.awt.MenuComponent, so they do not inherit the same superclass functionality)

getSize()

returns the size of a component

The return type is Dimension, which has public data members height and width

setForeground() and setBackground():

set the foreground and background colors of a component

Each method takes a single argument, which is an instance of java.awt.Color

Chapter 12 discusses how to use the Color class.

Generally the foreground color of a

component is used for rendering text, and the background color is used for rendering the non-textual area of the component

Example: a label

blue foreground color and black background color will show up as blue text on a black background

setFont():

The setFont() method determines the font that a component will use for rendering any text that it needs to display

The method takes a single argument, which is an instance of java.awt.Font

If you do not explicitly set a component's font, the component uses the font of its container, in the same way that the container's foreground and background colors are used if you do not explicitly call setForeground() or setBackground()

Thus, if you have an applet whose font is 48-point bold Serif, and you add a check box to the applet without calling setFont() on the check box, you will get a check box whose label appears in 48-point bold Serif

setEnabled():

takes a single argument of type boolean

If this argument is true, then the component has its normal appearance

If the argument is false, then the component is grayed out and does not respond to user input

This method replaces the 1.0 methods enable() and disable(), which are deprecated

setSize() and setBounds():

set a component's geometry, or rather, they attempt to set geometry. They replace the deprecated 1.0 methods resize() and reshape()

setSize(): takes two int arguments:

width and height

an overloaded form takes a single dimension

setBounds(): takes four int arguments: x, y, width, and height; an overloaded form

takes a single rectangle

If you have tried calling these methods, you know that it is usually futile:

the size and position that you attempt to give a component is overruled by a layout manager

In fact, these two methods exist mostly for the use of layout managers

The major exception to this rule is the Frame class, which is not under the thumb of a layout manager and is perfectly willing to have you set its size or bounds

setVisible(): takes a boolean argument that dictates

whether the component is to be seen on the screen

it only works for frames, unless you learn some techniques that are beyond the scope the Certification Exam

• Button• Canvas• Checkbox• Choice• FileDialog• Label• List• ScrollPane• Scrollbar• TextArea• TextField

The Visual ComponentsThe Visual Components

To use one of the components in a GUI:

1. create an instance by calling the appropriate constructor

2. add the component to a container

implements a push button

ButtonButton

new Button( "Apply" );

This constructor takes a string parameter that specifies the text of the button's label

When a button is pushed, it sends an Action event

Applet Viewer: Visual.class

Apply

It is a component that has no default appearance or behavior

You can subclass Canvas to create custom drawing regions, work areas, components, and so on

Canvases receive input events from the mouse and the keyboard; it is up to the programmer to transform those inputs into a meaningful look and feel

CanvasCanvas

The default size of a canvas is uselessly small

One way to deal with this problem is to use a layout manager that will resize the canvas

Another way is to call setSize() on the canvas yourself; canvases are a rare case where this will actually work

1. Canvas canv = new Canvas();2. canv.setBackground( Color.black );3. canv.setSize( 100, 50 );


A check box is a two-state button

The two states are true (checked) and false (not checked)

CheckboxCheckbox

Checkbox( String label )Checkbox( String label, boolean initialState )


Use secure server

boolean getState()void setState( boolean state )

If you do not specify an initial state, the default is false

Two methods support reading and setting the state of a check box:

1. CheckboxGroup cbg = new CheckboxGroup();2. add( new Checkbox( "Cinnamon", false, cbg ) );3. add( new Checkbox( "Nutmeg", false, cbg ) );4. add( new Checkbox( "All spice", true, cbg ) );


Cinnamon Nutmeg Allspice

Checkbox getSelectedCheckbox()void setSelectedCheckbox (Checkbox newSelection)

Two methods support reading and setting the currently selected member of the group

Check boxes send Item events when they are selected

A choice is a pull-down list

ChoiceChoice

To create a choice:

1. call the constructor, 2. populate the choice by repeatedly

calling addltem()

Next figure shows two choices, both of which present the same options

The choice on the left is in its normal state; the choice on the right has been mouse-clicked

1. Choice ch = new Choice();2. ch.addltem( "Alligators" );3. ch.addltem( "Crocodiles" );4. ch.addItem( "Gila Monsters" );5. ch.addltem( "Dragons" );


Applet started.

