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Computer Science and Computer Engineering
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GUI Programming
- Event-driven programming
- GUI event loop
- Event listeners
- Components can generate events
Graphical User Interfaces (GUIs) CS 144 John Goettsche Computer - - PowerPoint PPT Presentation
Graphical User Interfaces (GUIs) CS 144 John Goettsche Computer - - PowerPoint PPT Presentation
Graphical User Interfaces (GUIs) CS 144 John Goettsche Computer Science and Computer Engineering 1 GUI Programming Event-driven programming GUI event loop Event listeners Components can generate events 2 CSCE Event
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Event Driven Programming
Event loop Event Listener
Event
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Example: Push Counter
- Update counter when button is pushed.
- Components: JButton, JLabel
- Event: button push
- Listener: actionListener
After 6 clicks: Initial GUI:
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Push Counter Components
Panel (JPanel object) Button (JButton object) Label (JLabel object) Frame (JFrame object)
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Building the GUI: Setting up Components
panel.add( button ); panel.add( label ); frame.add( panel ); The panel
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Building the GUI: Event Listeners
public void actionPerformed(ActionEvent e) { ... } Code to be executed when event occurs This method is contained in a class. We use
- bjects of this class to serve as "listener" objects.
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Building the GUI: Connecting Events to Event Listeners
button.addActionListener( listenerObject ); Object containing code for listener. (The actionPerformed method from the previous slide.)
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public class MyGUI { private int x; public MyGUI() { ... } private class MyInnerClass { } } Class within a class.
Inner Classes
Available to both MyGUI and MyInnerClass Inner class has access to outer class's fields.
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public class PushCounter extends JFrame { // The "Push Me" button private JButton pushButton; public PushCounter() { ... pushButton = new JButton( "Push Me!"); pushButton.addActionListener(new PushCounterButtonListener()); ... } private class PushCounterButtonListener implements ActionListener { public void actionPerformed(ActionEvent e) { .... } } }
Code to execute when event occurs.
A GUI Framework
Inner class implements the ActionListener interface. Extend JFrame class. Connect button click event to listener object.
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Inheritance
- Gain all methods and fields from inherited class.
- PushCounter is-a JFrame
public class PushCounter extends JFrame { ... } All methods/fields that are in class JFrame are now available in class PushCounter.
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Implementation of Interfaces
- Require the creation of certain methods (depending on the interface)
- ActionListener requires that the actionPerformed method exist.
private class PushCounterButtonListener implements ActionListener { public void actionPerformed(ActionEvent e) { .... } }
Must create the actionPerformed method
- r this will not compile.
Interface
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Starting Things Up
- To start a GUI-based program we need only instantiate a JFrame object.
- Remember, PushCounter is-a JFrame.
- JVM will automatically enter the event loop.
- main method may exist anywhere you like.
public static void main( String[] args ) { PushCounter frame = new PushCounter(); } Instantiate PushCounter, which extends JFrame. In GUI programs, the main method is often quite short.
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Code: PushCounter.java
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