CS4273: Distributed System Technologies and Programming I

Lecture 2: Introduction to Java and GUI

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Important References for Java Manuals

• Web site for Java Swing (GUI)

http://download.oracle.com/javase/tutorial/uiswing/

• Web site for Java Swing Event handler

http://download.oracle.com/javase/tutorial/uiswing/events/index.html

• Web site for Java 2 Standard Edition

http://www.oracle.com/technetwork/java/javase/overview/index.html

• Sample Programs

Source codes of demo programs are in unix system: ~jia/www/java/

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Java: an Object-Oriented Programming Language

Object-Oriented Programming:

• Encapsulation

• Inheritance

• Polymorphism

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Class and Method Definition

• class consists of fields and methods:class class-name [extends superclass]

[implements interface] {

variable declarations;

method declarations;

}

• method declaration:

[modifiers] returnT method (args) {

statements;

}

public class dummyButton extends JApplet { String font; int style, size; private JButton bold, italic; public void init() { bold = new JButton("BOLD"); add(bold); add(italic = new JButton("ITALIC")); font = "Helvetica"; style = Font.PLAIN; size = 48; } public void paint(Graphics g) { super.paint(g); g.setFont(new Font(font, style, size)); g.drawString("Hello Java!", 50, 100); } class butnhandler implements ActionListener { public void actionPerformed (ActionEvent e) {

………}

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Object Initialization and Constructor

• All variables must be initialized before use. Java automatically sets some initial values for variables of the class, but not variables in methods.

• A constructor is a special method, with the same name as the class, for initialization. Java provides an empty & no-argument constructor if a class does not have one.class Rabbit { int Age; Rabbit (int Age) { //constructor this.Age = Age; }//differentiate from the Age in the

parameter void run(int duration,boolean zigzag){ ... }}class RabbitGame { public static void main(string[] args) { Rabbit bunny = new Rabbit(3); ….}

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Class Inheritance and Method Overriding

• When a method has the same name as in the superclass, the method overrides the one in superclass. Otherwise the superclass methods are inherited with no change.

• The top class in Java is “object”. Every class is a descendent of object (implicitly inherited).

• Keyword super refers to the superclass’ methods or variables.

class Animals { int Age; Animals (int Age) { this.Age = Age; } void wish () { System.out.println ("I want to eat"); }}class Rabbit extends Animals {

.. .. ..Rabbit (int Age, char Color) {

super (Age); // call super constructor

this.Color = Color; }

void wish () { // method over-ridesuper.wish(); // call a super method

System.out.println("I want a carrot"); }}

Rabbit my_rabbit = new Rabbit(3, ‘B’);my_rabbit.wish(); // ? output ?

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Modifier: static

• A variable or method defined as static means that the variable or the method belongs to the class, and they are not with objects.

• Static variables / methods are class variables / methods. There is only one copy of them in the system.

• Class methods can only access class variables. It cannot use “this” reference, cannot call normal methods, etc. But normal methods can access class variables.

class Rabbit {static int rabbits_count = 0;int Age;

Rabbit (int Age) {rabbits_count++; // access static var

this.Age = Age; } static int count() {// only use class variable

return rabbits_count; } }

object object

Object Templatestatic vars,…

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Modifiers: public, private, protected, final

• public: can be used everywhere.• private: can be used only in its own class, not even from its subclass.• protected: can be accessed within its own package.• final defines:

– a class can’t be extended – a method can’t be overridden – a variable cannot be modified (constant)

final class Rabbit extends Animal { final float = 3.14;

…. final int run(int duration, boolean zigzag) { …. } // method 1

}

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Abstract Methods and Abstract Classes

• An abstract method is a method that has no implementation. It allows subclasses to implement the method according to their own needs.

• An abstract class is a class that contains at least one abstract method.• An abstract class must be FULLY implemented before it can be

instantiated.abstract class Animals {

int Age; Animals (int Age) { this.Age = Age; } abstract void wish ();

}class Rabbit extends Animals {

.. .. ..void wish () { // implement the abstract method

System.out.println("I want a carrot"); }}

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Interface and its Implementation

• An interface is a class that all its methods are abstract.

