Chapter 14

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Chapter 14

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Title: Chapter 14

1
Chapter 14 Using Layout Managers and Events

Dr. James Burns

2
Layout Manager

A layout manager is an object that controls the
size and position of components inside a
Container object
The layout manager that you assign to the window
determines how the components are sized and
positioned within the window
Layout managers are interface classes that are
part of the Java SDK
They align your components
Then the components do not crowd each other or
overlap

3
Learning about Layout Managers

If you are placing more than one component on a
JFrame, JApplet or any other container, you spend
a lot of time determining exactly where to place
each component so the layout is attractive and no
component obscures another
The alternative is to use a layout manager

4
Examples

One layout manager arranges components in equally
spaced columns and rows
Another layout manager centers components within
their Container
Each layout manager defines methods that arrange
components within a Container itself, so you can
assign layout managers within layout managers

5
More on Layout Managers

Layout managers can also be a Container itself,
so you can assign layout managers within layout
managers
The Java platform provides very simple layout
managers like FlowLayout and GridLayout to the
special purpose BorderLayout and CardLayout to
the very flexible GridBagLayout and BoxLayout

6
Table showing when each layout manager should be
used
Layout Manager When to Use
BorderLayout Use when you add components to a maximum of five sections arranged in north, south, east, west, and center positions
FlowLayout Use when you need to add components from left to right, Flowlayout automatically moves to the next row when needed, and each component takes its preferred size
GridLayout Use when you need to add components into a grid of rows and columns each component is the same size
7
Table, Continued
Layout Manager When to Use
CardLayout Use when you need to add components that are displayed one at a time
BoxLayout Use when you need to add components into a single row or a single column
GridBagLayout Use when you need to set size, placement, and alignment constraints for every component that you add

8
Using Border Layout

The BorderLayout manager is the default manager
class for all content panes
You can use the BorderLayout class with any
container that has five or fewer components
The components will fill the screen in five
regionsnorth, south, east, west, and center

9
Using add() to add components

When you add a component to a container that uses
BorderLayout, the add() method uses two
argumentsthe component and the region to which
the component is added
The BorderLayout class provides five named
constants for the regionsBorderLayout.NORTH,
.SOUTH, .EAST, .WEST, and .CENTER

import javax.swing.
import java.awt.
public class JDemoBorderLayout extends JApplet
private JButton nb new JButton("North
Button")
private JButton sb new JButton("South
Button")
private JButton eb new JButton("East
Button")
private JButton wb new JButton("West
Button")
private JButton cb new JButton("Center
Button")
public void init()
setLayout(new BorderLayout())
add(nb, BorderLayout.NORTH)
add(sb, BorderLayout.SOUTH)
add(eb, BorderLayout.EAST)
add(wb, BorderLayout.WEST)
add(cb, BorderLayout.CENTER)

11
Html doc to host the applet

lthtmlgt
ltobject code JDemoBorderLayout.class width
375 Height 200gt
lt/objectgt
lt/htmlgt

12
Using FlowLayout

You can use the FlowLayout manager class to
arrange components in rows across the width of a
Container
With FlowLayout, each Component that you add is
placed to the right of previously added
components in a row, or if the current row is
filled, the Component is placed to start a new row

13
Comparing BorderLayout

With BorderLayout, the Components are sized to
fit the pane
With FlowLayout, each Component retains its
default size, or preferred size.
If there is text in the Component, that text may
not be visible when it gets resized with
BorderLayout

14
FlowLayout class contains three constants you can
use to align Components

FlowLayout.LEFT
FlowLayout.CENTER
FlowLayout.RIGHT
If you do not specify alignment, Components are
center-aligned in a FlowLayout Container by
default

import javax.swing.
import java.awt.
import java.awt.event.
public class JDemoFlowLayout extends JApplet
implements ActionListener
private JButton lb new JButton("L Button")
private JButton rb new JButton("R Button")
private Container con getContentPane()
private FlowLayout layout new FlowLayout()
public void init()
con.setLayout(layout)
con.add(lb)
con.add(rb)
lb.addActionListener(this)
rb.addActionListener(this)
public void actionPerformed(ActionEvent
event)

16
You can Dynamically reposition Components

As illustrated in Figures 14-5, 6 and 7
Involving use of
con.invalidate()
con.validate()
The invalidate() call marks the container as
needing to be laid out
The validate() call causes the components to be
rearranged based on the newly assigned layout

17
Using GridLayout

Use GridLayout if you want to arrange components
into equal rows and columns
When you create a GridLayout object, you indicate
the numbers of rows and columns you want, and
then the container surface is divided into a grid

18
For example,

The following statement establishes an anonymous
GridLayout with four horizontal rows and five
vertical columns in a Container named con
Con.setLayout(new GridLayout(4, 5))

