Unit VIII - Event Programming.

2.1 AWT Packages

AWT is huge! It consists of 12 packages of 370 classes (Swing is even bigger, with 18 packages of 737 classes as of JDK 8). Fortunately, only 2 packages - java.awt and java.awt.event - are commonly-used.

The java.awt package contains the core AWT graphics classes:
- GUI Component classes, such as Button, TextField, and Label.
- GUI Container classes, such as Frame and Panel.
- Layout managers, such as FlowLayout, BorderLayout and GridLayout.
- Custom graphics classes, such as Graphics, Color and Font.
The java.awt.event package supports event handling:
- Event classes, such as ActionEvent, MouseEvent, KeyEvent and WindowEvent,
- Event Listener Interfaces, such as ActionListener, MouseListener, MouseMotionListener, KeyListener and WindowListener,
- Event Listener Adapter classes, such as MouseAdapter, KeyAdapter, and WindowAdapter.

AWT provides a platform-independent and device-independent interface to develop graphic programs that runs on all platforms, including Windows, macOS, and Unixes.

2.2 AWT Containers and Components

There are two groups of GUI elements:

Component (Widget, Control): Components are elementary GUI entities, such as Button, Label, and TextField. They are also called widgets, controls in other graphics systems.
Container: Containers, such as Frame and Panel, are used to hold components in a specific layout (such as FlowLayout or GridLayout). A container can also hold sub-containers.

In the above figure, there are three containers: a Frame and two Panels. A Frame is the top-level container of an AWT program. A Frame has a title bar (containing an icon, a title, and the minimize/maximize/close buttons), an optional menu bar and the content display area. A Panel is a rectangular area used to group related GUI components in a certain layout. In the above figure, the top-level Frame contains two Panels. There are five components: a Label (providing description), a TextField (for users to enter text), and three Buttons (for user to trigger certain programmed actions).

In a GUI program, a component must be kept (or added) in a container. You need to identify a container to hold the components. Every container has a method called add(Component c). A container (say aContainer) can invoke aContainer.add(aComponent) to add aComponent into itself. For example,

Panel pnl = new Panel();          // Panel is a container
Button btn = new Button("Press"); // Button is a component
pnl.add(btn);                     // The Panel container adds a Button component

GUI components are also called controls (e.g., Microsoft ActiveX Control), widgets (e.g., Eclipse's Standard Widget Toolkit, Google Web Toolkit), which allow users to interact with (or control) the application.

2.3 AWT Container Classes

Top-Level Containers: Frame, Dialog and Applet

Each GUI program has a top-level container. The commonly-used top-level containers in AWT are Frame, Dialog and Applet:

A Frame provides the "main window" for your GUI application. It has a title bar (containing an icon, a title, the minimize, maximize/restore-down and close buttons), an optional menu bar, and the content display area. To write a GUI program, we typically start with a subclass extending from java.awt.Frame to inherit the main window as follows:

import java.awt.Frame;  // Using Frame class in package java.awt

// A GUI program is written as a subclass of Frame - the top-level container
// This subclass inherits all properties from Frame, e.g., title, icon, buttons, content-pane
public class MyGUIProgram extends Frame {

   // private variables
   ......
 
   // Constructor to setup the GUI components and event handlers
   public MyGUIProgram() { ...... }

   // The entry main() method
   public static void main(String[] args) {
      // Invoke the constructor (to setup the GUI) by allocating an instance
      new MyGUIProgram();
   }
}

An AWT Dialog is a "pop-up window" used for interacting with the users. A Dialog has a title-bar (containing an icon, a title and a close button) and a content display area, as illustrated.
An AWT Applet (in package java.applet) is the top-level container for an applet, which is a Java program running inside a browser. Applet is no longer supported in most of the browsers.

Secondary Containers: Panel and ScrollPane

Secondary containers are placed inside a top-level container or another secondary container. AWT provides these secondary containers:

Panel: a rectangular box used to layout a set of related GUI components in pattern such as grid or flow.
ScrollPane: provides automatic horizontal and/or vertical scrolling for a single child component.
others.

Hierarchy of the AWT Container Classes

The hierarchy of the AWT Container classes is as follows:

As illustrated, a Container has a LayoutManager to layout the components in a certain pattern, e.g., flow, grid.

2.4 AWT Component Classes

AWT provides many ready-made and reusable GUI components in package java.awt. The frequently-used are: Button, TextField, Label, Checkbox, CheckboxGroup (radio buttons), List, and Choice, as illustrated below.

AWT GUI Component: java.awt.Label

A java.awt.Label provides a descriptive text string. Take note that System.out.println() prints to the system console, NOT to the graphics screen. You could use a Label to label another component (such as text field) to provide a text description.

Check the JDK API specification for java.awt.Label.

Constructors

public Label(String strLabel, int alignment); // Construct a Label with the given text String, of the text alignment
public Label(String strLabel);                // Construct a Label with the given text String
public Label();                               // Construct an initially empty Label

The Label class has three constructors:

The first constructor constructs a Label object with the given text string in the given alignment. Note that three static constants Label.LEFT, Label.RIGHT, and Label.CENTER are defined in the class for you to specify the alignment (rather than asking you to memorize arbitrary integer values).
The second constructor constructs a Label object with the given text string in default of left-aligned.
The third constructor constructs a Label object with an initially empty string. You could set the label text via the setText() method later.

Constants (final static fields)

public static final LEFT;    // Label.LEFT
public static final RIGHT;   // Label.RIGHT
public static final CENTER;  // Label.CENTER

These three constants are defined for specifying the alignment of the Label's text, as used in the above constructor.

Public Methods

// Examples
public String getText();
public void setText(String strLabel);
public int getAlignment();
public void setAlignment(int alignment);  // Label.LEFT, Label.RIGHT, Label.CENTER

The getText() and setText() methods can be used to read and modify the Label's text. Similarly, the getAlignment() and setAlignment() methods can be used to retrieve and modify the alignment of the text.

Constructing a Component and Adding the Component into a Container

Three steps are necessary to create and place a GUI component:

Declare the component with an identifier (name);
Construct the component by invoking an appropriate constructor via the new operator;
Identify the container (such as Frame or Panel) designed to hold this component. The container can then add this component onto itself via aContainer.add(aComponent) method. Every container has a add(Component) method. Take note that it is the container that actively and explicitly adds a component onto itself, NOT the other way.

Example

Label lblInput;                     // Declare an Label instance called lblInput
lblInput = new Label("Enter ID");   // Construct by invoking a constructor via the new operator
add(lblInput);                      // this.add(lblInput) - "this" is typically a subclass of Frame
lblInput.setText("Enter password"); // Modify the Label's text string
lblInput.getText();                 // Retrieve the Label's text string

An Anonymous Label Instance

You can create a Label without specifying an identifier, called anonymous instance. In the case, the Java compiler will assign an anonymous identifier for the allocated object. You will not be able to reference an anonymous instance in your program after it is created. This is usually alright for a Label instance as there is often no need to reference a Label after it is constructed.

Example

// Allocate an anonymous Label instance.
// "this" container adds the instance.
// You CANNOT reference an anonymous instance to carry out further operations.
add(new Label("Enter Name: ", Label.RIGHT));
 
// Same as
Label xxx = new Label("Enter Name: ", Label.RIGHT)); // xxx assigned by compiler
add(xxx);

AWT GUI Component: java.awt.Button

A java.awt.Button is a GUI component that triggers a certain programmed action upon clicking.

Constructors

public Button(String btnLabel);
   // Construct a Button with the given label
public Button();
   // Construct a Button with empty label

The Button class has two constructors. The first constructor creates a Button object with the given label painted over the button. The second constructor creates a Button object with no label.

