Event Driven Programming

Event-driven Programming

• In the early days of computing communication with the outside world was accomplished using a technique called Polling.

• This is the technique you used with your Mom and Dad when you were little and went on a long trip

• Are we there yet? • Are we there yet?• Are we there yet?• Are we there yet?• Are we there yet?• Are we there yet?

Event-driven Programming

• Eventually your Dad said something like, “I’ll tell you when we get there, sweetie.”

• In a like manner early programs would check the keyboard to find out if a key had been pressed and then take action.

• As the user interface (keyboard, mouse, screen) became more complex this became problematic.

• There was simply too much activity to keep track of what was going on efficiently.

Event-driven Programming

• So a system was developed where some very fast-efficient code is included as either part of the operating system or the JVM

• In this approach the programmer can say when a certain event occurs a certain piece of code should be executed

• This is refined and expanded with modern languages by providing programmers with a Graphical User Interface Toolkit

• This means that items (referred to as widgets) are pre-programmed in to the system. These include buttons, text areas, etc.

Event-driven Programming

• Programmers now have tools to design screen layouts and make them respond as desired.

• Today we will show you a small demo and the code that runs it.

Adding Machine

• We want a small window that will display

• Two editable numbers• One non-editable sum• An ADD Button• A Clear Button

How it worksUnlike the otherprograms you havewritten event-drivenprogramming doesn’t follow a single path

The functionality of the text windows isbuilt-in

Different objects will be constructed to handle the actionwhen each buttonis pressed

The Code

import java.awt.event.*;

import javax.swing.*;

import java.awt.*;

class Adder extends WindowAdapter {

// The widget references

public JFrame f;

public JPanel p;

public JTextField top;

public JTextField bottom;

public JTextField sum;

public JButton add;

public JButton clear;

The Code

// Constructor

public Adder()

{

// Top level window on screen

f = new JFrame("Adding Machine");

f.setSize(200, 200);

// Panel to hold components

p = new JPanel();

// 3 text fields to hold 2 numbers and sum

top = new JTextField("0.0", 20);

top.setHorizontalAlignment(JTextField.RIGHT);

bottom = new JTextField("0.0", 20);

The Code

// Still in constructor

bottom.setHorizontalAlignment

(JTextField.RIGHT);

sum = new JTextField("0.0", 20);

sum.setHorizontalAlignment(JTextField.RIGHT);

sum.setEditable(false);

// Add and clear buttons

add = new JButton("ADD");

add.addActionListener(new

AddButtonHandler(top, bottom, sum));

clear = new JButton("CLEAR");

clear.addActionListener(new

ClearButtonHandler(top, bottom, sum));

The Code

// Still in constructor

// How it looks

p.setLayout

(new BoxLayout(p, BoxLayout.Y_AXIS));

f.getContentPane().add(p);

p.add(top);

p.add(bottom);

p.add(sum);

p.add(add);

p.add(clear);

f.addWindowListener(this);

f.show();

}

The Code

// Handle clicking on window close X

public void windowClosing(WindowEvent e) {

System.exit(0);

}

// Start it up

public static void main(String args[]) {

new Adder();

}

}

Event Handlers

import java.awt.event.*;

import javax.swing.*;

import java.awt.*;

class ClearButtonHandler implements ActionListener {

// Widgets

JTextField top, bottom, sum;

// Constructor

public ClearButtonHandler(JTextField top,

JTextField bottom,

JTextField sum)

{

this.top = top;

this.bottom = bottom;

this.sum = sum;

}

Event Handlers

import java.awt.event.*;

import javax.swing.*;

import java.awt.*;

// Clear all the text fields

public void actionPerformed(ActionEvent e) {

top.setText("0.0");

bottom.setText("0.0");

sum.setText("0.0");

}

}

Event Handlers

import java.awt.event.*;

import javax.swing.*;

import java.awt.*;

class AddButtonHandler implements ActionListener {

// Widget References

JTextField top, bottom, sum;

// Constructor

public AddButtonHandler(JTextField top,

JTextField bottom,

JTextField sum)

{

this.top = top;

this.bottom = bottom;

this.sum = sum;

}

Event Handlers

// Get text field contents, convert, add and disp

public void actionPerformed(ActionEvent e) {

double t, b, total;

String tString = top.getText();

String bString = bottom.getText();

try {

t = Double.parseDouble(tString);

}

catch(Exception e1) {

t = 0.0;

top.setText("Error!");

}

Event Handlers

try

{

b = Double.parseDouble(bString);

}

catch(Exception e2)

{

b = 0.0;

bottom.setText("Error!");

}

total = t + b;

sum.setText((new Double(total)).toString());

}

}