lec6

CS396N - Web Programming

Spring 2002

Chapter 13 - AWT Components

Components

User interface controls:

scroll bars
labels
text fields
buttons

Component class:

in awt package
provides mechanism for creating components

Class	Description
Button	A labeled button
TextField	A single editable line
Label	A component for placing text
TextArea	A multiline area for displayingtext
Checkbox	A graphical component thathas true/false state
CheckboxGroup	A graphics componentwhich works like radio buttons
Choice	A class used to present a pop-upmenu of choices
List	Used to design a scrolling listof text items
Canvas	A component used for paintinggraphics
Scroll pane	Containers with scrollbars
Scroll bars	Scroll bars to input values

Component methods

setFont, setBackground() and setForeground()
Other methods:

Method name	Arguments	Description
setBounds(x,y,w,h)	x,y,w,h - integers	Component will be placed at (x,y)(pixels). Component's width will be w, height h (pixels).
setLocation(x,y)	x,y - integers	Component will be placed at (x,y) (pixels).
setSize(w,h)	w,h - integers	Component's width will be w, height h (pixels).
setVisible(b)	b - true or false	Component is visible if b is true; invisible otherwise.
isVisible()	None	Returns true if the component is visible.

To use setBounds(), setLocation() and setSize(), you must declare setLayout(null).

Changing fonts and colors of components

setBackground() of the Component class is used to change the background color of a component.

componentobject.setBackground(color_object)

where color_object is one of

Color.black	Color.blue	Color.cyan
Color.darkGray	Color.gray	Color.green
Color.lightGray	Color.magenta	Color.orange
Color.pink	Color.red	Color.white
Color.yellow

Also, using the RGB values:

Color mycolor_object = new Color(R,G,B);

where R,G,B are integers, between 0 and 255, corresponding to red, green and blue.

componentobject.setForeground(color_object)

setFont() of the Component class is used to change the font of the component.

componentobject.setFont(font_name,font_style,size)

font_name is a string, specifying the name of the font. Possible names are:

Serif, SanSerif, Monospaced, Helvetica, TimesRoman, Courier, Dialog, DialogInput.

font_style is an integer specifying the style. Possible values are:

Font.PLAIN (0), Font.BOLD(1), Font.ITALIC(2) and Font.BOLD+Font.ITALIC (3).

Example - ColoredButtons

import java.awt.*;
import java.applet.*;

public class ColoredButtons extends Applet{

public void init(){
Button b1 = new Button("one");
b1.setFont(new Font("Helvetica",Font.BOLD,12));
b1.setBackground(Color.magenta);
b1.setForeground(Color.yellow);
Button b2 = new Button("two");
b2.setFont(new Font("TimesRoman",Font.ITALIC,12));
b2.setBackground(Color.blue);
Button b3 = new Button("three");
b3.setFont(new Font("Dialog",Font.BOLD+Font.ITALIC,12));
b3.setBackground(Color.green);
Button b4 = new Button("four");
b4.setFont(new Font("Courier",1,12));
b4.setBackground(Color.pink);
Panel p1 = new Panel();
p1.setLayout(new GridLayout(2,2,15,15));
p1.setBackground(Color.red);
p1.add(b1);
p1.add(b2);
p1.add(b3);
p1.add(b4);
add(p1);
}

}

The Canvas Component

A Canvas component represents a blank rectangular area of the screen into which the application may draw.
The Canvas class must be subclassed.
The paint method (of Component class) must be overridden in order to perform custom graphics on the canvas.
Constructor: Canvas() creates a canvas.

Example - Creates a canvas and places it a particular location on screen

import java.awt.*;
import java.applet.*;
import java.awt.event.*;
public class CanvasExample1 extends Applet implements ActionListener{

    Button button = new Button("click");
    Canvas myCanvas = new Canvas();
    TextField t1 = new TextField(20);

public void init(){

setLayout(null);
myCanvas.setSize(100,100);
myCanvas.setBackground(Color.magenta);
myCanvas.setLocation(100,200);
t1.setSize(100,50);
t1.setBackground(Color.green);
t1.setLocation(100,5);
button.setSize(50,50);
button.setBackground(Color.red);
button.setLocation(5,110);
add(t1);
add(myCanvas);
add(button);
button.addActionListener(this);