Public Methods

public String getLabel();
   // Get the label of this Button instance
public void setLabel(String btnLabel);
   // Set the label of this Button instance
public void setEnable(boolean enable);   
   // Enable or disable this Button. Disabled Button cannot be clicked.

The getLabel() and setLabel() methods can be used to read the current label and modify the label of a button, respectively.

Note: The latest Swing's JButton replaces getLabel()/setLabel() with getText()/setText() to be consistent with all the components. We will describe Swing later.

Event

Clicking a button fires a so-called ActionEvent and triggers a certain programmed action. I will explain event-handling later.

Example

Button btnColor = new Button("Red"); // Declare and allocate a Button instance called btnColor
add(btnColor);                       // "this" Container adds the Button
...
btnColor.setLabel("Green");          // Change the button's label
btnColor.getLabel();                 // Read the button's label

AWT GUI Component: java.awt.TextField

A java.awt.TextField is single-line text box for users to enter texts. (There is a multiple-line text box called TextArea.) Hitting the "ENTER" key on a TextField object fires an ActionEvent.

Constructors

public TextField(String initialText, int columns);
   // Construct a TextField instance with the given initial text string with the number of columns.
public TextField(String initialText);
   // Construct a TextField instance with the given initial text string.
public TextField(int columns);
   // Construct a TextField instance with the number of columns.

Public Methods

public String getText();
   // Get the current text on this TextField instance
public void setText(String strText);
   // Set the display text on this TextField instance
public void setEditable(boolean editable);
   // Set this TextField to editable (read/write) or non-editable (read-only)

Event

Hitting the "ENTER" key on a TextField fires a ActionEvent, and triggers a certain programmed action.

Example

TextField tfInput = new TextField(30); // Declare and allocate an TextField instance called tfInput
add(tfInput);                          // "this" Container adds the TextField
TextField tfResult = new TextField();  // Declare and allocate an TextField instance called tfResult
tfResult.setEditable(false) ;          // Set to read-only
add(tfResult);                         // "this" Container adds the TextField
......
// Read an int from TextField "tfInput", square it, and display on "tfResult".
// getText() returns a String, need to convert to int
int number = Integer.parseInt(tfInput.getText());
number *= number;
// setText() requires a String, need to convert the int number to String.
tfResult.setText(number + "");

Take note that getText()/SetText() operates on String. You can convert a String to a primitive, such as int or double via static method Integer.parseInt() or Double.parseDouble(). To convert a primitive to a String, simply concatenate the primitive with an empty String.

2.5 Example 1: AWTCounter

Let's assemble a few components together into a simple GUI counter program, as illustrated. It has a top-level container Frame, which contains three components - a Label "Counter", a non-editable TextField to display the current count, and a "Count" Button. The TextField shall display count of 0 initially.

Each time you click the button, the counter's value increases by 1.

import java.awt.*;        // Using AWT container and component classes
import java.awt.event.*;  // Using AWT event classes and listener interfaces

// An AWT program inherits from the top-level container java.awt.Frame
public class AWTCounter extends Frame {
   private Label lblCount;    // Declare a Label component
   private TextField tfCount; // Declare a TextField component
   private Button btnCount;   // Declare a Button component
   private int count = 0;     // Counter's value

   // Constructor to setup GUI components and event handlers
   public AWTCounter () {
      setLayout(new FlowLayout());
         // "super" Frame, which is a Container, sets its layout to FlowLayout to arrange
         // the components from left-to-right, and flow to next row from top-to-bottom.

      lblCount = new Label("Counter");  // construct the Label component
      add(lblCount);                    // "super" Frame container adds Label component

      tfCount = new TextField(count + "", 10); // construct the TextField component with initial text
      tfCount.setEditable(false);       // set to read-only
      add(tfCount);                     // "super" Frame container adds TextField component

      btnCount = new Button("Count");   // construct the Button component
      add(btnCount);                    // "super" Frame container adds Button component

      BtnCountListener listener = new BtnCountListener();
      btnCount.addActionListener(listener);
         // "btnCount" is the source object that fires an ActionEvent when clicked.
         // The source object adds an instance of BtnCountListener as an ActionEvent listener,
         //   which provides an ActionEvent handler called actionPerformed().
         // Clicking "Count" button calls back actionPerformed().

      setTitle("AWT Counter");  // "super" Frame sets its title
      setSize(300, 100);        // "super" Frame sets its initial window size

      // For inspecting the Container/Components objects
      // System.out.println(this);
      // System.out.println(lblCount);
      // System.out.println(tfCount);
      // System.out.println(btnCount);
      setVisible(true);         // "super" Frame shows
      // System.out.println(this);
      // System.out.println(lblCount);
      // System.out.println(tfCount);
      // System.out.println(btnCount);
   }

   // The entry main() method
   public static void main(String[] args) {
      // Invoke the constructor to setup the GUI, by allocating an instance
      AWTCounter app = new AWTCounter();
         // or simply "new AWTCounter();" for an anonymous instance
   }

   // Define an inner class to handle the "Count" button-click
   private class BtnCountListener implements ActionListener {
      // ActionEvent handler - Called back upon button-click.
      @Override
      public void actionPerformed(ActionEvent evt) {
         ++count; // Increase the counter value
         // Display the counter value on the TextField tfCount
         tfCount.setText(count + ""); // Convert int to String
      }
   }
}

To exit this program, you have to close the CMD-shell (or press "control-c" on the CMD console); or push the "red" close button in Eclipse's Application Console. This is because we have yet to write the handler for the Frame's close button. We shall do that in the later example.

Dissecting the AWTCounter.java

The import statements (Lines 1-2) are needed, as AWT container and component classes, such as Frame, Button, TextField, and Label, are kept in the java.awt package; while AWT events and event-listener interfaces, such as ActionEvent and ActionListener are kept in the java.awt.event package.
A GUI program needs a top-level container, and is often written as a subclass of Frame (Line 5). In other words, this class AWTCounter is a Frame, and inherits all the attributes and behaviors of a Frame, such as the title bar and content pane.
Lines 11 to 47 define a constructor, which is used to setup the GUI components and event handlers.
In Line 13, the setLayout() (inherited from the superclass Frame) is used to set the layout of the container. FlowLayout is used which arranges the components in left-to-right and flows into next row in a top-to-bottom manner.
A Label, TextField (non-editable), and Button are constructed. We invoke the add() method (inherited from the superclass Frame) to add these components into container.
In Line 34-35, we invoke the setSize() and the setTitle() (inherited from the superclass Frame) to set the initial size and the title of the Frame. The setVisible(true) method (Line 42) is then invoked to show the display.
Line 27-28 is used to setup the callback event-handler, which will be discussed in length later. In brief, whenever the button is clicked, the actionPerformed() will be called. In the actionPerformed() (Lines 59-64), the counter value increases by 1 and displayed on the TextField.
In the entry main() method (Lines 50-54), an instance of AWTCounter is constructed. The constructor is executed to initialize the GUI components and setup the event-handlers. The GUI program then waits for the user action.

