AWT Controls

Control Fundamentals

The AWT supports the following types of controls:

·         Labels 

·         Push buttons

·         Check boxes

·         Choice lists

·         Lists

·         Scroll bars

·         Text editing

 

Adding and Removing Controls

·         To include a control in a window, you must add it to the window

o   first create an instance of the desired control

o   add it to a window by calling add( )

o   Component add(Component compObj)

·         when the control is no longer needed, we can remove it by calling remove( )

·         void remove(Component obj)

Labels

·         The easiest control to use is a label.

·         it contains a string, which it displays

·         Label defines the following constructors:

Label( )                                    :           creates a blank label.

Label(String str)                     :           creates a label that contains                                                                                                              the string specified by str

Label(String str, int how)      :           creates a label that contains                                                                                                       the string specified by str using the alignment

                                                            specified by how

·         setText( )                                :           used to set or change the text in a label

·         getText( )                                :           can obtain the current label

Demonstrating Labels

 

 

Using Buttons

·         A push button is a component that contains a label and that generates an event when it is pressed

·         Button defines these two constructors:

Button( )                : creates an empty button

Button(String str)  : creates a button that contains str as a label

·         setLabel( )       : sets the label of the button

·         getLabel( )       : retrive the label of the button

Handling Buttons

·         Each time a button is pressed, an action event is generated

·         This is sent to any listeners

·         Each listener implements the ActionListener interface

·         That interface defines the actionPerformed( ) method, which is called when an event occurs.

·         getActionCommand( ):label on the button is retrived

Demonstrating 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");

add(yes);

add(no);

add(maybe);

yes.addActionListener(this);

no.addActionListener(this);

maybe.addActionListener(this);

 }

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();

}

public void paint(Graphics g)

{

g.drawString(msg, 6, 100);

}

 }

output

 

Applying Check Boxes

·         A check box is a control that is used to turn an option on or off. It consists of a small box that can either contain a check mark or not. There is a label associated with each check box

·         Checkbox supports these constructors

Checkbox( ) : label is initially blank, The state of the check box is unchecked

Checkbox(String str): label is specified by str, The state of the check box is unchecked

Checkbox(String str, boolean on):allows to set the initial state of the check box

Checkbox(String str, boolean on, CheckboxGroup cbGroup)

Checkbox(String str, CheckboxGroup cbGroup, boolean on)

                                group is specified by cbGroup

·         common functions

getState( ): retrieve the current state of a check box

setState( ), getLabel( ), setLabel( )

Handling Check Boxes

·         Each time a check box is selected or deselected, an item event is generated

·         This is sent to any listeners

·         Each listener implements the ItemListener interface.

·         That interface defines the itemStateChanged( ) method

Demonstrating check boxes

Output

Choice Controls

     ·         The Choice class is used to create a pop-up list of items from which the user may choose 

     ·         Choice control is a form of menu. 

     ·         When the user clicks on it, the whole list of choices pops up

 

          ·             To add a selection to the list, call add( ) 

          ·         getSelectedItem( ): returns a string containing the name of the item 

          ·         getSelectedIndex( ): returns the index of the item 

          ·         getItemCount( ): number of items in the list   

          ·         select( )           :           can set the currently selected item

Handling Choice Lists 

            ·         Each time a choice is selected, an item event is generated 

            ·         This is sent to any listeners 

            ·         Each listener implements the ItemListener interface. 

            ·         That interface defines the itemStateChanged( ) method

Demonstrating Choice lists

import java.awt.*;

import java.awt.event.*;

import java.applet.*;

/*<applet code="ChoiceDemo" width=300 height=180>

</applet > */

public class ChoiceDemo extends Applet implements ItemListener {

 Choice os, browser;

 String msg = "";

public void init() {

os = new Choice();

browser = new Choice();

 // add items to os list

 os.add("Windows XP");

os.add("Windows Vista");

os.add("Solaris");

os.add("Mac OS");

 // add items to browser list browser.add("Internet Explorer"); browser.add("Firefox"); browser.add("Opera");

// add choice lists to window

add(os);

add(browser);

// register to receive item events os.addItemListener(this); browser.addItemListener(this);

}

public void itemStateChanged(ItemEvent ie) {

 repaint();

}

// Display current selections.

public void paint(Graphics g) {

 msg = "Current OS: ";

 msg += os.getSelectedItem(); g.drawString(msg, 6, 120);

msg = "Current Browser: ";

msg += browser.getSelectedItem(); g.drawString(msg, 6, 140);

}

 }

Using Lists      

        ·         The List class provides a compact, multiple-choice, scrolling selection list  

        ·         a List object can be constructed to show any number of choices in the visible window

 

            ·         List provides these constructors:

List( ) throws HeadlessException :allows only one item to be selected

 List(int numRows) throws HeadlessException: number of entries in the list

List(int numRows, boolean multipleSelect) throws HeadlessException

Handling Lists   

        ·         To add a selection to the list, call add( ).

void add(String name) : Here, name is the name of the item added to the list.

void add(String name, int index) : adds the item at the index specified by index   

item is selected by calling

getSelectedItem( )     -  getSelectedItems( )

getSelectedIndex( )   -   getSelectedIndexes( )

·                                                 getItemCount( )

·                                                 select(int index)

DEMO PROGRAM

// Demonstrate Lists.

import java.awt.*;

import java.awt.event.*;

import java.applet.*;

 /* 

<applet code="ListDemo" width=300 height=180> </applet >

 */

public class ListDemo extends Applet implements ActionListener {

List os, browser;

String msg = "";

public void init() {

 os = new List(4, true);

browser = new List(4, false);

// add items to os list

os.add("Windows XP");

os.add("Windows Vista");

 os.add("Solaris");

os.add("Mac OS");

// add items to browser list

browser.add("Internet Explorer");

browser.add("Firefox");

browser.add("Opera");

browser.select(1);

// add lists to window

add(os);

add(browser);

// register to receive action events

os.addActionListener(this);

browser.addActionListener(this);

}

 

public void actionPerformed(ActionEvent ae)

{

 repaint();

}

// Display current selections.

public void paint(Graphics g)

 {

int idx[];

msg = "Current OS: ";

 idx = os.getSelectedIndexes();

for(int i=0; i<idx.length; i++)

            msg += os.getItem(idx[i]) + " ";

g.drawString(msg, 6, 120);

 msg = "Current Browser: ";

msg += browser.getSelectedItem();

 g.drawString(msg, 6, 140);

}  

}