• There are many interfaces in Java, especially for event handlers, that require implementations.

• When defining a class which implements an interface, uses the format:

class xxx implements interface {

method declaration;

…….

method declaration;

}

• If a class implements some methods of an interface, not all of them, then this class becomes an abstract class.

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Graphical User Interface (GUI)

Containers and Components

• Containers provide a rectangular display area in which components are positioned. A container can also contain other containers.

• Components are positioned relative to the top left corner (0,0) of its container and are located using the container’s coordinate space.

• Components are added to a container using the add() method.

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Containers/Components

Containers: JFrame, equivalent to windows.JPanel, applet’s display area is JPanel. It is often used to organize

components together in display.

Atomic Components:JButtons (JCheckBoxes, JRadioButtons)JComboBoxJLabelsJTextField (JTextArea)Popup menusScrollPane

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A Simple Example of Buttons

public class dummyButton extends JApplet {public void init() { setLayout(new FlowLayout(FlowLayout.CENTER)); bold = new JButton("bold");

add(bold); add(italic = new JButton("italic")); } public void paint(Graphics g) { super.paint(g); g.setFont(new Font(MyFont, MyStyle, MySize)); g.drawString("Hello Java!", 50, 100);}………..

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Java Event Handling Model

• When user clicks a button or presses a key, an event is generated. The button clicked is called “event source”.

• For each event source, you need to delegate a Listener object to listen to it.

• When the listener object hears an event, it invokes a method to process the event.

button Listener Object

Event

bold = new JButton(“bold”);bold.addActionListener(new BtnHandler());

public class BtnHandler implements ActionListener { …….

public void actionPerformed (ActionEvent e) { // code to handler start button }}

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Make Buttons Responsive

The steps making buttons responsive:

1. write an event handler which implements interface ActionListener. Interface ActionListener has only one method actionPerformed.

2. implement method actionPerformed in the interface.

3. add action listener (your event handler) to each event source.

public class dummyButton extends JApplet {public void init() { add(bold = new Button("bold")); bold.addActionListener (new butnhandler()); …. }

class butnhandler implements ActionListener { public void actionPerformed (ActionEvent e) { if (e.getSource() == bold) style = Font.BOLD; if (e.getSource() == italic) style = Font.ITALIC; repaint(); } }}

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TextField

Textfiled is used for getting Text Input from users.

• TextField is constructed as:JTextField tf = new JTextField(“input here”, 20);

add(tf);

• an “action” event is generated when the user types “return” in a TextField. You need to add an event-handler to the JTextField object by:

tf.addActionListener (new actionAdapter());

• get the input text by:

String InputStr = tf.getText();

• set the text field (for display) by:tf.setText(“wrong input”);

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TextArea

TextArea is used to display or edit a number of rows and cols.

There is no event handler associated with it. It can be used

with ScrollPane to make it scrollable.

• TextArea can be constructed by:JTextArea ta = new JTextArea(row, col);

• display text in a TextArea by:ta.setText(String str); OR

ta.append(String str);

• get the whole text in TextArea by:ta.getText();

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Event handling for TextField

JTextField event is “action” type:// jia/www/java/gui/textField.javapublic class textFieldTest extends JApplet {JTextField textin, textout;JTextArea textArea;public void init() { setLayout(new FlowLayout()); add(textin = new JTextField("",20)); textin.addActionListener (

new TextHandler ()); textArea = new JTextArea(10,20); add(textArea); add(textout = new

JTextField("initial",20));}

class TextHandler implements ActionListener { public void actionPerformed (ActionEvent e){ if (e.getSource() == textin) { textArea.setText(textin.getText()); textout.setText(textin.getText()); } }}}

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GUI Event Handlings

• Action Events. Event sources:– JButton, List, – JTextField, – JMenuItem, etc.