}

public void actionPerformed(ActionEvent event){

    Graphics g = myCanvas.getGraphics();
    g.drawString("NEW YORK",10,10);
    Graphics g1 = getGraphics();
    t1.setText("You clicked...");

}
}

Example - Component (canvas) is hidden and then displayed

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class CanvasExample2 extends Applet implements ActionListener{

Button button = new Button("click");
Canvas myCanvas = new Canvas();
TextField t1 = new TextField(20);
public void init(){
setLayout(null);
myCanvas.setSize(100,100);
myCanvas.setBackground(Color.magenta);
myCanvas.setLocation(100,200);
t1.setSize(100,50);
t1.setBackground(Color.green);
t1.setLocation(100,5);
button.setSize(50,50);
button.setBackground(Color.red);
button.setLocation(5,110);
add(t1);
add(myCanvas);
myCanvas.setVisible(false);
add(button);
button.addActionListener(this);
}
public void actionPerformed(ActionEvent event){
if (myCanvas.isShowing())
myCanvas.setVisible(false);
else
myCanvas.setVisible(true);
t1.setText("You clicked...");
}

}

Example

//The following example extends the Canvas class and writes to it.

import java.awt.*;
import java.applet.*;

public class CanvasExample3 extends Applet{

public void init(){

myCanvas can1 = new myCanvas();
can1.setSize(200,100);
can1.setBackground(Color.blue);
add(can1);
repaint();
}

}

class myCanvas extends Canvas{

    public void paint(Graphics g){
        g.setColor(Color.cyan);
        g.drawString("New York... Westchester",10,10);
    }
}

The Panel Class

The Container Class

Container is an abstract class that extends the Component class.
A container can contain multiple components.
A Layout manager determines how components are organized in a container.
add() method is used to add a component to the Container class.

add(component)

The Applet Class

The Scrollpane Class

The ScrollPane class allows creation of a container with scroll bars that includes only one component (called the child component).
Scroll bars are used to view the contents of the component.
This viewable area is called the viewport.

Field	Type
SCROLLBARS_ALWAYS	static final int
SCROLLBARS_AS_NEEDED	static final int
SCROLLBARS_NEVER	static final int

Constructors:

Constructor Arguments Description

ScrollPane() None Creates scroll pane with scroll bars as needed.

ScrollPane(x) x - int one of the three fields Creates a scroll panewith the specified scroll bar display policy

.
Methods:

Method Arguments Description

getViewportSize() None Returns dimensions (Dimension) of the viewport.

Example - Displays a text field in a ScrollPane

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class ScrollPane1 extends Applet{

    ScrollPane scrBar1;
    TextField text1;
    TextField text2;

public void init(){

    scrBar1 = new ScrollPane();
    text1 = new TextField("New York, . . . . . . . . . . Westchester");
    text2 = new TextField("Pace University, . . . . . . . . . ,NY");
    scrBar1.add(text1);
    add(text2);
    add(scrBar1);
}

    public void paint(Graphics g){
        Dimension d = scrBar1.getViewportSize();
        g.drawString("height is = "+ d.height+" width is = "+d.width,200,200);
    }
}

The Checkbox Class

The Checkbox class is a subclass of Component class.
A check box is a graphical component that has an "on" (true) and "off" (false) state.
Constructors:

Constructor	Arguments	Description
Checkbox()	None	Creates a checkbox with no labels
Checkbox(str)	str - String	Creates a checkbox with label str

Methods:

Method	Description
getLabel()	Returns the label (String) (null, if no label)
getState()	Returns true if the check box is checked; otherwise returns false

Example - Displays check boxes for programming languages and asks the user to select all languages he/she knows