19
Similarly, within a JApplet, you can establish
the same layout with either of the following

This.setLayout(new GridLayout(4, 5))
setLayout(new GridLayout(4, 5))
As you add new components to a Gridlayout, they
are positioned left-to-right across each row in
sequence

20
More on GridLayouts

You cannot skip a position or specify an exact
position for a component
You can add a blank label to a grid position and
give the illusion of skipping a position

21
Gaps

You can specify vertical and horizontal gaps
measured in pixels, using two additional
arguments.
Private GridLayout layout new GridLayout(3, 2,
5, 5)

import javax.swing.
import java.awt.
public class JDemoGridLayout extends JApplet
private JButton b1 new JButton("Button 1")
private JButton b2 new JButton("Button 2")
private JButton b3 new JButton("Button 3")
private JButton b4 new JButton("Button 4")
private JButton b5 new JButton("Button 5")
private GridLayout layout new GridLayout(3,
2, 5, 5)
public void init()
setLayout(layout)
add(b1)
add(b2)
add(b3)
add(b4)
add(b5)

23
Using CardLayout

You use CardLayout when you want multiple
compnoents to share the same display space
The CardLayout manager generates a stack of
containers or components, one on top of another
Each component is a card and each card can be
any component typeex., JButton, JLabel, or JPanel

24
CardLayout uses one of two constructors

CardLayout() creates a card layout without a
horizontal or vertical gap
CarLayout(int hgap, int vgap) creates a card
layout with the specified horizontal and vertical
gaps
Horizontal gaps are placed at the left and right
edges
Vertical gaps are placed at the bottom and top
edges

import javax.swing.
import java.awt.
import java.awt.event.
public class JDemoCardLayout extends JApplet
implements ActionListener
private CardLayout cards new CardLayout()
private JButton b1 new JButton("Ace of
Hearts")
private JButton b2 new JButton("Three of
Spades")
private JButton b3 new JButton("Queen of
Clubs")
public void init()
setLayout(cards)
add("ace", b1)
b1.addActionListener(this)
add("three", b2)
b2.addActionListener(this)
add("queen", b3)
b3.addActionListener(this)

26
Using Advanced Layout Managers

Professioinal Java programmers create new
components and new layout managers
Examples include
GridBagLayout managerallows you to add
Components to precise locations within the grid,
as well as indicate that specific Components
should span multiple rows or columns within the
grid
If you want to create a JPanel with six JButtons,
in which two are twice as wide as the others, use
GridBagLayout

27
Using JPanels to Increase Layout Options

You can use the JPanel class to greatly increase
the number of possible component arrangements
JPanel is similar to a JWindow or a JFrame, in
that a JPanel is a surface on which you can place
components
But JPanel is not a child of JWindow or JFrame

28
JPanel is a type of JComponent.

JPanel is a Container, meaning it can contain
other components
For example, you can create a JApplet using
BorderLayout and place a JPanel in any of the
five regions
Then within the north JPanel, you can place four
JButtons using GRidLayout and within the east
JPanel, you can place three JLabels using
FlowLayout.

29
JPanels.

By using JPanels within JPanels, you can create
an infinite variety of screen layouts
JPanels support double buffering, which is the
default buffering strategy resulting in an
updated screen only when it is complete so that
no flickering takes place while the JPanel is
being redrawn

30
JPanel Constructors

JPanel() creates a JPanel with double buffering
and a flow layout
JPanel(LayoutManager layout) creates a JPanel
with the specified layout manager and double
buffering
JPanel(Boolean isDoubleBuffered) creates a JPanel
with flow layout and the specified
double-buffering strategy
JPanel(LayoutManager layout, Boolean
isDoubleBuffered) creates a JPanel with the
specified layout manager and the specified
doublebuffering strategy

31
Examples

The following statements create a JPanel that
uses a named BorderLayout manager
BorderLayout border new BorderLayout()
JPanel myPanel new JPanel(border)

The following statement accomplishes the same
thing as the previous two, combining
bothstatements by using an anonymous layout
manager
JPanel myPanel new JPANEL(new BorderLayout())

import javax.swing.
import java.awt.
public class JDemoManyPanels extends JApplet
// Twelve buttons
JButton button01 new JButton("One")
JButton button02 new JButton("Two")
JButton button03 new JButton("Three")
JButton button04 new JButton("Four")
JButton button05 new JButton("Five")
JButton button06 new JButton("Six")
JButton button07 new JButton("Seven")
JButton button08 new JButton("Eight")
JButton button09 new JButton("Nine")
JButton button10 new JButton("Ten")
JButton button11 new JButton("Eleven")
JButton button12 new JButton("Twelve")
// Four panels
JPanel panel01 new JPanel(new GridLayout(2,
0))

public void init()
setLayout(new BorderLayout())
add(panel01, BorderLayout.WEST)
add(panel02, BorderLayout.CENTER)
add(panel03, BorderLayout.SOUTH)
add(panel04, BorderLayout.EAST)
panel01.add(button01)
panel01.add(button02)
panel01.add(button03)
panel02.add(button04)
panel02.add(button05)
panel02.add(button06)

