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I/O Streams and Files

// Example for File I/O

import java.lang.System;

import java.io.FileInputStream;

import java.io.FileOutputStream;

import java.io.File;

import java.io.IOException;

public class FileIOApp {

public static void main ( String [] arg0 ) throws IOException {

FileOutputStream outStream =

new FileOutputStream ("test.txt");

String s = "This is a test.";

for ( int i =0; i

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then uses the method delete( ) to delete the file.

The program's output is given by:

The inStream has 15 bytes available.

15 bytes were read.

They are: This is a test.

Chapter 11 Exception Handling

An exception is an event that occurs during the execution of a

program and makes program continuation impossible or undesirable.

Examples of exception are: integer division by zero, array reference

with index out of bounds, loading a file that cannot be found on disk,

printer out of paper in an attempt to print, and so on.

If any of the code in a method throws an exception of a type other

than the ones named in the catch clauses, Java exits this method

immediately. (Hopefully, one of its callers has already coded a catch

clause for that type). Any exception that occurred during the executionof doFileProcessing() or displayResults() would be caught and

processed. If an exception occurred during doFileProcessing(),

displayResults() would never get called, and execution would proceed

directly to the catch block.

The finallystatement can be used in conjunction with a tryblock.

Basically, it ensures that if

an exception occurs, any cleanup work that is necessary is taken care of.

code in a finallystatement is guaranteed to run no matter whether an exception occurs o

not.

Example:

Graphics g = image.getGraphics();

try

{

// section 1

code that might throw exceptions// section 2

}

catch ( IOException e )

{

// section 3

show error dialog

// section 4

}

finally

{

// section 5

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g.dispose();

}

// section 6

Scenarios:

1. The code throws no exceptions(Execution path: section 1, 2, 5, 6 )

2. The code throws an exception that is caught in a catch clause, in our

case, an IOException. In this scenario, execution passes through points

1, 3, 4, 5, and 6. If the catch clause throws an exception, then the

exception is thrown back to he caller of this method, and execution

passes through points 1, 3, and 5 only (no 6).

3. The code throws an exception that is not caught in any catch clause.

(Execution path: 1 and 5 only, no 6)

Exception (all checked, except for RuntimeException)

RuntimeException (unchecked) Other subclasses of

Exception, all checked.

ArithmeticException IOException (checked)IndexOutOfBoundsException

FileNotFoundException

ArrayIndexOutOfBoundsException

MalformedURLException

IllegalArgumentException InterruptedException

(checked)

NumberFormatException

The argument to throw can be any expression that returns an

object of a subclass of Throwable. You usually use new and a

constructorto create the object. To throw an exception, we find an

appropriate exception class in the class library if there is one.

Otherwise, we define our own class by extending the Throwable class

or one of its subclasses. We then make an object of that class and

throw it.

Users of a method need to know the exceptions it might throw sothey can be prepared to handle them.Java requires that a method

definition includes a list of the checked exceptions the method throws

in the header.

import java.lang.*;

import java.io.*;

public class example2

{ public static void main(String[] args) throws IOException

{

System.out.println("Within main\n");

test1();

}

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public static void test1() throws IOException

{

try {

System.out.println("Within test1\n");

test2();

}

catch( RuntimeException e ) {

System.out.println("Caught RuntimeException\n");

}

}

public static void test2() throws IOException

// it can throw spuerclass for checking but in test1 and main need to

check spuerclass {

try {

System.out.println("Within test2\n");

throw new IOException();}

catch ( NumberFormatException e ){

System.out.println("Caught NumberFormatException\n");

}

}

}

Case 1: A checked exception

public void openFile()try {

infile = new BufferedReader ( new

FileReader ("myFile.txt") );

}

catch ( FileNotFoundException e) {

errorField.setText ("Missing file, please try again!");

}

// The FileReader constructor throws a checked exception which wemust deal with. We can either use try-catch, or specify that the

method which contains the above statement can throw the exception.

The second approach is to pass the exception up to the invoking

method as follows:

public void openFile() throws IOException {

infile = new BufferedReader ( new

FileReader ( "myFile.txt") );

}

Case 2: An unchecked exception

Because NumberFormatException (a subclass of RuntimeException)

is unchecked, the compiler will not force us to check it. We can catch it

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or ignore it (but there is no advantage to pass it upward).