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class CheckBoxes extends Applet implements ActionListener{
    int total;
    TextField result = new TextField(40);
    Button button = new Button("Submit");
    Checkbox Pascal = new Checkbox("Pascal");
    Checkbox FORTRAN = new Checkbox("FORTRAN");
    Checkbox C = new Checkbox("C");
    Checkbox CPLUS = new Checkbox("CPLUS");
    Checkbox LISP = new Checkbox("LISP");
    Checkbox ADA = new Checkbox("ADA");
    Checkbox Java = new Checkbox("Java");
    Checkbox cobol = new Checkbox("COBOL");
    Panel panel = new Panel();

    public void init(){
        button.addActionListener(this);
        add( new Label("Click all computer programming languages you know:",Label.CENTER));
        panel.setLayout(new GridLayout(2,4,15,15));
        panel.add(Pascal);
        panel.add(FORTRAN);
        panel.add(C); panel.add(CPLUS);
        panel.add(LISP); panel.add(ADA);
        panel.add(Java); panel.add(cobol);
        add(panel); add(button); add(result);
    }

    public void actionPerformed(ActionEvent event){
        total = 0;
        if(Pascal.getState()) total++;
        if(FORTRAN.getState()) total++;
        if(C.getState()) total++;
        if(CPLUS.getState()) total++;
        if(LISP.getState()) total++;
        if(ADA.getState()) total++;
        if(Java.getState()) total++;
        if(cobol.getState()) total++;
        result.setText("You know "+total+" computer programming languages");
    }
}

Check Box Group

CheckboxGroup is used to group together a set of check boxes
Check boxes behave like radio buttons - the selections you make are mutually exclusive
Constructor:

CheckboxGroup();

Creates a new CheckboxGroup.

Method:

getSelectedCheckbox().getLabel();

Returns the label of the check box in this check box group that is currently "on"

if all are off returns NULL.

In practice:

Create a CheckboxGroup object, using the following Syntax:

CheckboxGroup groupname = new CheckboxGroup();

Create each Checkbox item, using the syntax:

Checkbox boxname = new Checkbox("label", groupname, false);

Example - Check boxes for programming languages. User asked to check his/her strongest language

Choice Menus

The Choice class provides a way of presenting a pop-up menu of choices.
The user can select any item from the menu.
Constructor:

Choice();

Creates a new choice menu.

Methods:

Method	Argument	Description
addItem(item)	item - String	adds item to menu
getItemCount()	None	returns the numberof items (int) in the menu
getItem(index)	index - int	returns theitem at index in the menu
getSelectedIndex()	None	returns theindex (int) of the selected item
getSelectedItem()	None	returns the selected item (String).

Example - Choice Menu

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class ChoiceMenu extends Applet implements ActionListener {
       Button button = new Button("Submit");
       TextField result = new TextField(50);
       Choice languageMenu = new Choice();
       String str= " ";

public void init(){
      button.addActionListener(this);
      add( new Label("Select your strongest programming
      language:",Label.CENTER));
      languageMenu.addItem("Pascal");
      languageMenu.addItem("FORTRAN");
      languageMenu.addItem("C");
      languageMenu.addItem("CPLUS");
      languageMenu.addItem("LISP");
      languageMenu.addItem("ADA");
      languageMenu.addItem("Java");
      languageMenu.addItem("COBOL");
      languageMenu.addItem("Perl");
      languageMenu.addItem("PL1");
      languageMenu.addItem("RPG");
      languageMenu.addItem("BASIC");
      languageMenu.addItem("JavaScript");
      languageMenu.addItem("VBScript");
      languageMenu.addItem("ActiveX");
      add(languageMenu);
      add(button);
      add(result);
}
public void actionPerformed(ActionEvent event){
      str = "number of items ="+languageMenu.getItemCount();
      str = str + "   item selected =
      "+languageMenu.getSelectedItem();
      str = str + "   index of item = " +
      languageMenu.getSelectedIndex();
      result.setText(str);
}

}

List Boxes

Provides a scrolling list of text of items.
User can select multiple items from the list
Constructor:

List(rows,true)

rows is an integer specifying the number of items to show.
true - multiple selections.
false - only one selection.

Methods:

Method	Argument	Description
add(item)	item - String	Adds item tothe list
getItemCount()	None	Returns the number(int) of items in the list
getItem(index)	index - int	Returnsthe item (String) at position index
getSelectedIndex()	None	Returns theindex (int) of selected item.
getSelectedItem()	None	Returns selecteditem (String).
getSelectedIndexes()	None	Returns anarray (int) of the index values of selected items.
GetSelectedItems()	None	Returns an array(String) of selected items.