Inspecting Container/Components via toString()

It is interesting to inspect the GUI objects via the toString(), to gain an insight to these classes. (Alternatively, use a graphic debugger in Eclipse/NetBeans or study the JDK source code.) For example, if we insert the following code before and after the setvisible():

System.out.println(this);
//AWTCounter[frame0,93,0,300x100,invalid,hidden,layout=java.awt.FlowLayout,title=AWT Counter,resizable,normal]
   // name (assigned by compiler) is "frame0"; top-left (x,y) at (93,0); width/height is 300x100 (via setSize());
System.out.println(lblCount);
//java.awt.Label[label0,0,0,0x0,invalid,align=left,text=Counter]
   // name is "Label0"; align is "Label.LEFT" (default); text is "Counter" (assigned in constructor)
System.out.println(tfCount);
//java.awt.TextField[textfield0,0,0,0x0,invalid,text=0,selection=0-0]
   // name is "Textfield0"; text is "0" (assigned in constructor)
System.out.println(btnCount);
//java.awt.Button[button0,0,0,0x0,invalid,label=Count]
   // name is "button0"; label text is "Count" (assigned in constructor)

setVisible(true);  // "super" Frame shows

System.out.println(this);
//AWTCounter[frame0,93,0,300x100,invalid,layout=java.awt.FlowLayout,title=AWT Counter,resizable,normal]
System.out.println(lblCount);
//java.awt.Label[label0,31,35,57x23,align=left,text=Counter]
System.out.println(tfCount);
//java.awt.TextField[textfield0,93,35,124x23,text=0,selection=0-0]
System.out.println(btnCount);
//java.awt.Button[button0,222,35,46x23,label=Count]

2.6 Example 2: AWTAccumulator

In this example, the top-level container is again the typical java.awt.Frame. It contains 4 components: a Label "Enter an Integer", a TextField for accepting user input, another Label "The Accumulated Sum is", and another non-editable TextField for displaying the sum. The components are arranged in GridLayout of 2 rows 2 columns.

The program shall accumulate the number entered into the input TextField and display the sum in the output TextField.

import java.awt.*;        // Using AWT container and component classes
import java.awt.event.*;  // Using AWT event classes and listener interfaces

// An AWT GUI program inherits (customized) from the top-level container
//   java.awt.Frame
public class AWTAccumulator extends Frame {
   //private Label lblInput;   // Declare input Label (to use anonymous)
   //private Label lblOutput;  // Declare output Label (to use anonymous)
   private TextField tfInput;  // Declare input TextField
   private TextField tfOutput; // Declare output TextField
   private int sum = 0;        // Accumulated sum, init to 0

   // Constructor to setup the GUI components and event handlers
   public AWTAccumulator() {
      setLayout(new GridLayout(2, 2));
         // "super" Frame (Container) sets layout to GridLayout of 2 rows 2 columns.

      add(new Label("Enter an Integer: "));  // "super" Frame adds an anonymous Label

      tfInput = new TextField(10); // Construct TextField
      add(tfInput);                // "super" Frame adds TextField

      tfInput.addActionListener(new TFInputListener());
         // "tfInput" is the source object that fires an ActionEvent upon entered.
         // The source add an anonymous instance of TFInputListener as an ActionEvent
         //   listener, which provides an ActionEvent handler called actionPerformed().
         // Hitting "enter" on tfInput invokes actionPerformed().

      add(new Label("The Accumulated Sum is: "));  // "super" Frame adds an anonymous Label

      tfOutput = new TextField(10); // allocate TextField
      tfOutput.setEditable(false);  // read-only
      add(tfOutput);                // "super" Frame adds TextField

      setTitle("AWT Accumulator");  // "super" Frame sets title
      setSize(350, 120);  // "super" Frame sets initial window size
      setVisible(true);   // "super" Frame shows
   }

   // The entry main() method
   public static void main(String[] args) {
      // Invoke the constructor to setup the GUI, by allocating an anonymous instance
      new AWTAccumulator();
   }

   // Define an inner class to handle the input TextField.
   // An ActionEvent listener must implement ActionListener interface.
   private class TFInputListener implements ActionListener {
      // ActionEvent handler - Called back upon hitting "enter" key on TextField
      @Override
      public void actionPerformed(ActionEvent evt) {
         // Get the String entered into the TextField tfInput, convert to int
         int numberIn = Integer.parseInt(tfInput.getText());
         sum += numberIn;      // Accumulate numbers entered into sum
         tfInput.setText("");  // Clear input TextField
         tfOutput.setText(sum + ""); // Display sum on the output TextField
                                     // convert int to String
      }
   }
}

Dissecting the AWTAccumulator.java

An AWT GUI program extends from java.awt.Frame (Line 6) - the top-level window container.
In the constructor (Line 14), we constructs 4 components - 2 anonymous java.awt.Labels and 2 java.awt.TextFields. The Frame adds the components, in GridLayout.
tfInput (TextField) is the source object, which fires an ActionEvent upon hitting the Enter key. tfInput adds an anonymous instance of TFInputListener as an ActionEvent handler (Line 23). The listener class needs to implement ActionListener interface and provides implementation to method actionPerformed(). Whenever an user hits Enter on the tfInput (TextField), the actionPerformed() will be called back.

3. AWT Event-Handling

Java adopts the so-called "Event-Driven" (or "Event-Delegation") programming model for event-handling, similar to most of the visual programming languages like Visual Basic.

In event-driven programming, a piece of event-handling codes is executed (or called back by the graphics subsystem) when an event was fired in response to an user input (such as clicking a mouse button or hitting the ENTER key in a text field).

Callback Methods

In the above examples, the method actionPerformed() is known as a callback method. In other words, you never invoke actionPerformed() in your codes explicitly. The actionPerformed() is called back by the graphics subsystem under certain circumstances in response to certain user actions.

JavaScript can attach a Callback method to an Event Directly

In some languages, you can directly attach a method (or function) to an event (such as mouse-click). For example, the following JavaScript code (called JSCounter.html) implement a counter similar to the AWTCounter, with a text label, text field and button:

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>JavaScript Counter</title></head>
<body>
  <form>
  Count: <input id="tfCount" type="text" value="0">
  <input id="btnCount" type="button" value="Hello"
     onclick=myBtnOnClick() onmouseover=myBtnMouseOver() >
  </form>
<script>
function myBtnOnClick() { tfCount.value++; }
function myBtnMouseOver() { btnCount.value = "Count Up"; }
function myBtnMouseOut() { btnCount.value = "Click Me"; }
btnCount.onmouseout = myBtnMouseOut;  // assign a function to a variable
</script>
</body>
</html>

In Java, we CANNOT attach a method to a source object directly, as method is not a first-class object in Java. For example, a Java method cannot accept methods as its arguments and it cannot return a method; you cannot assign a method to a variable, etc. (JavaScript and C language CAN!).

Source, Event and Listener Objects

The AWT's event-handling classes are kept in package java.awt.event.

Three kinds of objects are involved in the event-handling: a source, listener(s) and an event object.

The source object (such as Button and Textfield) interacts with the user. Upon triggered, the source object creates an event object to capture the action (e.g., mouse-click x and y, texts entered, etc). This event object will be messaged to all the registered listener object(s), and an appropriate event-handler method of the listener(s) is called-back to provide the response. In other words, triggering a source fires an event to all its listener(s), and invoke an appropriate event handler of the listener(s).

To express interest for a certain source's event, the listener(s) must be registered with the source. In other words, the listener(s) "subscribes" to a source's event, and the source "publishes" the event to all its subscribers upon activation. This is known as subscribe-publish or observable-observer design pattern.

