Lab2: Event Handling

Most of the codes and activities source: Introduction to Programming Using Java, Sixth Edition

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Events

A GUI program must be prepared to respond to various kinds of events that can happen at any unpredictable time. Below is examples of simple events that the user can create:

• MouseEvent

  When the user presses one of the buttons on a mouse, moves the mouse, drag...

• KeyEvent

  When the user presses a key on the keyboard, a KeyEvent is created.

• WindowEvent

  If the window or the frame of your application is changed (opened, closed, activated, deactivated), WindowEvent is generated.

• ActionEvent

  This indicates the events generated by the component like Button, Checkboxes etc.

What we mean by "event is created" ? That means an object of the class Event is created that contains information about the event such as the source of the event (that is, the component on which the user clicked), the (x,y) coordinates of the point in the component where the click occurred, the exact time of the click, which button on the mouse was pressed...

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Event Listening

Listening to an event means some object(s) in our program detect the event and knows how to react to it. Depending on the type of the event that this object(s) are handling, they must contain instance methods that are specified in particular interfaces. Below is the list of interfaces that you might need for handling the above four types of events

• MouseListener

  Listens for mouse clicks, mouse presses, mouse releases and mouse movement into or out of the component's drawing area.

• MouseMotionListeners

  Listens for mous dragges, and mouse moves

• keyListener

  Listens for key presses.

• ActionListener

  Listens for when the user clicks a button, chooses a menu item, presses Enter in a text field

• WindowListener

  Listens for window events such as opening a window, closing a window, activating a window...

So, to be used as a listerer object, a class "implement"s an interface by doing two things:

• The class must be declared to implement the interface, as in "class XYZHandler implements MouseListener".
• The class must include a definition for each instance method specified in the interface. (This is a general rule in Java for whenever you want to use an interface)

For example, if a class implements MouseListener interface, it should incluse all the below methods in its body (they could be kept empty if the class does not need to implement them)

import java.awt.event.*;

public class xyz implements MouseListener {

public void mousePressed(MouseEvent evt){}
public void mouseReleased(MouseEvent evt){}
public void mouseClicked(MouseEvent evt){}
public void mouseEntered(MouseEvent evt)}
public void mouseExited(MouseEvent evt){}

}

Now depending of what the class wants to handle, it might impelemet one or more of the specified methods in the inreface that it implements. For example:

import java.awt.event.*;

public class xyz implements MouseListener {

public void mousePressed(MouseEvent evt){
System.out.println("Mouse is pressed!!!!"):
}
public void mouseReleased(MouseEvent evt){}
public void mouseClicked(MouseEvent evt){}
public void mouseEntered(MouseEvent evt){}
public void mouseExited(MouseEvent evt){}

}

Activity 1

Write a class that implements WindowListener interface (complete WindowHandler nested class in the code below). Put all the nessasary methods inside your class. Implement the body of all of them with simple methods such as printing some sentences when this action accures so that you can test the different behaviours of a window. Save the class for the next activity.

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

public class MainApplication{
	private static class WindowHandler ...{
	}
	public static void main(String[] args) {
	}
}

Hint: You could use Javadoc to find which methods you need to include in your classes.

Or

You could use Eclipse to put all these methods for you! Once you said implement ..., then there is an error. You can go on the error and it will put all the methods for you!!

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Event Registering

The last step in event handling is to register the listener object with the component(such as buttons,etc...) that produces the event (and you want your listener "hear"s that event)

xyz mylistener = new xyz();  // Create MouseListener object.
myButton.addMouseListener(mylistener); 

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Program Design Decision

When creating the event-listening objects, you have some options: The event-listener class could be a seperate class (or even a nested class), or you can choose that your main program handles these events. Lets say we want to print "Mouse is pressed!" whenever the user press the mouse inside our panel. The fllowing options demonstrate some code design decisions:

• The first code structure uses a seperaet class for handling the mouse event. This pattern seperates the mouse handling functionality from the rest of the code.

