Introduction to Java

Classes, events, GUI’s

Understand:

How to use TextPad

How to define a class or object

How to create a GUI interface

How event-driven programming works

How classes inherit methods

Overall program structure

Major Example

Red

colorMe

Green Blue

darker reSize

Red Green Blue

Each color is mixture of amounts of Red, Green, and Blue

( 200, 50 125 )

values in range: 0 … 255

Illustrate with Rectangle object

Class definition of rectangle

including constructor, properties, methods

Test program that creates instance of rectangle

Classes or Objects in Java

Class Rectangle

int height , width ;

{

}

public Rectangle (int h, int w ) {

}

height = h; width = w;

int findArea ( ) {

}

return height * width ;

instance variablesor properties

constructor

Class blueprint

int getHght ( ) { return height }

methods

public class TestRectangle {

}

public static void main ( String [ ] args ){

}

Rectangle r ;

r = new Rectangle (2, 4) ;

int area = r.findArea ( );

System.out.println ( "Area is: " + area )

Test program

instance rof rectangle

saved in file TestRectangle

say:r’s findArea

constructor

public class Rectangle {

}

public static void main ( String [ ] args ){

}

Rectangle r ;

r = new Rectangle (2, 4) ;

int area = r.findArea ( );

System.out.println ( "Area is: " + area ) ;anotherinstance

System.out.println ( "Area is: " + s.findArea ( ) );

Rectangle s = new Rectangle (5, 10) ;

Major Example

Red

colorMe

Green Blue

darker reSize

objectProgram Design

Design object

GUI componentsRecognize & handle events

Make Test program to create instances

GUI design

Build object on top of frame class

Add panel to frame

Put GUI elements in frame:

Add labels, textfields, & buttons

Make object listen for clicks on buttons

inherit propertiesof frames

Red

colorMe

Green Blue

darker reSize

object

labels

buttons

text fields

panel

Event-handler design

Recognize source of event

Take appropriate action

- grab data from textfield- convert data to integer

- use value to make RGB color

- RGB: ( r, g, b )

- change color of panel

Asynchronouslycalled

Program Structure for Demo

import java.awt.*;

public class TestColorizer{

}

public static void main (String[ ] args) { … }

class Colorizer

{

}

public void actionPerformed(ActionEvent e) { … }

public Colorizer( ) { … }

private JTextField Num1 … ;

implements ActionListenerextends JFrame

Program Structure

class Colorizer{

}

implements ActionListenerextends JFrame

private JTextField Num1 … ;

Instancevariables

public Colorizer( ) { … }

constructor

public void actionPerformed(ActionEvent e) { … }

Event-handler

Class

Content Pane

X

titlebar

JFrame

X

Red

colorMe

Green Blue

darker reSize

Colorizer objectextends Jframe

JFrame

Title Bar Content Pane

Panel

JTextFieldJLabel JButton

public Colorizer ( ) {

}

p1 = new JPanel ( );this.getContentPane().add(p1);

p1.add(new JLabel(“Red"));p1.add(Num1 = new JTextField(3 ));

p1.add(colorMe = new JButton("ColorMe"));

colorMe.addActionListener (this );

objects

Constructor

public Colorizer ( ) {

}

p1 = new JPanel ( );this.getContentPane().add(p1);

p1.add(new JLabel(“Red"));p1.add(Num1 = new JTextField(3 ));

p1.add(colorMe = new JButton("ColorMe"));

colorMe.addActionListener (this );

named so can refer to in event-handler

Constructor

un-named since not referred to in event-handler

system listensfor button click

class Colorizer

{

}

public Colorizer( ) { … }

private JTextField Num1 ;

implements ActionListenerextends JFrame

private JButton colorMe ;

private JPanel p1 ;

public void actionPerformed(ActionEvent e) { … }

instancevariables

Class…again

if (e.getSource() == colorMe ){

}

public void actionPerformed (ActionEvent e) {

}

p1.setBackground ( c );

Integer.parseInt ( );int R = Num1.getText( )

recognize eventsource

Get & convertnumber

c = new Color (R, 100, 150);

Create color

Event-handlermethodsignature

import java.awt.*;

public class TestColorizer{

}

public static void main (String[ ] args) {

}

Colorizer f = new Colorizer( );

f.setSize (400,400);

f.setVisible(true);

import javax.swing.*;import java.awt.event.*;

create instanceof object

use inheritedmethods

Test program

public class TestColorizer{

}

public static void main (String[ ] args) {

}

Colorizer f = new Colorizer( );f.setSize (400,400);f.setVisible(true);

import java.awt.*;

import javax.swing.*;import java.awt.event.*;

{

}

class Colorizer implements ActionListenerextends JFrame

if (e.getSource() == colorMe ){} c = new Color (R, 100, 150);

p1.setBackground ( c );

Integer.parseInt ( );int R = Num1.getText( )

public void actionPerformed (ActionEvent e) {

}

private JTextField Num1 ;

private JButton colorMe;private JPanel p1 ;

private Color c ;

p1 = new JPanel ( );this.getContentPane().add(p1);

p1.add(new JLabel(“Red"));p1.add(Num1 = new JTextField(3 ));

p1.add(colorMe = new JButton("ColorMe"));

colorMe.addActionListener (this );

public Colorizer ( ) {

}

prototypeprogram

Outline

Classes in Java

Rectangle exampleproperties or instance variablesinstances of classconstructor for class

Demo Colorizer Example

compilation executionRGB representation panel colorbuttons darken-button resize-buttonColorizer classmain program

TextPad

Colorizer Class Design

program structure

constructor

event-handler

GUI setup

constructor

recognize event-source

grab text

make Color

reset background color

reset window size

event-handler

instance variables

declarationslabels

textFields

buttons

panel

actionListeners

Outline

TextPad

Multiple instances

Terms and Concepts

GUI componentsJTextFields

JButtons

JLabels

Event-driven programming

AsynchronousInheriting properties & methods

Instance variables

Constructors

Method signature

ClassesObjectsMethodsInstances of objects

JFrames

import statements

extend class

JFrame’s setSize methodJFrame’s setVisible method

JPanelsJPanel’s add methodthis notationActionEvent

ActionEvent’e getSource() method

Color objectsRGB representation

Test programProgram structure

JPanel’s setBackground ( c) method