The sequence of steps is illustrated above:

The source object registers its listener(s) for a certain type of event.
A source fires an event when triggered. For example, clicking a Button fires an ActionEvent, clicking a mouse button fires MouseEvent, typing a key fires KeyEvent, and etc.
How the source and listener understand each other? The answer is via an agreed-upon interface. For example, if a source is capable of firing an event called XxxEvent (e.g., ActionEvent). Firstly, we need to declare an interface called XxxListener (e.g., ActionListener) containing the names of the handler methods (recall that an interface contains only abstract methods without implementation). For example, the ActionListener interface is declared as follows:
```
// An ActionListener interface, which declares the signature of the handlers
public interface ActionListener {
   public void actionPerformed(ActionEvent evt);  
}
```
Secondly, all XxxEvent listeners must implement the XxxListener interface. That is, the listeners must provide their own implementations (i.e., programmed responses) to all the abstract methods declared in the XxxListener interface. In this way, the listener(s) can response to these events appropriately. For example,
```
// An example of MouseListener,  which provides implementation to the event handler methods
class MyActionListener implement ActionListener {
   @Override
   public void actionPerformed(ActionEvent evt)  { 
      System.out.println("ActionEvent detected!"); 
   }
}
```
Thirdly, in the source, we need to maintain a list of XxxEvent listener object(s), and define two methods: addXxxListener() and removeXxxListener() to add and remove a XxxEvent listener from this list. For the ActionEvent, the signature of the methods are:
```
public void addActionListener(ActionListener lis);
public void removeActionListener(ActionListener lis);
```
Take note that the addXxxListenser() takes a XxxListener object as its sole parameter. In other words, it can only add objects of the type XxxListener, and its sub-type. Since XxxListener is an interface, you cannot create instance of XxxListener, but need to create instance of a subclass implementing the XxxListener interface.
In summary, we identify the source, the event-listener interface, and the listener object. The listener must implement the event-listener interface. The source object then registers listener object via the addXxxListener(XxxListener lis) method.
The source is triggered by a user.
The source create a XxxEvent object, which encapsulates the necessary information about the activation. For example, the (x, y) position of the mouse pointer, the text entered, etc.
Finally, for each of the XxxEvent listeners in the listener list, the source invokes the appropriate handler on the listener(s), which provides the programmed response.

In summary, triggering a source fires an event to all its registered listeners, and invoke an appropriate handler of the listener.

3.1 Revisit Example 1 AWTCounter: ActionEvent and ActionListener Interface

Clicking a Button (or hitting the "Enter" key on a TextField) fires an ActionEvent to all its ActionEvent listener(s). An ActionEvent listener must implement the ActionListener interface, which declares one abstract method called actionPerformed() as follow:

public interface ActionListener {
   public void actionPerformed(ActionEvent evt);
      // Called back upon button-click (on Button), enter-key pressed (on TextField)
}

Here are the event-handling steps:

We identify btnCount (of Button) as the source object.
Clicking Button fires an ActionEvent to all its ActionEvent listener(s).
The listener(s) is required to implement ActionListener interface, and override the actionPerformed() method to provide the response. In Line 56-65, we write an inner class called BtnCountListener, which override the actionPerformed() to increment and display the count. An inner class is a class defined inside an outer class, and it can access the private entities of the outer class. We will elaborate on the inner class in the next section.
The source object registers listener via the addActionListener(). In this example, the source btnCount (Button) adds an instance of BtnCountListener as a listener via:
```
BtnCountListener listener = new BtnCountListener();
btnCount.addActionListener(listener);
```
Note that addActionListener() takes an argument of the type ActionListener. BtnCountListener, which implements ActionListener interface (i.e., a subclass of ActionListener), is upcasted and passed to the addActionListener() method.
Upon button-click, the btnCount creates an ActionEvent object, and calls back the actionPerformed(ActionEvent) method of all its registered listener(s) with the ActionEvent object created:
```
ActionEvent evt = new ActionEvent( ...... );
listener.actionPerformed(evt);   // for all its listener(s)
```

The sequence diagram is as follows:

3.2 Revisit Example 2 AWTAccumulator: ActionEvent and ActionListener Interface

In this example,

We identify the tfInput (of TextField) as the source object.
Hitting the "Enter" key on a TextField fires an ActionEvent to all its ActionEvent listener(s).
In Line 46-59, we define an inner class called TFInputListener as the ActionEvent listener.The ActionEvent listener is required to implement the ActionListener interface, and override the actionPerformed() method to provide the programmed response upon activation.
The source object tfInput (of TextField) registers an anonymous instance of TFInputListener as its ActionEvent listener via the tfInput.addActionListener(new TFInputListener()) (Line 23).

3.3 Example 3: WindowEvent and WindowListener Interface

A WindowEvent is fired (to all its WindowEvent listeners) when a window (e.g., Frame) has been opened/closed, activated/deactivated, iconified/deiconified via the 3 buttons at the top-right corner or other means. The source of WindowEvent shall be a top-level window-container such as Frame.

A WindowEvent listener must implement WindowListener interface, which declares 7 abstract event-handling methods, as follows. Among them, the windowClosing(), which is called back upon clicking the window-close button, is the most commonly-used.

public void windowClosing(WindowEvent evt)
   // Called-back when the user attempts to close the window by clicking the window close button.
   // This is the most-frequently used handler.
public void windowOpened(WindowEvent evt)
   // Called-back the first time a window is made visible.
public void windowClosed(WindowEvent evt)
   // Called-back when a window has been closed as the result of calling dispose on the window.
public void windowActivated(WindowEvent evt)
   // Called-back when the Window is set to be the active Window.
public void windowDeactivated(WindowEvent evt)
   // Called-back when a Window is no longer the active Window.
public void windowIconified(WindowEvent evt)
   // Called-back when a window is changed from a normal to a minimized state.
public void windowDeiconified(WindowEvent evt)
   // Called-back when a window is changed from a minimized to a normal state.

The following program added support for "close-window button" to "Example 1: AWTCounter".

import java.awt.*;        // Using AWT containers and components
import java.awt.event.*;  // Using AWT events classes and listener interfaces

// An AWT GUI program inherits the top-level container java.awt.Frame
public class WindowEventDemo extends Frame {

   private TextField tfCount;  // Declare a TextField component
   private Button btnCount;    // Declare a Button component
   private int count = 0;      // Counter's value

   // Constructor to setup the GUI components and event handlers
   public WindowEventDemo() {
      setLayout(new FlowLayout()); // "super" Frame sets to FlowLayout

      add(new Label("Counter"));   // "super" Frame adds an anonymous Label

      tfCount = new TextField("0", 10); // Construct the TextField
      tfCount.setEditable(false);       // read-only
      add(tfCount);                     // "super" Frame adds TextField

      btnCount = new Button("Count");  // Construct the Button
      add(btnCount);                   // "super" Frame adds Button

      btnCount.addActionListener(new BtnCountListener());
        // btnCount (source object) fires ActionEvent upon clicking
        // btnCount adds an anonymous instance of BtnCountListener
        //   as an ActionEvent listener

      addWindowListener(new MyWindowListener());
        // "super" Frame (source object) fires WindowEvent.
        // "super" Frame adds an anonymous instance of MyWindowListener
        //   as a WindowEvent listener.

      setTitle("WindowEvent Demo"); // "super" Frame sets title
      setSize(300, 100);            // "super" Frame sets initial size
      setVisible(true);             // "super" Frame shows
   }

   // The entry main() method
   public static void main(String[] args) {
      new WindowEventDemo();  // Let the construct do the job
   }

   // Define an inner class to handle ActionEvent of btnCount
   private class BtnCountListener implements ActionListener {
      @Override
      public void actionPerformed(ActionEvent evt) {
         ++count;
         tfCount.setText(count + "");
      }
   }

   // Define an inner class to handle WindowEvent of this Frame
   private class MyWindowListener implements WindowListener {
      // Called back upon clicking close-window button
      @Override
      public void windowClosing(WindowEvent evt) {
         System.exit(0);  // Terminate the program
      }

      // Not Used, BUT need to provide an empty body to compile.
      @Override public void windowOpened(WindowEvent evt) { }
      @Override public void windowClosed(WindowEvent evt) { }
      // For Debugging
      @Override public void windowIconified(WindowEvent evt) { System.out.println("Window Iconified"); }
      @Override public void windowDeiconified(WindowEvent evt) { System.out.println("Window Deiconified"); }
      @Override public void windowActivated(WindowEvent evt) { System.out.println("Window Activated"); }
      @Override public void windowDeactivated(WindowEvent evt) { System.out.println("Window Deactivated"); }
   }
}

In this example, we shall modify the earlier AWTCounter example to handle the WindowEvent. Recall that pushing the "close-window" button on the AWTCounter has no effect, as it did not handle the WindowEvent of windowClosing(). We included the WindowEvent handling codes in this example.