Example - Menu List

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class menuList1 extends Applet implements ActionListener {

Button button = new Button("Submit");
TextArea result = new TextArea(15,40);
List languageList = new List(5,true);
public void init(){
button.addActionListener(this);
add( new Label("Select all programming languages you know:",Label.CENTER));
languageList.addItem("Pascal");languageList.addItem("FORTRAN");
languageList.addItem("C"); languageList.addItem("CPLUS");
languageList.addItem("LISP"); languageList.addItem("ADA");
languageList.addItem("Java"); languageList.addItem("COBOL");
languageList.addItem("Perl"); languageList.addItem("PL1");
languageList.addItem("RPG"); languageList.addItem("BASIC");
languageList.addItem("JavaScript");
languageList.addItem("VBScript");
languageList.addItem("ActiveX");
add(languageList); add(button); add(result);
}
public void actionPerformed(ActionEvent event){
int count = languageList.getItemCount();
String out = "Number of items on the list is = "+count+"\nYOU HAVE SELECTED ...\n";
String str[] = languageList.getSelectedItems();
int i = 0;
while(i < str.length ){
    out=out +"\n"+ str[i];
    i++;
}
    result.setText(out);
}

}

The TextArea Class

A text area is a multiline area for text.
It can be set to allow editing or to be read only.
Fields: ( static final int)

Field name	Meaning
SCROLLBARS_BOTH	Both horizontal and vertical scroll bars present.
SCROLLBARS_VERTICAL_ONLY	Only vertical scroll bar present.
SCROLLBARS_HORIZONTAL_ONLY	Only horizontal scroll bar present.
SCROLLBARS_NONE	No scroll bars present.

Constructors:

Constructor	Arguments	Meaning
TextArea()	None	Constructs a new text area.
TextArea(r,c)	r and c - int	Constructs a new text area with r rowsand c columns.
TextArea(str)	str - String	Constructs a new text area with text str.
TextArea(str,r)	str - String r and c - int	Constructs a new text area with r rows, c columns, and text str.
TextArea(str, r,c,b)	str – String, r,c,b - int	Constructs a new text areawith text str r rows, c columns, and b is one of the static fields.

Methods:

Method	Arguments	Description
getText()	None	Returns the string (String) from the text area.
setText(str)	str - String	Sets the str in the field.
replaceRange( str ,n ,m)	str – String n,m - int	Replaces string from position n through m by str.
insert(str, n)	str – String n - int	Inserts str at position n.
append(str)	str - String	Appends str to the text in the text area.

Example

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class TextArea1 extends Applet implements ActionListener{

    String s1 = "\nThank you for giving your input";
    String s2 = "Pace University,New York and Westchester";
    TextArea ta = new TextArea(s2,20,35);
    Button button = new Button("THANK YOU");

public void init(){

        ta.setBackground(Color.green);
        button.setBackground(Color.red);
        add(ta);
        add(button);
        button.addActionListener(this);
    }

    public void actionPerformed(ActionEvent event){
        ta.append(s1);
    }
}

The Label Class

A label is a tesxt component of a container.
Text may be changed by the application, but a user cannot edit directly.
Fields:

Static fields in the label class.
Use field names or numbers.

Field name	Number	Meaning
CENTER	1	Indicates that the labelshould be centered
LEFT	0	Indicates that the label shouldbe left justified
RIGHT	2	Indicates that the label shouldbe right justified

Constructors:

Constructor	Arguments	Description
Label()	None	Constructs an empty label
*Label(str)*	str – String	Constructs a label with str
*Label(str, n)*	str - String, n - int 0, 1, or 2 or Label.LEFT, Label.CENTER ,or Label.RIGHT	*Construct a label with str, with specified alignment*.