Case 3: A RuntimeException

If we want the origin of the exception to be indicated so that it can

be tracked down and fixed, the easy way is to ignore the exception.

(It's ok, because the compiler does not check for this kind of

exception).

Programming pitfalls: Failing to acknowledge a checked exception will produce a

compilation error. You must either catch an exception locally, or

specify that your method throws it. Do not micromanage exceptions, i.e., do not put each line or small

section of code in a separate try-catch block. Do not squelch exceptions, i.e., if something goes wrong, do not

shut up that error by doing

try

{ doing something wrong }catch ( Throwable e ) {}

Allowing an exception to be thrown from a method, when it can

sensibly be handled locally, dramatically slows down your code.

Chapter 9 User Interface Components with Swing

The flow layoutmanageris the default layout manager for apanel.

Flow layoutmanager: lines the components horizontally until there

is no more room and then starts a new row of components.e.g.,

setLayout (new FlowLayout(FlowLayout.LEFT));

// align to the left; default = center alignment

(The method setLayoutis defined injava.awt.Container.)

Border Layout(p.371) - using the borderlayoutmanager, you can

choose to place the component in the center, north, south, east, or

west of the container. It is the default layout manager of the content

pane of everyJFrame.e.g.,

//To change the layout manager of a

// JPanel object.

class MyPanel extends JPanel {

setLayout ( new BorderLayout() );

. . .

add ( yellowbutton, BorderLayout.SOUTH);

}

The Grid Layout Manager. All rows and columns have the same size.

public GridLayout(int rows, int cols)

public GridLayout(int rows, int cols, int hgap, int vgap) // hgap, vgap:

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horizontal and vertical gap

panel.setLayout( new GridLayout(5,4) );

JLabel

JLabels are components that hold text. They do not react to user

input.

JLabel label = new JLabel( User name: , SwingConstants.RIGHT );

panel.add( label );

Text Input

JTextField(text fields): single line of text

JTextArea (text areas): multiple lines of text

Both inherit from the abstract classJTextComponent.

For text fields (p.378):

JPanel p = new JPanel ();

JTextField tf = new JTextField ("Default input", 20);

// Adds a text field and initializes its content;sets the width equal to 20 columns.

p.add (tf);

tf.setText ( "HKBU");

//Changes the content of the text field.

tf.setColumn (10);

// Change the size of the text field.

p.validate();

// Recompute the size and layout of all components in a container.String s = tf.getText().trim();

// Return the exact text and trim any extraneous spaces for the data in

the text field.

interface DocumentListener(3 methods)

insertUpdate(..)

character(s) inserted

removeUpdate(..)

character(s) deletedchangedUpdate(..)

changed in formatting, etc.

DocumentListener listener = new ClockFieldListener();

// add a panel with text fields

JPanel panel = new JPanel();

panel.add(new JLabel("Hours:"));

hourField = new JTextField("12", 3);

panel.add(hourField);

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hourField.getDocument().addDocumentListener(listener);

panel.add(new JLabel("Minutes:"));

minuteField = new JTextField("00", 3);

panel.add(minuteField);

minuteField.getDocument().addDocumentListener(listener);

add(panel, BorderLayout.SOUTH);

// add the clock

clock = new ClockPanel();

add(clock, BorderLayout.CENTER);

pack();

}

/**

Set the clock to the values stored in the text fields.*/

publicvoid setClock()

{

try

{

int hours = Integer.parseInt(hourField.getText().trim());

int minutes = Integer.parseInt(minuteField.getText().trim());

clock.setTime(hours, minutes);}

catch (NumberFormatException e) {}

// don't set the clock if the input can't be parsed

}

publicstaticfinalint DEFAULT_WIDTH = 300;

publicstaticfinalint DEFAULT_HEIGHT = 300;

private JTextField hourField;private JTextField minuteField;

private ClockPanel clock;

privateclass ClockFieldListener implements DocumentListener

{

publicvoid insertUpdate(DocumentEvent event) { setClock(); }

publicvoid removeUpdate(DocumentEvent event) { setClock(); }

publicvoid changedUpdate(DocumentEvent event) {}

}

}

Note: The model for all text components is described by the Document

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interface. In the statement

minuteField.getDocument().addDocumentListener(listener);

the minuteFieldobtains the data model "document" through the meth

getDocument( ).