We identify the super Frame as the source object.
The Frame fires the WindowEvent to all its registered WindowEvent listener(s).
In Line 53-69, we define an inner class called MyWindowListener as the WindowEvent listener. It is required to implement the WindowListener interface, which declares 7 abstract methods: windowOpened(), windowClosed(), windowClosing(), windowActivated(), windowDeactivated(), windowIconified() and windowDeiconified().
We register an anonymous instance of MyWindowListener as the WindowEvent listener to the source Frame via method addWindowListener(new MyWindowListener()).
We override the windowClosing() handler to terminate the program using System.exit(0). We ignore the other 6 handlers, but required to provide an empty body for compilation.

The sequence diagram is as follow:

3.4 Example 4: MouseEvent and MouseListener Interface

A MouseEvent is fired when you press, release, or click (press followed by release) a mouse-button (left or right button) at the source object; or position the mouse-pointer at (enter) and away (exit) from the source object.

A MouseEvent listener must implement the MouseListener interface, which declares the following five abstract methods:

public void mouseClicked(MouseEvent evt)
   // Called-back when the mouse-button has been clicked (pressed followed by released) on the source.
public void mousePressed(MouseEvent evt)
public void mouseReleased(MouseEvent evt)
   // Called-back when a mouse-button has been pressed/released on the source.
   // A mouse-click invokes mousePressed(), mouseReleased() and mouseClicked().
public void mouseEntered(MouseEvent evt)
public void mouseExited(MouseEvent evt) 
   // Called-back when the mouse-pointer has entered/exited the source.

import java.awt.*;
import java.awt.event.*;

public class MouseEventDemo extends Frame {
   private TextField tfMouseX; // to display mouse-click-x
   private TextField tfMouseY; // to display mouse-click-y

   // Constructor - Setup the UI components and event handlers
   public MouseEventDemo() {
      setLayout(new FlowLayout()); // "super" frame sets its layout to FlowLayout

      // Label (anonymous)
      add(new Label("X-Click: ")); // "super" frame adds Label component

      // TextField
      tfMouseX = new TextField(10); // 10 columns
      tfMouseX.setEditable(false);  // read-only
      add(tfMouseX);                // "super" frame adds TextField component

      // Label (anonymous)
      add(new Label("Y-Click: ")); // "super" frame adds Label component

      // TextField
      tfMouseY = new TextField(10);
      tfMouseY.setEditable(false);  // read-only
      add(tfMouseY);                // "super" frame adds TextField component

      addMouseListener(new MyMouseListener());
          // "super" frame (source) fires the MouseEvent.
          // "super" frame adds an anonymous instance of MyMouseListener
          //   as a MouseEvent listener.

      setTitle("MouseEvent Demo"); // "super" Frame sets title
      setSize(350, 100);           // "super" Frame sets initial size
      setVisible(true);            // "super" Frame shows
   }

   public static void main(String[] args) {
      new MouseEventDemo();  // Let the constructor do the job
   }

   // Define an inner class to handle MouseEvent
   private class MyMouseListener implements MouseListener {
      // Called back upon mouse clicked
      @Override
      public void mouseClicked(MouseEvent evt) {
         tfMouseX.setText(evt.getX() + "");
         tfMouseY.setText(evt.getY() + "");
      }

      // Not used - need to provide an empty body to compile.
      @Override public void mousePressed(MouseEvent evt) { }
      @Override public void mouseReleased(MouseEvent evt) { }
      @Override public void mouseEntered(MouseEvent evt) { }
      @Override public void mouseExited(MouseEvent evt) { }
   }
}

In this example, we setup a GUI with 4 components (two anonynous Labels and two non-editable TextFields) inside a top-level container Frame, arranged in FlowLayout.

To demonstrate the MouseEvent:

We identity super Frame as the source object.
The Frame fires a MouseEvent to all its MouseEvent listener(s) when you click/press/release a mouse-button or enter/exit with the mouse-pointer.
In Line 42-56, we define an inner class called MyMouseListener as the MouseEvent listener. It is required to implement the MouseListener interface, which declares 5 abstract methods: mouseClicked(), mousePressed(), mouseReleased(), mouseEntered(), and mouseExit(). We override the mouseClicked() to display the (x, y) coordinates of the mouse click on the two displayed TextFields. We ignore all the other handlers (for simplicity - but you need to provide an empty body for compilation).
We register an anonymous instance of MyMouseListener as the MouseEvent listener to super Frame (source) via the method addMouseListener(new MyMouseListener()).

Try: Include a WindowListener to handle the close-window button.

3.5 Example 5: MouseEvent and MouseMotionListener Interface

A MouseEvent is also fired when you move and drag the mouse pointer at the source object. But you need to use MouseMotionListener to handle the mouse-move and mouse-drag. The MouseMotionListener interface declares the following two abstract methods:

public void mouseDragged(MouseEvent e)
   // Called-back when a mouse-button is pressed on the source component and then dragged.
public void mouseMoved(MouseEvent e)
   // Called-back when the mouse-pointer has been moved onto the source component but no buttons have been pushed.

import java.awt.*;
import java.awt.event.*;

// An AWT GUI program inherits from the top-level container java.awt.Frame
public class MouseMotionDemo extends Frame {

   // To display the (x, y) of the mouse-clicked
   private TextField tfMouseClickX;
   private TextField tfMouseClickY;
   // To display the (x, y) of the current mouse-pointer position
   private TextField tfMousePositionX;
   private TextField tfMousePositionY;

   // Constructor to setup the GUI components and event handlers
   public MouseMotionDemo() {
      setLayout(new FlowLayout()); // "super" frame sets to FlowLayout

      add(new Label("X-Click: "));
      tfMouseClickX = new TextField(10);
      tfMouseClickX.setEditable(false);
      add(tfMouseClickX);
      add(new Label("Y-Click: "));
      tfMouseClickY = new TextField(10);
      tfMouseClickY.setEditable(false);
      add(tfMouseClickY);

      add(new Label("X-Position: "));
      tfMousePositionX = new TextField(10);
      tfMousePositionX.setEditable(false);
      add(tfMousePositionX);
      add(new Label("Y-Position: "));
      tfMousePositionY = new TextField(10);
      tfMousePositionY.setEditable(false);
      add(tfMousePositionY);

      MyMouseListener listener = new MyMouseListener();
      addMouseListener(listener);
      addMouseMotionListener(listener);
        // "super" frame (source) fires MouseEvent.
        // "super" frame adds an instance of MyMouseListener
        //   as MouseListener and MouseMotionListener.

      setTitle("MouseMotion Demo"); // "super" Frame sets title
      setSize(400, 120);            // "super" Frame sets initial size
      setVisible(true);             // "super" Frame shows
   }

   // The entry main() method
   public static void main(String[] args) {
      new MouseMotionDemo();  // Let the constructor do the job
   }

   // Define an inner class as both the MouseListener and MouseMotionListener
   // A Java class can extend one superclass but implement many interfaces
   private class MyMouseListener implements MouseListener, MouseMotionListener {
      /* MouseListener handlers */
      // Called back when a mouse-button has been clicked
      @Override
      public void mouseClicked(MouseEvent evt) {
         tfMouseClickX.setText(evt.getX() + "");
         tfMouseClickY.setText(evt.getY() + "");
      }

      // Not Used, but need to provide an empty body for compilation
      @Override public void mousePressed(MouseEvent evt) { }
      @Override public void mouseReleased(MouseEvent evt) { }
      @Override public void mouseEntered(MouseEvent evt) { }
      @Override public void mouseExited(MouseEvent evt) { }

      /* MouseMotionEvent handlers */
      // Called back when the mouse-pointer has been moved
      @Override
      public void mouseMoved(MouseEvent evt) {
         tfMousePositionX.setText(evt.getX() + "");
         tfMousePositionY.setText(evt.getY() + "");
      }

      // Not Used, but need to provide an empty body for compilation
      @Override public void mouseDragged(MouseEvent evt) { }
   }
}

In this example, we shall illustrate both the MouseListener and MouseMotionListener.

