Event Driven Programming

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Event Driven Programming
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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?

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Event-driven Programming

• Eventually your Dad said something like, Ill
  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.

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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.

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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.

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Adding Machine

• We want a small window that will display
• Two editable numbers
• One non-editable sum
• An ADD Button
• A Clear Button

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How it works
Unlike the other programs you have written
event-driven programming doesnt follow a single
path
The functionality of the text windows is built-in
Different objects will be constructed to handle
the action when each button is pressed
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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

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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.RIGH
  T)
• bottom new JTextField("0.0", 20)

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The Code

• // Still in constructor
• bottom.setHorizontalAlignment
• 
  (JTextField.RIGHT)
• sum new JTextField("0.0", 20)
• sum.setHorizontalAlignment(JTextField.RIGH
  T)
• 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))

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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()

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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()

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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

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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")

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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

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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!")

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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()
  )

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