This document discusses labels and buttons in Java. It covers adding controls to windows, the different types of controls including labels and buttons, and how to handle button events. Labels are used to display text while buttons generate action events when pressed. The code examples demonstrate how to create labels and buttons, set their text/labels, add them to windows, and handle button press events.

1. Labels and buttons
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2. Objectives
On completion of this period, you would be
able to know
• Control fundamentals
• Adding and removing controls
• Labels
• Buttons
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3. Recap
In the previous class, you have leant
• Displaying information within a window
• Working with color and font
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4. Control Fundamentals
• The AWT supports the following types of
controls:
• Labels
• Push buttons
• Check boxes
• Choice lists
• Lists
• Scroll bars
• Text editing
• These controls are subclasses of Component
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5. Adding and Removing Controls
• To include a control in a window, you must add it
to the window
• To do this, you must first create an instance of
the desired control
• Then add it to a window by calling add()
• The syntax is
– Component add(Component compObj);
– Here, compObj is an instance of the control that you
want to add
– A reference to compObj is returned
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6. Adding and Removing Controls contd..
• Sometimes you want to remove a control from a
window when the control is no longer needed
• To do this, call remove( )
• It has this general form:
– void remove(Component obj)
– Here, obj is a reference to the control you want to
remove
• You can remove all controls by calling
removeAll( )
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7. Labels
• The easiest control to use is a label
• 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 defines the following constructors:
– Label( )
– Label(String str)
– Label(String str, int how)
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8. Labels contd..
• 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’
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9. Labels contd..
• The alignment is specified by one of these three
constants
– Label.LEFT
– Label.RIGHT
– Label.CENTER
• You can set or change the text in a label by
using the setText( ) method
– void setText(String str)
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10. Labels contd..
• You can obtain the current label by calling
getText( )
– String getText( )
• You can set the alignment of the string within the
label by calling setAlignment()
– void setAlignment(int how)
• To obtain the current alignment, call
getAlignment( )
– int getAlignment( )
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11. Labels contd..
// Demonstrate Labels
import java.awt.*;
import java.applet.*;
/* The sample output is shown here
<applet code="LabelDemo" width=300
height=200>
</applet>
*/
public class LabelDemo extends Applet {
public void init() {
Label one = new Label("One");
Label two = new Label("Two"); Fig. 70.1 LabelDemo
Label three = new Label("Three");
// add labels to applet window
add(one);
add(two);
add(three);
}
} http://improvejava.blogspot.in/ 11

12. Buttons
• The most widely used control is the push button
• A push button is a component that contains a
label and that generates an event when it is
pressed
• Push buttons are objects of type Button
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13. Buttons contd..
• Button defines these two constructors:
– 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( )
– void setLabel(String str)
– Here, str becomes the new label for the button
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14. Handling Buttons
• Each time a button is pressed, an action event is
generated
• This is sent to any listeners that previously registered
• Each listener implements the ActionListener interface
• That interface defines the actionPerformed() method,
which is called when an event occurs
• We have to write our code in this method to handle the
button’s event
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15. Handling Buttons contd..
// Demonstrate Buttons
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
/*
<applet code="ButtonDemo" width=250 height=150>
</applet>
*/
public class ButtonDemo extends Applet implements ActionListener {
String msg = "";
Button yes, no, maybe;
public void init() {
yes = new Button("Yes");
no = new Button("No");
maybe = new Button("Undecided");
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16. Handling Buttons contd..
add(yes);
add(no);
add(maybe);
yes.addActionListener(this);
no.addActionListener(this);
maybe.addActionListener(this);
}
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17. Handling Buttons contd..
public void actionPerformed(ActionEvent ae) {
String str = ae.getActionCommand();
if(str.equals("Yes")) {
msg = "You pressed Yes.";
} else if(str.equals("No")) {
msg = "You pressed No.";
} else {
msg = "You pressed Undecided.";
}
repaint();
}
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18. Handling Buttons contd..
public void paint(Graphics g) {
g.drawString(msg, 6, 100);
}
} The sample output is shown here
Fig. 70.2 Handling Buttons
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19. Summary
• AWT supports several GUI components
• In this class we have seen how to use
– Labels
– Buttons
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20. Quiz
1. The event generated by button is
– FocusEvent
– ActionEvent
– ContainerEvent
– KeyEvent
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21. Quiz contd..
2. Which method is used get the caption of a label
– getCaption()
– getLabel()
– getText()
– None
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22. Frequently Asked Questions
1. List the constructors and methods of Label
2. List the constructors and methods of Button
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