GUI components in Java

M.SUJITHA,
II-M.SC(CS&IT),
Nadar Saraswathi College Of Arts and Science, Theni

 Java includes libraries to provide multi-platform
support for Graphic User Interface objects.
 Java's GUI components include labels, text fields,
text areas, buttons.
 The Abstract Windowing Toolkit (AWT) also
includes containers which includes these
components.
 Containers include frames (windows), canvases
which are used to draw and panels which are used
to group components.
 Panels and canvases are contained in frames while
buttons and other components can be placed either
directly on frames or in panels inside the frames.

 These GUI components are automatically drawn
whenever the window is drawn.
 These GUI components are handled using Java's
event model.
 When a user interacts with a component, an event is
generated by the component that you interact with.
 For each component of your program, the
programmer is required to designate one or more
objects to "listen" for events from that component.
 If the program has a button labelled "start" you
must assign one or more objects which will be notified
when a user clicks on the button.

BUTTONS
 Button is a class in package java.awt which
represents buttons on the screen.
 The constructor is: public Button(String label)
which, when executed, creates a button with "label"
printed on it.
 The button is large enough to display the label.
 There is also a parameter less constructor that
creates an unlabeled button.
 Buttons respond to a variety of messages.
 "Action Listener" to a button with the method:
public void addActionListener(ActionListener
listener);

ADDING BUTTONS TO A FRAME OR PANEL
 A command to add a button to a frame or panel:
 my Frame .add(start Button);
 This code is used in the constructor for the frame or
panel. add(start Button) or simply add(start Button).
 The class extends Frame, which is part of java.awt.
 The constructor for Frame takes a String parameter and
creates a window with the string in the title bar.
 The constructor for Button Demo calls the super class
constructor, and then sets the size of the new Frame to be 400
x 200.
 The set Layout command tells the new Frame that new
components should be added from left to right across the panel.

EXAMPLE CODING:
import java.awt.*;
public class Button Demo extends Frame
{
protected Button start Button, stop Button;
// Constructor sets features of the frame, creates
buttons, //
public Button Demo()
{
super("Button demo");
// calls Frame constructor //
which adds title to window setSize(400,200);
// sets the size of the window //

Grid Layout. setLayout(new FlowLayout());
// create two new buttons
labels start and stop startButton = new Button("Start");
stopButton = new Button("Stop");
add(startButton); //
add buttons to frame add(stopButton);
// create an object to listen to both buttons:
ButtonListener myButtonListener = new
ButtonListener();

startButton.addActionListener(myButtonListener);
stopButton.addActionListener(myButtonListener);
setVisible(true);
// Show the window on the screen. //
Trivial main program associated with ButtonDemo //
public static void main(String args[])
{
// Create an instance of Buttons ButtonDemo app = new
ButtonDemo()
}
}

ACTION LISTENERS FOR BUTTONS
 Objects which implement the Action Listener
interface are required to implement a method action
Performed which takes a parameter of type Action
Event.
 When an action occurs to a button, all objects
which have been added as Action Listener's for that
button are notified by calling their action Performed
method with a parameter with information on the
exact event that occurred.
 The system automatically creates the ActionEvent
object and sends it along to the listener.
 It is need to manually create an ActionEvent
object in this course.The most useful methods of
ActionEvent are

INNER CLASSES
A bit heavy to have to create a completely
separate class in order to create a listener for the
two buttons in our Button Demo class.
 Two alternatives are possible.
 1.One is to let the frame itself be the
Action Listener for the button.
 2.public class Button Demo extends Frame
implements Action Listener .
 This is the style suggested in Core Java for
handling action events in simple cases.
 There is another style which is almost as simple,
but more general. It involves the use of what are
called "inner classes".

EXAMPLE SOURCE CODE:
import java.awt.*;
import java.awt.event.*;
public class ButtonDemo extends Frame
{
protected Button startButton, stopButton; public ButtonDemo()
{
startButton.addActionListener(myButtonListener);
stopButton.addActionListener(myButtonListener);
}
public static void main(String args[])
{
{
if (source == startButton) System.out.println("Start button");
else if (source == stopButton) System.out.println("Stop button");
} } }

 ThetButton Listener is declared to be protected, it
can only be used inside the containing class, Button
Demo. The method Performed is still public. If it is
declared as protected.
 These nested classes would now be contained within
a single file named ButtonDemo.java.
 Another advantage of using nested classes is that
all of the instance variables (and methods) of the
outer class are visible inside the inner class.
 This can be handy for picking up information from
other components of the outer class.

OTHER GUI COMPONENTS
LABELS
 A Label is a very simple component which contains a
string.
 The constructors are public Label()
 // creates label with no text public Label//
 The methods available are public String getText()
 The return label text public void setText(String s)
 It sets the label text the user can change the text
in Labels.

TEXT FIELDS
 A TextField is a single line area that the user can
type into.
 It is a good way of getting text input from the
user.
 The constructors are public Text Field () .
 When the user types into a text field and then
hits the return or enter key, it generates an event
which can be handled by the same kind of Action
Listener used with Buttons.
 If for some reason the user likes to be notified
every time any change is made to the Text Field one
can associate a Text Listener to the field.

TEXT AREAS
 Text Area is a class that provides an area to hold
multiple lines of text.
 It is fairly similar to Text Field except that no
special event is generated by hitting the return key.
 The constructors are
public Text Area(int rows, int columns)
// create text area with rows, columns, and displaying s
Methods public void set Editable(booleans)
// if false the Text Area is not user editable public
String get Text() //
return text in Text Area public void set Text(String s)
// sets the text

GUI components in Java

More Related Content

GUI components in Java