Alligators

Crocodiles

Gila MonstersDragons

Gila Monsters 0 Alligators0

Choices send Item events when they are selected

The FileDialog class represents a file open or file save dialog

The appearance of these dialogs varies greatly from platform to platform

A file dialog is modal; this means that input from the dialog's parent frame will be directed exclusively to the dialog, as long as the dialog remains visible on the screen

The dialog is automatically removed when the user specifies a file or clicks the Cancel button

FileDialogFileDialog

The dialog's parent is the frame over which the dialog will appear

The title string appears in the dialog's title bar (on most platforms)

FileDialog(Frame parent, String title, int mode)

The most useful FileDialog constructor

The mode should be • FileDialog.LOAD or • FileDialog.SAVE

After the user has specified a file, the name

of the file or its directory can be retrieved:

String getFile()String getDirectory()

1. FileDialog fidi = new FileDialog( f, "Choose!",

FileDialog.LOAD );2. fidi.setVisible( true );3. System.out.println( fidi.getFile() );

The simplest AWT component

Labels do not respond to user input, and they do not send out any events

Constructors:

LabelLabel

• Label()• Label( String text )• Label( String text, int alignment )

The default alignment for labels is to the left

To set the alignment, use the third form of the constructor and pass in one of the following:

• Label.LEFT• Label.CENTER• Label.RIGHT

Two methods support reading and setting the text of a label:

String getText() void setText( String newText )

new Label( "I'm a label, Mabel" );


Applet started.

I'm a label, Mabel

A list is a collection of text items, arranged vertically

If a list contains more items than it can display, it acquires a vertical scroll bar

Constructors:

ListList

• List()• List( int nVisibleRows )• List( int nVisibleRows, boolean bMultiSelectOk )

The number of visible rows (parameter nVisibleRows) dictates the height of a list

The first version of the constructor does not specify a number of visible rows, so presumably the height of such list will be dictated by a layout manager

If the version of the third constructor is used and multiSelectOk is true, then the list supports multiple selection

If multiple selection is not enabled, then selecting a new item causes the old selected item to be deselected

1. List list = new List( 4, true );2. list.addltem( "Augustus" );3. list. addltem( "Tiberius" );4. list.addltem( "Caligula" );5. list.addltem( "Claudius" );6. list.addltem( "Nero" );7. list.addltem( "Otho" );8. list.addItem( "Galba" );

Applet started.

Nero

Otho

Claudius

Galba

Applet Viewer: Vi...

The List class provides a large number of support methods

• void addItem( String text ): adds an item to the bottom of the list

• void addItem( String text, int index ):inserts an item at the specified index

• String getltem( int index ): returns the item with the specified index

• int getltemCount():returns the number of items in the list

• int getRows(): returns the number of visible lines in the list

• int getSelectedlndex(): returns the index of the currently selected item (the

list should be in single-selection mode)

• int[] getSelectedlndexes(): returns an array containing the index of every

currently selected item (the list should be in multiple-selection mode)

• String getSelectedltem():returns a string that reflects the cur rently selected

item (the list should be in single-selection mode)

• String[] getSelectedltems(): returns an array containing a string for every currently selected item (the list should be in multiple-selection mode)

It was introduced in Java 1.1

A scroll pane can contain a single component, which may be taller or wider than the scroll pane itself

If the contained component is larger than the scroll pane, then the default behavior of the scroll pane is to acquire horizontal and/or vertical scroll bars as needed

ScrollPaneScrollPane

Constructors:

• ScrollPane():ScrollPane(): constructs a scroll pane with default scroll bar behavior

• ScrollPane( int scrollbarPolicy ):ScrollPane( int scrollbarPolicy ):constructs a scroll pane with the specified scroll bar behavior