  import java.awt.*;
  import java.awt.event.*;
  import javax.swing.*;
  
  public class MainApp {
     
     public static void main(String[] args) {
        JFrame window = new JFrame("Random Strings");
        JPanel content = new JPanel();
        mouseHandler myMouseHandler=new mouseHandler();
        content.addMouseListener( myMouseHandler );  // Register mouse listener.
        window.setContentPane(content);
     }
  
     private static class mouseHandler implements MouseListener {
  
        public void mousePressed(MouseEvent evt) {
        System.out.println("Mouse is pressed!");
        }
        public void mouseClicked(MouseEvent evt) { }
        public void mouseReleased(MouseEvent evt) { }
        public void mouseEntered(MouseEvent evt) { }
        public void mouseExited(MouseEvent evt) { }
  
     }
  }
  

• The following code implements the mouse listening behaviour in the main application:

  import java.awt.*;
  import java.awt.event.*;
  import javax.swing.*;
  
  public class MainApp implements MouseListener {
     
     public static void main(String[] args) {
        JFrame window = new JFrame("Random Strings");
        JPanel content = new JPanel();
        MainApp myMainApp = new MainApp();//an object of the MainApp class handles events
        content.addMouseListener( myMainApp );  // Register mouse listener.
        window.setContentPane(content);
     }
  
        public void mousePressed(MouseEvent evt) {
         System.out.println("Mouse is pressed!");
        }
        public void mouseClicked(MouseEvent evt) { }
        public void mouseReleased(MouseEvent evt) { }
        public void mouseEntered(MouseEvent evt) { }
        public void mouseExited(MouseEvent evt) { }
  
     }
  }
  

• The third patterns is similar to the second one, but here the main function is used to create an instance of the class. The event registering happens whenever a new object of the class is created (inside class construction)

  import java.awt.event.*;
  import javax.swing.*;
  
  public class MainApp implements MouseListener {
  	public MainApp(){
  	JFrame window = new JFrame("Random Strings");
  	JPanel content = new JPanel();
  	content.addMouseListener( this );  // Register mouse listener.
  	window.setContentPane(content);
  	window.setVisible(true);
  	}
  	public static void main(String[] args) {
  		MainApp myMainApp=new MainApp ();
  	}
  
  	public void mousePressed(MouseEvent evt) {
  	System.out.println("Mouse is pressed!");
  	}
  	public void mouseClicked(MouseEvent evt) { }
  	public void mouseReleased(MouseEvent evt) { }
  	public void mouseEntered(MouseEvent evt) { }
  	public void mouseExited(MouseEvent evt) { }
  
  	}
  

Activity2

Make sure that you understand the above codes and patterns. (And why they are correct)

Activity3

Write a program that could compute and display the sum of a set of numbers entered by the user.

• The panel should include a JTextField where the user enters a number. Beside your JTextField there is a text says: "Enter a new Number";
• The user can enter a new number and press "Enter" button.
• It should have another label that display the sum for the numbers that have been entered. Every time the user enters a new number, the sum displayed on the labels should change.
• There should be a "Reset" button that clears out all the data.
• When the user press the mouse inside your frame, all the text colors in your application set to a random color.
• When the user presses the exit icon on the main window, the program be closed and it will open a new window says "GoodBye!";

For this activity, you need to implement three interfaces. Implement one of the listeres in your main program (pattern 3) and two of them in a seperate nested class (pattern 1). Code below shows the structure of the program:

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

public class MainApplication implements MouseListener, ActionListener{

	public MainApplication(){
                 // your constructor
	}

	private static class WindowHandler implements WindowListener{
		// use activity 1 class here
	}

	public static void main(String[] args) {
		MainApplication() myApplication= new  MainApplication();
	}

}

Hints:

• This program handles the actionListener for both of the buttons inside just one method. You can understand which buttons is pushed by useing the events that it created.

  event.getSource()
  

• To understand the Listeners that Swing Components Support go to: Listeners For example: an object handles Window Listener can just be added to a Dialog or a Frame.