For text areas, the user can enter any number of lines of text, using

the ENTER key to separate them.

JTextArea textArea = new JTextArea (8, 40);

// 8 lines of 40 columns

frame.add (textArea);

Internally, each line ends with '\n'.

textArea.setLineWrap (true); // long lines are wrapped

visually; no '\n' characters are inserted into the text.Automatic Scrolling

textArea = new JTextArea ( 8, 40 );

JScrollPane scrollPane = new JScrollPane( textArea );

frame.add( scrollPane, BorderLayout.CENTER );

JPasswordField

The characters that the user entered are not actually displayed, bu

represented by an echo character (typically *).

JPasswordField( String text, int columns)

// add button to append text into the text area

JButton insertButton = new JButton("Insert");

buttonPanel.add(insertButton);

insertButton.addActionListener(new

ActionListener()

{

publicvoid actionPerformed(ActionEvent event){

textArea.append("The quick brown fox jumps over the lazy

dog. ");

}

});

// add button to turn line wrapping on and off

wrapButton = new JButton("Wrap");buttonPanel.add(wrapButton);

wrapButton.addActionListener(new

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ActionListener()

{

publicvoid actionPerformed(ActionEvent event)

{

boolean wrap = !textArea.getLineWrap();

textArea.setLineWrap(wrap);

scrollPane.revalidate();

wrapButton.setText(wrap ? "No Wrap" : "Wrap");

}

});

Radio Buttons

ButtonGroup group = new ButtonGroup();

JRadioButton smallButton = newJRadioButton( Small, false );group.add( smallButton );

JRadioButton mediumButton = newJRadioButton( Medium,

false );

group.add( mediumButton );

Sliders

JSlider slider = new JSlider(min, max, initialValue); // default: 0, 100,

JSlider slider = new JSlider(SwingConstants.VERTICAL, min, max,

initialValue);ChangeEvent, addChangeListener(), stateChanged()

snap to ticks move slider to closest tick

// common listener for all sliders

listener = new ChangeListener()

{

public void stateChanged(ChangeEvent event)

{

// update text field when the slider value changesJSlider source = (JSlider) event.getSource();

textField.setText("" + source.getValue());

}

};

// add a plain slider

JSlider slider = new JSlider();

addSlider(slider, "Plain");

// add a slider with major and

minor ticks

slider = new JSlider();

slider.setPaintTicks(true);

slider.setMajorTickSpacing(20

);

slider.setMinorTickSpacing(5);

addSlider(slider, "Ticks");

// add a slider that snaps to

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ticks



slider.setSnapToTicks(true);


);


addSlider(slider, "Snap to

ticks");

// add a slider with no track




);


slider.setPaintTrack(false);addSlider(slider, "No track");

// add an inverted slider




);


slider.setInverted(true);addSlider(slider, "Inverted");

// add a slider with numeric

labels



slider.setPaintLabels(true);


);slider.setMinorTickSpacing(5);

addSlider(slider, "Labels");

// add a slider with alphabetic

labels





);


Dictionary labelTable = new

Hashtable();

labelTable.put(0, new

JLabel("A"));


JLabel("B"));

labelTable.put(40, newJLabel("C"));


JLabel("D"));


JLabel("E"));


JLabel("F"));

slider.setLabelTable(labelTable);

addSlider(slider, "Custom

labels");

// add a slider with icon labels




slider.setSnapToTicks(true);slider.setMajorTickSpacing(20

);


labelTable = new Hashtable();

// add card images

labelTable.put(0, new JLabel(new ImageIcon("nine.gif")));

labelTable.put(20, new JLabel(new ImageIcon("ten.gif")));

labelTable.put(40, new JLabel(new ImageIcon("jack.gif")));

labelTable.put(60, new JLabel(new ImageIcon("queen.gif")));

labelTable.put(80, new JLabel(new ImageIcon("king.gif")));

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labelTable.put(100, new JLabel(new ImageIcon("ace.gif")));

slider.setLabelTable(labelTable);

addSlider(slider, "Icon labels");

// add the text field that displays the slider value

textField = new JTextField();

add(sliderPanel, BorderLayout.CENTER);

add(textField, BorderLayout.SOUTH);

}

Chapter 8 Event Handling

What is an event?