If you use the second form of the

constructor, then scrollbarPolicy should be one of:

1. ScrollPane.SCROLLBARS_AS_NEEDED2. ScrollPane.SCROLLBARS_ALWAYS3. ScrollPane.SCROLLBARS_NEVER

1. ScrollPane spane = new ScrollPane();2. Button bigButton = new Button( "What big teeth you

have, Grandmother" );3. bigButton.setFont( new Font( "Serif", Font.ITALIC, 80 ) );4. spane.add( bigButton );

Applet

OthoWhat big teeth y

Applet Viewer: Visual class

Applet started.

The scroll bar component that adjusts lists and scroll panes is available as a component in its own right

Scrollbar Scrollbar

Constructors:

1.1. Scrollbar(): Scrollbar():constructs a vertical scroll bar 2.2. Scrollbar( int orientation ): Scrollbar( int orientation ): constructs a scroll bar with the specified orientation 3.3. Scrollbar( int orientation, int initialValue, Scrollbar( int orientation, int initialValue, int sliderSize, int minValue, int sliderSize, int minValue, int maxValue ):int maxValue ): constructs a scroll bar with the specified parameters

For constructors that take an orientation parameter, this value should be one of:

• Scroll bar.HORIZONTAL• Scroll bar.VERTICAL

In the third form of the constructor, the sliderSize parameter is a bit confusing

The Java terminology for the piece of the scroll bar that slides is the slider, which in itself is confusing because in some window systems the entire component is called a slider

The sliderSize parameter controls the size of the slider, but not in pixel units. The units of sliderSize parameter are the units defined by the spread between the minimum and maximum value of the scroll bar

Scrollbar sbar = new Scrollbar( Scrollbar.HORIZONTAL, 625, 25, 600, 700 );

Applet

Applet Viewer: Visual class

The TextField and TextArea classes implement one-dimensional and two-dimensional components for text input, display, and editing

Both classes have a variety of constructors, which offer the option of specifying or not specifying an initial string or a size

The constructors that do not specify size are for use with layout managers that will enforce a size

TextField and TextAreaTextField and TextArea

Constructors for TextField

1.1. TextField(): TextField(): constructs an empty text field 2.2. TextField( int nCols ): TextField( int nCols ): constructs an empty text field with the specified number of columns 3.3. TextField( String text ): TextField( String text ): constructs a text field whose initial content is text 4.4. TextField( String text, int nCols ): TextField( String text, int nCols ): constructs a text field whose initial content istext, with the specified number of columns

Constructors for TextArea

1.1. TextArea(): TextArea(): constructs an empty text area

2.2. TextArea( int nRows, int nCols ): TextArea( int nRows, int nCols ): constructs an empty text area with the specified number of rows and columns

3.3. TextArea( String text): TextArea( String text): constructs a text area whose initial content is text

4.4. TextArea( String text, int nRows, int nCols ): TextArea( String text, int nRows, int nCols ): constructs a text area whose initial content is text, with the specified number of rows and columns

Constructors for TextArea ...

5.5. TextArea( String text, int nRows, int nCols, TextArea( String text, int nRows, int nCols, int scrollbarPolicy ):int scrollbarPolicy ): same as above, but the scroll bar placement policy is determined by the last parameter, which should be one of the following: • TextArea.SCROLLBARS_BOTH• TextArea.SCROLLBARS_NONE• TextArea.SCROLLBARS_HORIZONTAL_ONLY• TextArea.SCROLLBARS_VERTICAL_ONLY

the number-of-columns parameter: 1. the number of columns is a measure of width in

terms of columns of text, as rendered in a particular font

A 25-column text area with a tiny font will be very narrow, while a 5-column text area with a huge font will be extremely wide

2. There is the problem of proportional fonts

• For a fixed-width font, it is obvious what the column width should be

• For a proportional font, the column width is taken to be the average of all the font's character widths

what happens when a user types beyond the rightmost character column in one of these components?