We identify the super Frame as the source, which fires the MouseEvent to its registered MouseListener and MouseMotionListener.
In Line 53-80, we define an inner class called MyMouseListener as both the MouseListener and MouseMotionListner.
We register an instance of MyMouseListener as the listener to super Frame via method addMouseListener() and addMouseMotionListener().
The MouseMotionListener needs to implement 2 abstract methods: mouseMoved() and mouseDragged() declared in the MouseMotionListener interface.
We override the mouseMoved() to display the (x, y) position of the mouse pointer. We ignore the MouseDragged() handler by providing an empty body for compilation.

Try: Include a WindowListener to handle the close-window button.

3.6 Example 6: KeyEvent and KeyListener Interface

A KeyEvent is fired when you pressed, released, and typed (pressed followed by released) a key on the source object. A KeyEvent listener must implement KeyListener interface, which declares three abstract methods:

public void keyTyped(KeyEvent e)
   // Called-back when a key has been typed (pressed and released).
public void keyPressed(KeyEvent e)
public void keyReleased(KeyEvent e)
   // Called-back when a key has been pressed or released.

import java.awt.*;
import java.awt.event.*;

// An AWT GUI program inherits from the top-level container java.awt.Frame
public class KeyEventDemo extends Frame {

   private TextField tfInput;  // Single-line TextField to receive tfInput key
   private TextArea taDisplay; // Multi-line TextArea to taDisplay result

   // Constructor to setup the GUI components and event handlers
   public KeyEventDemo() {
      setLayout(new FlowLayout()); // "super" frame sets to FlowLayout

      add(new Label("Enter Text: "));
      tfInput = new TextField(10);
      add(tfInput);
      taDisplay = new TextArea(5, 40); // 5 rows, 40 columns
      add(taDisplay);

      tfInput.addKeyListener(new MyKeyListener());
         // tfInput TextField (source) fires KeyEvent.
         // tfInput adds an anonymous instance of MyKeyListener
         //   as a KeyEvent listener.

      setTitle("KeyEvent Demo"); // "super" Frame sets title
      setSize(400, 200);         // "super" Frame sets initial size
      setVisible(true);          // "super" Frame shows
   }

   // The entry main() method
   public static void main(String[] args) {
      new KeyEventDemo();  // Let the constructor do the job
   }

   // Define an inner class to handle KeyEvent
   private class MyKeyListener implements KeyListener {
      // Called back when a key has been typed (pressed and released)
      @Override
      public void keyTyped(KeyEvent evt) {
         taDisplay.append("You have typed " + evt.getKeyChar() + "\n");
      }

      // Not Used, but need to provide an empty body for compilation
      @Override public void keyPressed(KeyEvent evt) { }
      @Override public void keyReleased(KeyEvent evt) { }
   }
}

In this example:

We identify the tfInput (of TextField) as the source object.
The source fires a KeyEvent when you press/release/type a key to all its KeyEvent listener(s).
In Line 35-46, we define an inner class called MyKeyListener as the KeyEvent listener.
We register an anonymous instance of MyKeyListener as the KeyEvent listener to the source TextField via method input.addKeyListener().
The KeyEvent listener needs to implement the KeyListener interface, which declares 3 abstract methods: keyTyped(), keyPressed(), keyReleased().
We override the keyTyped() to display key typed o

4.6 Example 10: Using the Same Listener Instance for All the Buttons

If you use the same instance as the listener for all the 3 buttons, you need to determine which button has fired the event. It is because all the 3 buttons trigger the same event-handler method.

Using ActionEvent's getActionCommand()

In the following example, we use the same instance of a "named" inner class as the listener for all the 3 buttons. The listener needs to determine which button has fired the event. This can be accomplished via the ActionEvent's getActionCommonad() method, which returns the button's label.

import java.awt.*;
import java.awt.event.*;
 
// An AWT GUI program inherits the top-level container java.awt.Frame
public class AWTCounter3Buttons1Listener extends Frame {
   private TextField tfCount;
   private Button btnCountUp, btnCountDown, btnReset;
   private int count = 0;
 
   // Constructor to setup the GUI components and event handlers
   public AWTCounter3Buttons1Listener () {
      setLayout(new FlowLayout());
      add(new Label("Counter"));
      tfCount = new TextField("0", 10);
      tfCount.setEditable(false);
      add(tfCount);
 
      // Construct Buttons
      btnCountUp = new Button("Count Up");
      add(btnCountUp);
      btnCountDown = new Button("Count Down");
      add(btnCountDown);
      btnReset = new Button("Reset");
      add(btnReset);
 
      // Allocate an instance of the "named" inner class BtnListener.
      AllButtonsListener listener = new AllButtonsListener();
      // Use the same listener instance for all the 3 Buttons.
      btnCountUp.addActionListener(listener);
      btnCountDown.addActionListener(listener);
      btnReset.addActionListener(listener);
 
      setTitle("AWT Counter");
      setSize(400, 100);
      setVisible(true);
   }
 
   // The entry main method
   public static void main(String[] args) {
      new AWTCounter3Buttons1Listener();  // Let the constructor do the job
   }
 
   /**
    * AllButtonsListener is an named inner class used as ActionEvent listener for all the Buttons.
    */
   private class AllButtonsListener implements ActionListener {
      @Override
      public void actionPerformed(ActionEvent evt) {
         // Need to determine which button fired the event.
         // the getActionCommand() returns the Button's label
         String btnLabel = evt.getActionCommand();
         if (btnLabel.equals("Count Up")) {
            ++count;
         } else if (btnLabel.equals("Count Down")) {
            --count;
         } else {
            count = 0;
         }
         tfCount.setText(count + "");
      }
   }
}

Using getSource() of EventObject

Besides the getActionCommand(), which is only available for ActionEvent, you can use the getSource() method, which is available to all event objects, to retrieve a reference to the source object that has fired the event. getSource() returns a java.lang.Object. You may need to downcast it to the proper type of the source object. For example,

import java.awt.*;
import java.awt.event.*;
 
public class AWTCounter3ButtonsGetSource extends Frame {
   private TextField tfCount;
   private Button btnCountUp, btnCountDown, btnReset;
   private int count = 0;
 
   // Constructor to setup the GUI components and event handlers
   public AWTCounter3ButtonsGetSource () {
      setLayout(new FlowLayout());
      add(new Label("Counter"));
      tfCount = new TextField("0", 10);
      tfCount.setEditable(false);
      add(tfCount);
 
      // Construct Buttons
      btnCountUp = new Button("Count Up");
      add(btnCountUp);
      btnCountDown = new Button("Count Down");
      add(btnCountDown);
      btnReset = new Button("Reset");
      add(btnReset);
 
      // Allocate an instance of inner class BtnListener.
      AllButtonsListener listener = new AllButtonsListener();
      // Use the same listener instance to all the 3 Buttons.
      btnCountUp.addActionListener(listener);
      btnCountDown.addActionListener(listener);
      btnReset.addActionListener(listener);
 
      setTitle("AWT Counter");
      setSize(400, 100);
      setVisible(true);
   }
 
   // The entry main method
   public static void main(String[] args) {
      new AWTCounter3ButtonsGetSource();  // Let the constructor do the job
   }
 