35
Checker Board Panel

The following program creates a checkerboard
panel, as shown in figure 14-16

import java.awt.
import javax.swing.
public class Checkerboard extends JFrame
final int ROWS 8
final int COLS 8
final int GAP 2
final int NUM ROWS COLS
int x
JPanel pane new JPanel(new GridLayout(ROWS,
COLS, GAP, GAP))
JPanel panel new JPanelNUM
Color color1 Color.WHITE
Color color2 Color.BLUE
Color tempColor
public Checkerboard()
super("Checkerboard")
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
add(pane)

if(x COLS 0)
tempColor color1
color1 color2
color2 tempColor
if(x 2 0)
panelx.setBackground(color1)
else
panelx.setBackground(color2)
public static void main(String arguments)
Checkerboard frame new Checkerboard()
final int SIZE 300
frame.setSize(SIZE, SIZE)
frame.setVisible(true)

38
Understanding Events and Event Handling

Now that you understand inheritance and abstract
classes, you can take a deeper look at event
handling
Like all Java classes, events are Objects.
Specifically, events are objects that the user
initiates, such as key presses or mouse clicks

39
EventObject

EventObject is the parent class for all event
objects, which descends from the Object class
EventObject is the parent of AWTEvent, which in
turn is the parent of specific event classes,
such as ActionEvent and ComponentEvent

40
ComponentEvent

ComponentEvent is a child of EventObject
But is also a parent of KeyEvent and MouseEvent

41
Rules

ActionEvents are generated by components that
users can click, such as JButtons and JCheckBoxes
TextEvents are generated by components into which
the user enters text, such as a JTextField
MouseEvents include determining the location of
the mouse and distinguishing between a single-
and double-click.

42
User Action Resulting Event type
Click a Button ActionEvent
Click a component MouseEvent
Click an item in a list box ItemEvent
Click an item in a check box ItemEvent
Change text in a text field TextEvent
Open a window WindowEvent
Iconify a window WindowEvent
Press a key KeyEvent

43
ActionEvents and MouseEvents

Because ActionEvents and MouseEvents involve the
mouse, it is easy to confuse ActionEvents and
MouseEvents
Use ActionEvents when a mouse click changes a
component, like a JButton
Use MouseEvents when your program is focusing on
what the user is doing manually with the mouse,
such as clicking the left mouse button

44
For events in which the program has no interest

No listeners have to be set up for such events
For events that you care about, your must set up
a listener
Every Listener interface method has the return
type void and each takes one argumentthe object
thatis an instanceof the corresponding event
class.

45
ActionListener

Is an interface that has a method named
actionPerformed()
Interface methods such as actionPerformed(),
which are called automatically when an
appropriate event occurs, are called event
handlers

import javax.swing.
import java.awt.
import java.awt.event.
public class JDemoKeyFrame extends JFrame
implements KeyListener
JLabel prompt new JLabel("Type keys
below")
JLabel outputLabel new JLabel("Key Typed")
JTextField textField new JTextField(10)
public JDemoKeyFrame()
setTitle("JKey Frame")
setDefaultCloseOperation(JFrame.EXIT_ON_CLOS
E)
setLayout(new BorderLayout())
add(prompt, BorderLayout.NORTH)
add(textField, BorderLayout.CENTER)
add(outputLabel, BorderLayout.SOUTH)
addKeyListener(this)
textField.addKeyListener(this)

public void keyTyped(KeyEvent e)
char c e.getKeyChar()
outputLabel.setText("Last key typed "
c)
public void keyPressed(KeyEvent e)
public void keyReleased(KeyEvent e)
public static void main(String args)
JDemoKeyFrame keyFrame new
JDemoKeyFrame()
final int WIDTH 250
final int HEIGHT 100
keyFrame.setSize(WIDTH, HEIGHT)
keyFrame.setVisible(true)

48
An Event-Handling Example KeyListener

You use the KeyListener interface when you are
interested in actions the user initiates from the
keyboard
The KeyListener interface contains three methods
keyPressed()
keyTyped()
keyReleased()

49
Using AWTEvent Class Methods

Methods in these classes will tell you which
component was involved in an event
You use the getComponent() method to find the
specific component involved

50
Handling Mouse Events

Users spend most of their time operating a mouse
rather than keying on a keyboard
The MouseMotionListener interface provides you
with methods named mouseDragged() and
mouseMoved() that detect the mouse being rolled
or dragged across a component surface

51
MouseListener interface

Provides methods, such as
mousePressed()
mouseClicked()
mouseReleased()
mouseEntered()
mouseExited()
The last two methods tell you when a component
has a mouse pointer on it or extied from it

52
MouseInputListener Interface