The visible text scrolls to the left

The insertion point remains in place, at the rightmost column

The component now contains more text than it can display, so scrolling is required

Text areas support scroll bars. Text fields can be scrolled by using the and keys

Inheritance of TextField and TextArea

TextField TextArea

TextComponent

Object

Component

1 .1 . String getSelectedText(): String getSelectedText():returns the currently selected text2.2. String getText(): String getText():returns the text contents of the component3.3. void setEditable( boolean editable ): void setEditable( boolean editable ): if editable is true, permits the user to edit the component4.4. void setText( String text ): void setText( String text ): sets the text contents of the component

Both classes inherit some functionality from their common superclass, TextComponent

1. TextField tf1 = new TextField( 5 );2. tf1.setFont( new Font( "Serif", Font.PLAIN, 24 ) );3. tf1.setText( "12345" );4. TextField tf2 = new TextField( 5 );5. tf2.setFont( new Font("SansSerif , Font.PLAIN, 24 ) );6. tf2.setText( "12345" );7. TextField tf3 = new TextField( 5 );8. tf3.setFont( new Font( "Monospaced", Font.PLAIN, 24 ) );9. tf3.setText( "12345" );

1234


1234

1234

1. TextArea ta1 = new TextArea( 6, 5 );2. ta1.setFont( new Font( "Serif", Font.PLAIN, 24 ) );3. ta1.setText( "Serif\n12345\nabcde\niiiiiiiiii\nWWWWW" );4. TextArea ta2 = new TextArea( 6, 5 );5. ta2.setFont( new Font( "SansSerif", Font.PLAIN, 24 ) );6. ta2.setText( "Sans\n12345\nabcde\niiiiiiiiii\nWWWWW" );7. TextArea ta3 = new TextArea( 6, 5 );8. ta3.setFont(new Font( "Monospaced", Font.PLAIN, 24 ) );9. ta3.setText( "Mono\n12345\nabcde\niiiiiiiiii\nWWWWW" );

Applet started.

Serif1234abcdeiiiiiiiiWWW

Sans1234abcdeiiiiiiiiWWW

Mono1234abcdeiiiiiWWW


Containers are components capable of holding other components within their boundaries

• Applet• Frame• Panel• Dialog

Technically, ScrollPane is also a container, because it inherits from the Container superclass, but it does not present the issues that the other three

The Container Components The Container Components

Inheritance of Applet, Frame and Panel

Applet Frame

Component

Container

Panel Window

The only issue that needs attention here is the problem of resizing

Applets, by virtue of inheriting from Component, have setSize() and setBounds() methods.

Applets only exist in browsers. Changing the size of an applet is permitted or forbidden by the applet's browser, and during the development cycle you cannot know which brand of browser will be running your applet

AppletApplet

The easiest browser for development is the applet viewer, which allows resizing of applets

It is common for an applet to have a temporary setSize() call in its init() method, because this provides an easy way to play with different sizes

If you use this technique, remember to delete the setSize() call before final delivery and set the size in your HTML tag.

A frame is an independent window, decorated by the underlying window system and capable of being moved around on the screen independent of other GUI windows

Any application that requires a GUI must use one or more frames to contain the desired components

FrameFrame

Constructors

1.1. Frame(): Frame():constructs a frame with an empty title bar

2.2. Frame( String title ): Frame( String title ):constructs a frame with the specified title

When a frame is constructed, it has no size and is not displayed on the screen

To give a frame a size, call one of the inherited methods setSize() or setBounds()

If you call setBounds(), the x and y parameters tell the frame where it will appear on the screen. Once a frame has been given a size, you can display it by calling setVisible( true )

To remove an unwanted frame from the screen, you can call setVisible( false )

This does not destroy the frame or damage it in any way; you can always display it again by calling setVisible( true )

When you are finished with a frame, you need to recycle its non-memory resources

Memory will be harvested by the garbage collector

Non-memory resources are system-dependent; suffice it to say that it takes a lot to connect a Java GUI to an underlying window system