• Item Events. Event sources:– JCheckbox,– JButtonGroup (RadioButton),– JComboBox– JChoice (Pop-up menu)

• Mouse Events.– Mouse press / release– Mouse drag / move

• Key Events– Key press / release– Key typed

…………

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CheckBox, ButtonGroup (RadioButton)

Checkbox allows users to tick (true / false). A checkbox can be used independently.• a Checkbox is created by:

new JCheckbox();

ButtonGroup is a group of checkboxes, only one of them can be selected.• a ButtonGroup is created by:

g = new ButtonGroup();• a RadioButton is created & added to Group by:

btn = new JRadioButton();g.add(btn); // add btn to group g

• When a checkbox is clicked, an event of type ItemEvent is generated. A handler for ItemEvent type of events implements the interface ItemListener.

• Interface ItemListener has only one method itemStateChanged(). You need to implement it.

• For each checkbox, an event handler (an object of type addItemListener) should be added to it.

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Choice and Event Handler

• Choice is a drop-down menu of choices, created by c = Choice().

• A choice item is added in by: c.addItem(“item”); // c is a choice

• An “item” event is generated when an item of the Choice is selected and you can get the item by:String e.getItem(); // e is the event

• Choice event handler is the same as CheckBox, which implements interface ItemListener. ItemListener has a method itemStateChanged (ItemEvent e).

• An event handler object must be added to a Choice (the entire drop-down menu) by:addItemListener()

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Mouse Event and Handler

• Two interfaces, MouseListener and MouseMotionListener, are for mouse events. Two adapters MouseAdapter and MouseMotionAdapter are provided for the interfaces. You may either implement the interfaces or extend the adapters.

• Mouse events include click, press, release, move, drag (press-move), etc.

• You need to implement all the methods in interfaces or redefine (override) the necessary methods in adapters.

• Add an Adapter object to the window in which you want to catch mouse events.

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Event Listener Interface and Adapter

• For easy programming, some listener interfaces have Adapter classes, which have default implementations of all methods in the respective interfaces.

• If the Adapter of a listener interface is provided, you don’t have to implement the whole interface. You only need to override some methods necessary to perform your work.

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Example of Mouse Event Handler

//file:mouseDrawAline.javaimport java.awt.event.*;public class mouseDrawAline extends JApplet { Point start = new Point(0,0), end = new Point(10,10); public void init() { addMouseListener(new

myMouseAdapter()); addMouseMotionListener(

new myMouseMotionAdapter()); } public void paint(Graphics g) {

super.paint(g); // clear existing paintings g.drawLine(start.x, start.y, end.x, end.y); }

class myMouseAdapter extends MouseAdapter { public void mousePressed(MouseEvent e) {

start.x = e.getX(); start.y = e.getY(); }

public void mouseReleased(MouseEvent e) { end.x = e.getX(); end.y = e.getY(); repaint(); } }

class myMouseMotionAdapter extendsMouseMotionAdapter {

public void mouseDragged(MouseEvent e) { end.x = e.getX(); end.y = e.getY();

repaint(); }}

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Keyboard Event

• The interface for keyboard event handler is KeyListener.

• KeyListener has an adapter KeyAdapter. A keyevent handler class can either implement KeyListener interface, or extend KeyAdapter class.

• Three methods defined in KeyListener:– keyPressed (keyEvent), called when any key is pressed

– keyTyped (keyEvent), called only when a non-function key is pressed

– keyReleased (keyEvent), called when a key is released after keyPressed or keyTyped event.

• Three methods on keyEvent allows you to get keycode:– event.getKeyCode()

– event.getKeyChar(), get printable letter of the key

– event.getKeyText(keyCode), get string name of the key

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Keyboard Event

public class keyTest extends JApplet { public void init() { setLayout (new BorderLayout ());

JTextArea textArea = new JTextArea( 30, 30 );add("Center", textArea);textArea.addKeyListener(new myKeyAdapter());

}

class myKeyAdapter extends KeyAdapter { public void keyPressed( KeyEvent event ) {

int keycode = event.getKeyCode();String line = "Key pressed code: "+keycode;line = line+ " & Text: "+ event.getKeyText(keycode);textArea.setText(line+"\n");

}

public void keyTyped( KeyEvent event ){textArea.append("Key typed: "+ event.getKeyChar()+"\n");

} public void keyReleased(KeyEvent event ) { textArea.append("Key released: "+

event.getKeyChar()); }

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Event Processing Model in Java: Event Types and Adapters

Java.lang.object

Java.awt.event.ComponenetAdapter

Java.awt.event.KeyAdapter

Java.awt.event.MouseAdapter

Java.awt.event.MouseMotionAdapter

Java.awt.event.WindowAdapter

Java.awt.event.ContainerAdapter

……………..