Methods:

Method	Argument	Meaning
setText(str)	str – String	Sets the label with specified string
getText()	None	Returns the label string (String)
getAlignment()	None	Returns (int) the alignment (0, 1, or 2)
setAlignment(n)	n – int	Sets the label with the specified alignment

Example - Puts out three labels in three alignments

import java.awt.*;
import java.applet.*;

public class Labels1 extends Applet{

public void init(){

        Label label1,label2, label3;
        label1 = new Label("left label",0);
        label2 = new Label("center label",1);
        label3 = new Label("right label",2);
        label1.setBackground(Color.green);
        label2.setBackground(Color.blue);
        label3.setBackground(Color.magenta);

        add(label1);
        add(label2);
        add(label3);
        add(new Label("Another label"));
    }
}

Example - Displays two buttons and one label. When the user presses a button, the label changes

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class Labels2 extends Applet implements ActionListener{

    Label label = new Label("Pace university");
    Button button1 = new Button("New York");
    Button button2 = new Button("Westchester");

public void init(){

button1.setBackground(Color.cyan);
button2.setBackground(Color.yellow);

        add(label);
        add(button1);
        add(button2);
        button1.addActionListener(this);
        button2.addActionListener(this);
    }

public void actionPerformed(ActionEvent event){

    if(event.getSource() == button1) label.setText("New York");
    if(event.getSource() == button2) label.setText("Westchester");
    }
}

Scrollbars and Sliders

The Scrollbar class enables us to include scroll bars.
The scroll bar allows the user to vary input values within a range.
The scroll bar will have a familiar "slider" and arrows .
To create a scroll bar:

Declare an object of the type Scrollbar.
Initialize the object using new, and a constructor.
Add the object.

Constructor:

Scrollbar(alignment, initvalue, visible, minimum, maximum)

alignment is one of the int static fields:

Scrollbar.VERTICAL (1)

Scrollbar.HORIZONTAL (0)

initvalue is an integer, specifying the initial value of the scroll bar.
visible is an integer, specifying the width (in pixels) of the scroll bar bubble.
minimum is an integer, specifying the minimum value of the scroll bar.
maximum is an integer, specifying the maximum value of the scroll bar.

Methods:

Method name	Arguments	Description
getMaximum()	---	Returns (int) the maximum value of the scroll bar.
getMinimum()	---	Returns (int) the minimum value of the scroll bar.
getOrientation()	---	Returns (int) the orientation of the scroll bar (0-horizontal, 1 – vertical).
getValue()	---	Returns (int) the current value of the scroll bar.

Events generated by scroll bar:

Clicking the arrows at the ends of the scroll bar or moveing the bubble in the scroll bar generates an event
Handle a generated event by :

import java.awt.event.*;

Step 2: Include the interface AdjustmentListener:
public class name extends Applet implements AdjustmentListener{
}

Step 3: Register the scroll bar object as the event source:

scroll_bar_object.addAdjustmentListener(this);

Step 4: Define the AdjustmentListener (interface) method adjustmentValueChanged():
public void adjustmentValueChanged(AdjustmentEvent e){
}

Methods of AdjustmentEvent class:

Method name	Description
getAdjustable()	Returns the scroll bar object
getValue()	Returns the current value of the scroll bar

Example - Scrollbars select RGB components of color

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class Scrollbar1 extends Applet implements AdjustmentListener{

TextField box;
String string;
Scrollbar sbr,sbg,sbb;
Canvas canvas;
int r=0,g=0,b=0;
public void init(){
box = new TextField();
sbr = new Scrollbar(Scrollbar.HORIZONTAL,0,1,0,255);
sbg = new Scrollbar(Scrollbar.HORIZONTAL,0,1,0,255);
sbb = new Scrollbar(Scrollbar.HORIZONTAL,0,1,0,255);
canvas = new Canvas();
sbr.addAdjustmentListener(this);
sbg.addAdjustmentListener(this);
sbb.addAdjustmentListener(this);
setLayout(null);
add(canvas);
add(sbr);
add(sbg);
add(sbb);
add(box);
sbr.setBounds(40,250,200,15);
sbg.setBounds(40,270,200,15);
sbb.setBounds(40,290,200,15);
canvas.setBounds(40,10,200,200);
box.setBounds(40,230,200,25);
}
public void adjustmentValueChanged(AdjustmentEvent evt){
if(evt.getAdjustable() == sbr) r = sbr.getValue();
if(evt.getAdjustable() == sbg) g = sbg.getValue();
if(evt.getAdjustable() == sbb) b = sbb.getValue();
Color color = new Color(r,g,b);
canvas.setBackground(color);
string = "R = "+r+" G = "+g+" B = "+b;
box.setText(string);
}

}