On a UNIX/Motif platform, for example, a frame's non-memory resources would include at least one file descriptor and X window

1. // Construct and display2. Frame f = new Frame( "This is a frame" );3. f.setBounds( 10, 10, 500, 350 );4. f.setVisible( true );5. 6. // delay 7. try {8. Thread.sleep( 30*1000 ); 9. } catch ( InterruptedException e ) { }10. 11.// Remove and dispose 12.f.setVisible( false ); 13.f.dispose();

Applets and frames serve as top-level or outermost GUI components

Panels provide an intermediate level of spatial organization for GUIs

Panel Panel

You are free to add all the components of a GUI directly into an applet or a frame, but you can provide additional levels of grouping by adding components to panels and adding panels to a top-level applet or frame

This process is recursive:

the components that you add to panels can themselves be panels, and so on, to whatever depth of containment you like

A dialog is a pop-up window that accepts user input

Dialogs may optionally be made modal

The Dialog class is the superclass of the FileDialog class. The default layout manager for this class is border layout

DialogDialog

Java supports two kinds of menu:

1. pull-down2. pop-up

The certification exam does not cover pop-up menus

The Menu ComponentsThe Menu Components

Pull-down menus are accessed via a menu bar, which may contain multiple menus

Menu bars may only appear in frames

Therefore pull-down menus also may only appear in frames

To create a frame with a menu bar containing a pull-down menu:

1.1. CreateCreate aa menu barmenu bar and attach it to the and attach it to the frameframe

2.2. Create Create andand populatepopulate the menuthe menu3.3. AttachAttach the menu to the menu barthe menu to the menu bar

To create a menu bar, just construct an instance of the MenuBar class

To attach it to a frame, pass it into the frame's setMenuBar() method.

There are four kinds of element that can be mixed and matched to populate a menu:

• Menu itemsMenu items• Check-box menu itemsCheck-box menu items• SeparatorsSeparators• MenusMenus

A menu item is an ordinary textual component available on a menu

MenuItem( String text )

where text is the label of the menu item

A menu item is very much like a button that happens to live in a menu

Like buttons, menu items generate Action events

A check-box menu item looks like a menu item with a check box to the left of its label. When a check-box menu item is selected, the check box changes its state

CheckboxMenuItem( String text )

where text is the label of the item

A check-box menu item is very much like a check box that happens to live in a menu;

you can read and set an item's state by calling getState() and setState() just as you would with a plain check box.

Check-box menu items generate Item events.

1. Frame frame;2. MenuBar bar;3. Menu fileMenu, subMenu, helpMenu;4. 5. // create frame and install menu bar 6. frame = new Frame( 'Menu demo" );7. frame.setSize( 400, 300 );8. bar = new MenuBar(); 9. frame.setMenuBar( bar );10. 11.// create submenu12.subMenu = new Menu( "Pull me" ); 13.subMenu.add( new Menultem( "Sub-This" ) ); 14.subMenu.add( new Menultem( "Sub-That" ) );15. 16.// create and add file menu.17.fileMenu = new Menu( "File" );18.fileMenu.add( new MenuItem( "New" ) );

18. fileMenu.add( new MenuItem( "New" ) );19. fileMenu.add( new Menultem( "Open" ) );20. fileMenu.addSeparator();21. fileMenu.add( new CheckboxMenuItem( "Print Preview Mode" ) );22. fileMenu.add( subMenu );23. bar.add( fileMenu );24. 25. // create help menu26. helpMenu = new Menu( "Help" ); 27. helpMenu.add( new Menultem( "Contents..." ) ); 28. helpMenu.add( new Menultem( "About this program..." ) ); 29. bar.setHelpMenu( helpMenu );30. 31. // now that the frame is completely built, display it 32. frame.setVisible( true );

Menu demo

NewOpen

Print Preview Mode

Pull me Sub-This

File Help

Sub-That

Frame with file menu and submenuFrame with file menu and submenu

Menu demo

About this program ...

Contents ...

HelpFile

Frame with help menuFrame with help menu