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Intro to "Event-driven Programming": making components in the GUI active

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• Interacting with a Graphical User Interface
  • A graphical program waits for user input as one of the following events:

    button press ENTER key press Mouse click etc

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  • Re-acting to an event:

    When an event is detected, a method is invoked The invoked method is called a call-back method

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  • This style of programming is called:

    event-driven programming

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• Events and Call-backs
  • Common events:

    User clicks a mouse button, presses Return key while typing in a text field, or select a menu item

  • Less common events:

    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 Component becomes visible Component gets the keyboard focus Table or list selection changes

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  • How to write event-driven programs:

    When you want a GUI component in the Java program to re-act when a certain event occurs in that GUI component: You must first write an appropriate "Listener" class for the event The "Listener" class is always an implementation class for an Java interface (Recall that the methods in an interface are polymorphic that must be implemented !!!) The interface defines the call-back methods !!! The "Listener" class will contain all the call-back methods defined in Then you register the "Listener" class that contains the necessary call-back methods with the GUI component. Now, when a given event occurs inside the GUI component, the registered call-back method will be invoked.

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  • The call-back methods will perform the requested action (that is associated with the event)

    Summary:

    The programmer (= you) must write the call-back function (stored in a Java class) and then register the Java class with a GUI component

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• Java's Interface (brief review)

  • Recall from Java that:

    An interface is an abstract class that contains only method declarations (Methods without a method body) A interface must be implemented by some Java class The implementation class must define all methods that have been declared in the interface Methods in the implementation class will "override" the methods in the interface The interface can (just like inheritance between parent and child classes) implement polymorphism

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  • The call-back mechanism in Java is implemented through polymorphism using interfaces

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• The different call-back interfaces that are available in GUI programming

  • Types of Call-back Interfaces:

    There are different groups of events that GUI programs need to handle Correspondingly: There are many types of call-back interfaces defined in Java

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  • Java's event classes and their corresponding call-back interface:

    Events that are handled	Name of Interface
    User clicks a button, presses the RETURN key in a text field, or chooses a menu item	ActionListener
    User moves mouse into/out of a component, presses a mouse button while the cursor is over a component	MouseListener
    User closes a frame (main window)	WindowListener
    User moves the mouse over a component	MouseMotionListener
    Component becomes visible	ComponentListener
    Component gets the keyboard focus	FocusListener
    Table or list selection changes	ListSelectionListener

    I have highlighted the interfaces ActionListener and MouseListener because I will discuss these in this webpage

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    Java programming note:

    One Java class can be used implement multiple interfaces Example: public class myCallBackClass implements ActionListener, MouseListener { // This class must define all methods declared // in all the implemented classes }

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  • The most commonly used interfaces are:

    ActionListener: Very basic and simple Interface has only one single function MouseListener: Monitors mouse events, such as mouse enters a component, mouse exists a component, mouse buttom pressed, etc.

    I will discuss how to use them next....