   /**
    * AllButtonsListener is a named inner class used as ActionEvent listener for all the Buttons.
    */
   private class AllButtonsListener implements ActionListener {
      @Override
      public void actionPerformed(ActionEvent evt) {
         // Need to determine which button has fired the event.
         Button source = (Button)evt.getSource();
               // Get a reference of the source that has fired the event.
               // getSource() returns a java.lang.Object. Downcast back to Button.
         if (source == btnCountUp) {
            ++count;
         } else if (source == btnCountDown) {
            --count;
         } else {
            count = 0;
         }
         tfCount.setText(count + "");
      }
   }
}

5. Event Listener's Adapter Classes

5.1 Example 11: WindowAdapter for WindowListener

Using WindowListener Interface

Refer to the WindowEventDemo, a WindowEvent listener is required to implement the WindowListener interface, which declares 7 abstract methods. Although we are only interested in windowClosing(), we need to provide an empty body to the other 6 abstract methods in order to compile the program. This is tedious, e.g., we can rewrite the WindowEventDemo using an inner class implementing ActionListener as follows:

import java.awt.*;
import java.awt.event.*;
 
// An AWT GUI program inherits the top-level container java.awt.Frame
public class WindowEventDemoWithInnerClass extends Frame {
   private TextField tfCount;
   private Button btnCount;
   private int count = 0;
 
   // Constructor to setup the GUI components and event handlers
   public WindowEventDemoWithInnerClass () {
      setLayout(new FlowLayout());
      add(new Label("Counter"));
      tfCount = new TextField("0", 10);
      tfCount.setEditable(false);
      add(tfCount);
 
      btnCount = new Button("Count");
      add(btnCount);
      btnCount.addActionListener(new ActionListener() {
         @Override
         public void actionPerformed(ActionEvent evt) {
            ++count;
            tfCount.setText(count + "");
         }
      });
 
      // Allocate an anonymous instance of an anonymous inner class
      // that implements WindowListener.
      // "super" Frame adds this instance as WindowEvent listener.
      addWindowListener(new WindowListener() {
         @Override
         public void windowClosing(WindowEvent evt) {
            System.exit(0);  // terminate the program
         }
         // Need to provide an empty body for compilation
         @Override public void windowOpened(WindowEvent evt) { }
         @Override public void windowClosed(WindowEvent evt) { }
         @Override public void windowIconified(WindowEvent evt) { }
         @Override public void windowDeiconified(WindowEvent evt) { }
         @Override public void windowActivated(WindowEvent evt) { }
         @Override public void windowDeactivated(WindowEvent evt) { }
      });
 
      setTitle("WindowEvent Demo");
      setSize(250, 100);
      setVisible(true);
   }
 
   // The entry main method
   public static void main(String[] args) {
      new WindowEventDemoWithInnerClass();   // Let the constructor do the job
   }
}

Using WindowAdapter Superclass

An adapter class called WindowAdapter is therefore provided, which implements the WindowListener interface and provides default implementations to all the 7 abstract methods. You can then derive a subclass from WindowAdapter and override only methods of interest and leave the rest to their default implementation. For example,

import java.awt.*;
import java.awt.event.*;
 
// An AWT GUI program inherits the top-level container java.awt.Frame
public class WindowEventDemoAdapter extends Frame {
   private TextField tfCount;
   private Button btnCount;
   private int count = 0;
 
   // Constructor to setup the GUI components and event handlers
   public WindowEventDemoAdapter () {
      setLayout(new FlowLayout());
      add(new Label("Counter"));
      tfCount = new TextField("0", 10);
      tfCount.setEditable(false);
      add(tfCount);
 
      btnCount = new Button("Count");
      add(btnCount);
      btnCount.addActionListener(new ActionListener() {
         @Override
         public void actionPerformed(ActionEvent evt) {
            ++count;
            tfCount.setText(count + "");
         }
      });
 
      // Allocate an anonymous instance of an anonymous inner class
      // that extends WindowAdapter.
      // "super" Frame adds the instance as WindowEvent listener.
      addWindowListener(new WindowAdapter() {
         @Override
         public void windowClosing(WindowEvent evt) {
            System.exit(0);  // Terminate the program
         }
      });
 
      setTitle("WindowEvent Demo");
      setSize(250, 100);
      setVisible(true);
   }
 
   /** The entry main method */
   public static void main(String[] args) {
      new WindowEventDemoAdapter();   // Let the constructor do the job
   }
}

Clearly, the adapter greatly simplifies the codes.

5.2 Other Event-Listener Adapter Classes

Similarly, adapter classes such as MouseAdapter, MouseMotionAdapter, KeyAdapter, FocusAdapter are available for MouseListener, MouseMotionListener, KeyListener, and FocusListener, respectively.

There is no ActionAdapter for ActionListener, because there is only one abstract method (i.e. actionPerformed()) declared in the ActionListener interface. This method has to be overridden and there is no need for an adapter.

7. Layout Managers and Panel

A container has a so-called layout manager to arrange its components. The layout managers provide a level of abstraction to map your user interface on all windowing systems, so that the layout can be platform-independent.

AWT provides the following layout managers (in package java.awt): FlowLayout, GridLayout, BorderLayout, GridBagLayout, BoxLayout, CardLayout, and others. Swing added more layout manager in package javax.swing, to be described later.

Container's setLayout() method

A container has a setLayout() method to set its layout manager:

// java.awt.Container
public void setLayout(LayoutManager mgr)

To set up the layout of a Container (such as Frame, JFrame, Panel, or JPanel), you have to:

Construct an instance of the chosen layout object, via new and constructor, e.g., new FlowLayout())
Invoke the setLayout() method of the Container, with the layout object created as the argument;
Place the GUI components into the Container using the add() method in the correct order; or into the correct zones.

For example,

// Allocate a Panel (container)
Panel pnl = new Panel();  
// Allocate a new Layout object. The Panel container sets to this layout.
pnl.setLayout(new FlowLayout());
// The Panel container adds components in the proper order.
pnl.add(new JLabel("One"));
pnl.add(new JLabel("Two"));
pnl.add(new JLabel("Three"));
......

Container's getLayout() method

You can get the current layout via Container's getLayout() method.

Panel pnl = new Panel();
System.out.println(pnl.getLayout());
      // java.awt.FlowLayout[hgap=5,vgap=5,align=center]

Panel's Initial Layout

Panel (and Swing's JPanel) provides a constructor to set its initial layout manager. It is because a primary function of Panel is to layout a group of component in a particular layout.

public void Panel(LayoutManager layout)  
   // Construct a Panel in the given layout
   // By default, Panel (and JPanel) has FlowLayout
 
// For example, create a Panel in BorderLayout
Panel pnl = new Panel(new BorderLayout());

7.1 FlowLayout

In the java.awt.FlowLayout, components are arranged from left-to-right inside the container in the order that they are added (via method aContainer.add(aComponent)). When one row is filled, a new row will be started. The actual appearance depends on the width of the display window.

