1 CSC111H Graphical User Interfaces (GUIs) Introduction GUIs in Java Understanding Events A Simple...

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CSC111HGraphical User Interfaces (GUIs)

• Introduction • GUIs in Java • Understanding Events • A Simple Application • The Containment Hierarchy • Layout Managers • Programming buttons• Other GUI Components

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Programming Actions: Implementing Listeners

• In Java, Listeners “listen” for events from components.

1. Create a listener (an object)

2. Attach listener to source component

3. When event occurs, source lets listener know

Source Component

Event

Listener

actionPerformed ( ActionEvent e )

ActionListener

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• Can have multiple listeners listening for same event

Source Component

EventListener 2


ActionListener

Listener 3


ActionListener

Listener 1


ActionListener

Event

Event

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• Listener acting on multiple events (of the same type)

Source Components

Event

Listener


ActionListener

Event

Event

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Implementing Listeners• What does a listener class look like?

import java.awt.*;

import java.awt.event.*;

// Simple listener class.

class AL implements ActionListener

{

public void actionPerformed( ActionEvent e )

{

System.out.println( “Something happened!”);

}

}

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import java.awt.*;




{


{


}

}

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import java.awt.*;




{


{


}

}

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import java.awt.*;




{


{


}

}

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import java.awt.*;




{


{


}

} e.getActionCommand()

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import java.awt.*;




{


{

System.out.println(“Something happened!”);

}

}

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Implementing Listeners• How do you add a Listener to a component?

...

AL listener = new AL(); // create 1 listener

JButton b1 = new JButton( “Button 1” );

JButton b2 = new JButton( “Button 2”);

// Add listener to buttons

b1.addActionListener(listener);

b2.addActionListener(listener);...

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Implementing Listeners• What happens when I press a button?

• The Java system looks for the ActionListener object linked to the button and calls the actionPerformed method.

listener


AL

ActionEvent

“Something happened!”

ActionEvent

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Implementing Listeners: Shorthand• Don’t have to create a new class to implement listener.

• Can use an anonymous inner class:

JButton b = new JButton(“Button 1”);

b.addActionListener( new ActionListener() {

public void actionPerformed (ActionEvent event){ System.out.println(“Something happened!”);} // end of method

} // end of anon. inner class

); // end of addActionListener method

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JButton b = new JButton(“Button 1”);





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JButton b = new Jbutton(“Button 1”);





Note:

Look for “Sample$1.class” files

Note:

Look for “Sample$1.class” files

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Listener Types:

Act that results in the event type

• User clicks a button, presses Return while typing in a text field, or chooses a menu item.

• User moves a slider.

• User closes a frame (main window).

• User presses a mouse button while the cursor is over a component.

• User moves the mouse over a component.

Listener

• ActionListener

• ChangeListener

• WindowListener

• MouseListener

• MouseMotionListener

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Adapters: A Shortcut

• Listeners are interfaces• To implement listeners, must define every method

of the interface

• Adapters are classes that implement listeners with empty methods

• Simply extend adapter and override required method instead of implementing every method of listener

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E.g. Shutting Down a Window:

• WindowListener declares many methods that must be implemented (for opening, closing, activating, deactivating, iconifing, deiconifing).

• WindowAdapter implements WindowListener with empty methods. To use, inherit and overwrite those of interest.

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• JFrame.EXIT_ON_CLOSE only available for Java 1.3 and up. Another way:

frame.addWindowListener(new WindowAdapter()

{

public void windowClosing(WindowEvent e)

{

System.exit(0);

}

}

);

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{


{

System.exit(0);

}

}

);

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{


{

System.exit(0);

}

}

);

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{


{

System.exit(0);

}

}

);

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When to Use an Adapter

• If listener only has one method, no adapter is needed or defined. - e.g. ActionListener

• If you are going to implement every method, use either the listener or the adapter

• If you only need a few of the methods, use an adapter. - e.g. WindowAdapter

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Other Components:

• JTextField

• JSlider

• User defined - ColorBlock

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Input of Character Strings

• The JTextField class implements a small area into which the user can type a sequence of characters that the program can then extract and process in some way.

• The constructor for this class takes two arguments: the initial text string to appear in the text field, and an integer specifying the width.

• The listener interface to implement for JTextField is the ActionListener.

• The getText() method returns the string inside the field.

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Sliders• The JSlider class is used to let the user enter a numeric value

bounded by a minimum and maximum value (Horizontal or Vertical).

• The listener interface to implement for JSlider is the ChangeListener.

• The method called when the user scrolls is called: stateChanged.

• Use getValue method to return position of the slider.

• Use setValue method to change position of the slider.

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ColorBlock (RGB)

• extends JLabel

• JLabel methods:– setBackground, setForeground

(Color)– setText

• New methods: setColor, setRed,... ?

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Other components:

• Search the API Reference

• Go through the Swing Tutorials

• Sample Code

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Tutorial 1

1 CSC111H Graphical User Interfaces (GUIs) Introduction GUIs in Java Understanding Events A Simple...

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