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Event Dispatching

• Each component in a container is an object that has two methods, dispathEvent() and processEvent().

• When an event is generated by an external device (mouse, kbd, etc), the out-most container’s dispatchEvent() is invoked, which dispatches the event recursively layer by layer to the right component (i.e., event source).

• When a component finds itself is the source of the event, it calls processEvent(e) to process the event.

// dispatch an event to the right componentvoid dispatchEvent(AWTEvent e) { ……

if (newEventsOnly) {if (eventEnabled(e)) { processEvent(e);}

…….}

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Process Event

Method processEvent() of the component is the root of all of the event-type processing functions.

protected void processEvent(AWTEvent e) {if (e instanceof FocusEvent) {

processFocusEvent((FocusEvent) e);} elseif (e instanceof MouseEvent) {

switch(e.getId()) {case

MouseEvent.MOUSE_PRESSED:case

MouseEvent.MOUSE_RELEASED:case

MouseEvent.MOUSE_CLICKED:case

MouseEvent.MOUSE_ENTERED:

case MouseEvent.MOUSE_EIXTED:

processMouseEvent((MouseEvent)e);break;

case MouseEvent.MOUSE_MOVED:case MouseEvent.MOUSE_DRAGGED:

processMouseMotionEvent((MouseEvent)e);

break; }} else if (e instanceof KeyEvent) {

processKeyEvent((KeyEvent)e);} elseif (e instanceof ComponentEvent) {

processComponentEvent((CompEvent) e); }

}

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MouseMotion Event Handler (example)

• Method addMouseMotionListener(mymouseMotionAdapter) does:mouseMotionListener mymouseMotionAdapter

• Methods .mouseMoved or .mouseDragged are implemented by you.

protected void processMouseMotionEvent(MouseEvent e) {if (mouseMotionListener != null) { int id = e.getId(); switch(id) {

case Mouseevent.MOUSE_MOVED: mouseMotionListener.mouseMoved(e); break;case Mouseevent.MOUSE_DRAGGED: mouseMotionListener.mouseDragged(e); break;}

}}

processEvent

processMouseMotionEvent

userHandlerObj.mouseMoved

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action Event Processing

• Button’s event handler overrides processEvent(). For action event, the code in the right-hand box is executed.

• addActionListener(myactionAdapter) does:

actionListener myactionAdapter

protected void processEvent(AWTEvent e) {if (e instanceof ActionEvent) { processActionEvent((Actionevent) e); return;}super.processEvent(e);

}

processActionEvent(ActionEvent e) {if (actionListener != null)

actionListener.actionPerformed(e); }

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Window Layout

• The default layout manager of content-panes is BorderLayout(); the default layout for JPanel is FlowLayout();

• The default layout manager can be disabled by setLayout().

• The common layouts are FlowLayout, BorderLayout, and GridLayout.

• Use JPanel to effectively organize components in different layouts and then arrange the JPanels in top-layer Layouts.

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Flow Layout

• FlowLayout(align, hgap, vgap);

• setLayout(new FlowLayout(FlowLayout.CENTER));

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Border Layout

• BorderLayout()

• To add components into a container with BorderLayout, the add method needs to specify “North”, “South”, “East”, “West”, or “Center”.

• The component at the Center automatically fills the whole unoccupied rectangle area from the center.

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Grid Layout (Cont.)

• GridLayout(rows, cols, hgap, vgap)• When adding components into a frame with GridLayout, it fills the

row from left to the right before moving to the next row.

• Example of displaying a calculator pad:public class calculator extends JApplet {public void init() {

setLayout(new GridLayout(3,4,5,5));add(new Button(“0”));

……add(new Button(“9”));

}

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Use Panel to Organize Components

~jia/www/java/gui/layoutTest.java