Constructors

public FlowLayout();
public FlowLayout(int alignment);
public FlowLayout(int alignment, int hgap, int vgap);
  // alignment: FlowLayout.LEFT (or LEADING), FlowLayout.RIGHT (or TRAILING), or FlowLayout.CENTER
  // hgap, vgap: horizontal/vertical gap between the components
  // By default: hgap = 5, vgap = 5, alignment = FlowLayout.CENTER

Example

import java.awt.*;
import java.awt.event.*;
 
// An AWT GUI program inherits the top-level container java.awt.Frame
public class AWTFlowLayoutDemo extends Frame {
   private Button btn1, btn2, btn3, btn4, btn5, btn6;
 
   // Constructor to setup GUI components and event handlers
   public AWTFlowLayoutDemo () {
      setLayout(new FlowLayout());
         // "super" Frame sets layout to FlowLayout, which arranges the components
         // from left-to-right, and flow from top-to-bottom.
 
      btn1 = new Button("Button 1");
      add(btn1);
      btn2 = new Button("This is Button 2");
      add(btn2);
      btn3 = new Button("3");
      add(btn3);
      btn4 = new Button("Another Button 4");
      add(btn4);
      btn5 = new Button("Button 5");
      add(btn5);
      btn6 = new Button("One More Button 6");
      add(btn6);
 
      setTitle("FlowLayout Demo"); // "super" Frame sets title
      setSize(280, 150);           // "super" Frame sets initial size
      setVisible(true);            // "super" Frame shows
   }
 
   // The entry main() method
   public static void main(String[] args) {
      new AWTFlowLayoutDemo();  // Let the constructor do the job
   }
}

7.2 GridLayout

In java.awt.GridLayout, components are arranged in a grid (matrix) of rows and columns inside the Container. Components are added in a left-to-right, top-to-bottom manner in the order they are added (via method aContainer.add(aComponent)).

Constructors

public GridLayout(int rows, int columns);
public GridLayout(int rows, int columns, int hgap, int vgap);
      // By default: rows = 1, cols = 0, hgap = 0, vgap = 0

Example

import java.awt.*;
import java.awt.event.*;
 
// An AWT GUI program inherits the top-level container java.awt.Frame
public class AWTGridLayoutDemo extends Frame {
   private Button btn1, btn2, btn3, btn4, btn5, btn6;
 
   // Constructor to setup GUI components and event handlers
   public AWTGridLayoutDemo () {
      setLayout(new GridLayout(3, 2, 3, 3));
         // "super" Frame sets layout to 3x2 GridLayout, horizontal and vertical gaps of 3 pixels
 
      // The components are added from left-to-right, top-to-bottom
      btn1 = new Button("Button 1");
      add(btn1);
      btn2 = new Button("This is Button 2");
      add(btn2);
      btn3 = new Button("3");
      add(btn3);
      btn4 = new Button("Another Button 4");
      add(btn4);
      btn5 = new Button("Button 5");
      add(btn5);
      btn6 = new Button("One More Button 6");
      add(btn6);
 
      setTitle("GridLayout Demo"); // "super" Frame sets title
      setSize(280, 150);           // "super" Frame sets initial size
      setVisible(true);            // "super" Frame shows
   }
 
   // The entry main() method
   public static void main(String[] args) {
      new AWTGridLayoutDemo();  // Let the constructor do the job
   }
}

7.3 BorderLayout

In java.awt.BorderLayout, the container is divided into 5 zones: EAST, WEST, SOUTH, NORTH, and CENTER. Components are added using method aContainer.add(aComponent, zone), where zone is either BorderLayout.NORTH (or PAGE_START), BorderLayout.SOUTH (or PAGE_END), BorderLayout.WEST (or LINE_START), BorderLayout.EAST (or LINE_END), or BorderLayout.CENTER.

You need not place components to all the 5 zones. The NORTH and SOUTH components may be stretched horizontally; the EAST and WEST components may be stretched vertically; the CENTER component may stretch both horizontally and vertically to fill any space left over.

Constructors

public BorderLayout();
public BorderLayout(int hgap, int vgap);
      // By default hgap = 0, vgap = 0

Example

import java.awt.*;
import java.awt.event.*;
 
// An AWT GUI program inherits the top-level container java.awt.Frame
public class AWTBorderLayoutDemo extends Frame {
   private Button btnNorth, btnSouth, btnCenter, btnEast, btnWest;
 
   // Constructor to setup GUI components and event handlers
   public AWTBorderLayoutDemo () {
      setLayout(new BorderLayout(3, 3));
         // "super" Frame sets layout to BorderLayout,
         // horizontal and vertical gaps of 3 pixels
 
      // The components are added to the specified zone
      btnNorth = new Button("NORTH");
      add(btnNorth, BorderLayout.NORTH);
      btnSouth = new Button("SOUTH");
      add(btnSouth, BorderLayout.SOUTH);
      btnCenter = new Button("CENTER");
      add(btnCenter, BorderLayout.CENTER);
      btnEast = new Button("EAST");
      add(btnEast, BorderLayout.EAST);
      btnWest = new Button("WEST");
      add(btnWest, BorderLayout.WEST);
 
      setTitle("BorderLayout Demo"); // "super" Frame sets title
      setSize(280, 150);             // "super" Frame sets initial size
      setVisible(true);              // "super" Frame shows
   }
 
   // The entry main() method
   public static void main(String[] args) {
      new AWTBorderLayoutDemo();  // Let the constructor do the job
   }
}

7.4 Using Panels as Sub-Container to Organize Components

An AWT Panel is a rectangular pane, which can be used as sub-container to organized a group of related components in a specific layout (e.g., FlowLayout, BorderLayout). Panels are secondary containers, which shall be added into a top-level container (such as Frame), or another Panel.

For example, the following figure shows a Frame in BorderLayout containing two Panels - panelResult in FlowLayout and panelButtons in GridLayout. panelResult is added to the NORTH, and panelButtons is added to the CENTER.

import java.awt.*;
import java.awt.event.*;
 
// An AWT GUI program inherits the top-level container java.awt.Frame
public class AWTPanelDemo extends Frame {
   private Button[] btnNumbers;  // Array of 10 numeric Buttons
   private Button btnHash, btnStar;
   private TextField tfDisplay;
 
   // Constructor to setup GUI components and event handlers
   public AWTPanelDemo () {
      // Set up display panel
      Panel panelDisplay = new Panel(new FlowLayout());
      tfDisplay = new TextField("0", 20);
      panelDisplay.add(tfDisplay);
 
      // Set up button panel
      Panel panelButtons = new Panel(new GridLayout(4, 3));
      btnNumbers = new Button[10];  // Construct an array of 10 numeric Buttons
      btnNumbers[1] = new Button("1");  // Construct Button "1"
      panelButtons.add(btnNumbers[1]);  // The Panel adds this Button
      btnNumbers[2] = new Button("2");
      panelButtons.add(btnNumbers[2]);
      btnNumbers[3] = new Button("3");
      panelButtons.add(btnNumbers[3]);
      btnNumbers[4] = new Button("4");
      panelButtons.add(btnNumbers[4]);
      btnNumbers[5] = new Button("5");
      panelButtons.add(btnNumbers[5]);
      btnNumbers[6] = new Button("6");
      panelButtons.add(btnNumbers[6]);
      btnNumbers[7] = new Button("7");
      panelButtons.add(btnNumbers[7]);
      btnNumbers[8] = new Button("8");
      panelButtons.add(btnNumbers[8]);
      btnNumbers[9] = new Button("9");
      panelButtons.add(btnNumbers[9]);
            // You should use a loop for the above statements!!!
      btnStar = new Button("*");
      panelButtons.add(btnStar);
      btnNumbers[0] = new Button("0");
      panelButtons.add(btnNumbers[0]);
      btnHash = new Button("#");
      panelButtons.add(btnHash);
 
      setLayout(new BorderLayout());  // "super" Frame sets to BorderLayout
      add(panelDisplay, BorderLayout.NORTH);
      add(panelButtons, BorderLayout.CENTER);
 
      setTitle("BorderLayout Demo"); // "super" Frame sets title
      setSize(200, 200);             // "super" Frame sets initial size
      setVisible(true);              // "super" Frame shows
   }
 
   // The entry main() method
   public static void main(String[] args) {
      new AWTPanelDemo();  // Let the constructor do the job
   }
}