An event is (in most cases) an action taken by the user. For example,

the user presses a key, clicks the mouse, etc

In Java, you completely control how an event is transmitted from the

event source (buttons, scrollbars, etc.) to the event listener(an object

that can carry out the response you want for the event). This Eventdelegationmodelgives us much more flexibility than VB (in VB, the

listener is predetermined; in Java, you can designate any object to be

an event listener).

eventSourceObject.addEventListener( eventListenerObject);

e.g.,

ActionListener listener = ;

JButton b = new JButton ("OK");

b.addActionListener(listener);// the method addActionListener is defined in class Button.

E.g.,

. . .

ActionListener listener = ;

JButton b = new JButton ("OK");

b.addActionListener(listener);

. . .

class MyListener implements ActionListener{ . . .

public void actionPerformed ( ActionEvent evt )

{ // reaction to button click goes here

. . .

}

Listener Interfaces (See p.359 for a complete list.)

Name Methods Accessors Events

generated by

ActionListener actionPerformed getSource

JButton

isSelected CheckBox

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getActionCommand MenuItem

(getSource) JComboBox

Abstractbutton

JTextField

Timer

TextListener textValueChanged getText

TextComponent

#DocumentListener getText

TextComponent

insertUpdate

removeUpdate

changedupdate

MouseListener mousePressed MouseEventComponent

mouseReleased getClickCount

mouseEntered getX, getY

mouseExited getPoint

mouseClicked isPopupTrigger

WindowListener windowClosing getWindow Window

windowOpenedwindowIconified

windowDeconified

windowClosed

windowActivated

windowDeativated

KeyListener keyPressed getKeyCode Component

keyReleased getKeyCharkeyTyped getKeyModifiersText

#ListSelectionListener

valueChanged getSource List

Adapter Classes

Implementing six methods that do nothing is a tedious work. To

simplify this work, each of the listener interfaces that has more than

one method comes with a companion adapterclass that implements

all the methods in the interface but does nothing with them.

For example, the WindowAdapter class has seven do-nothing

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methods. This means the adapter class automatically satisfies the

technical requirements of using interface.

These are adapters:

WindowAdapter, ComponentAdapter, ContainerAdapter,

FocusAdapter,

KeyAdapter, MouseAdapter, MouseMotionAdapter.

addWindowListener ( new WindowAdapter()

{ public void windowClosing (WindowEvent e) { System.exit

(0);}

}

);

Handling Keyboard Events

Pressed the SHIFT key (keyPressedfor VK_SHIFT)

Pressed the "A" key (keyPressedfor VK_A)

Typed "A" key (keyTypedfor A)Released the "A" key (keyReleasedfor VK_A)

Released the SHIFT key (keyReleasedfor VK_SHIFT)

The keyTypedevent reports the characters that were typed (A)

whereas the keyPressesdand keyReleasedevents report on the raw

keystrokes (representing by the virtual key codes).

When we override the method keyPressedand keyReleased, we use

the getKeyCode method to obtain the virtual key code:void keyPressed( keyEvent evt) {

int keyCode = evt.getKeyCode ();

}

When we override the keyTypedmethod, we use the getKeyChar

method to obtain the actual character typed:

char keyChar = evt.getKeyChar();

Mouse Events (p.349)You do not need to handle mouse events explicitly if you just want

the user to be able to click on a button or menu.

These mouse operations are handled internally by the various

components in the GUI. If you want to draw with the mouse, you

need to trap mouse move, click, and drag events.

Methods of MouseListener interface:

mousePressed, mouseReleased, mouseClicked,

mouseEntered, mouseExitedMethods of MouseMotionListener interface:

mouseDragged, mouseMoved

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Method of MouseWheelListener interface:

mouseWheelMoved

A useful method: MouseEvent.getClickCount()

To detect if the right (nonprimary) mouse button is down:

if ((event.getModifiersEx() & InputEvent.BUTTON3_DOWN_MASK)

!= 0 ) {}

// Note that MouseEvent extends InputEvent.

// the square containing the mouse cursor

private class MouseHandler extends MouseAdapter

{

public void mousePressed(MouseEvent event)

{

// add a new square if the cursor isn't inside a square

current = find(event.getPoint());

if (current == null) add(event.getPoint());

}public void mouseClicked(MouseEvent event)

{

// remove the current square if double clicked

current = find(event.getPoint());

if (current != null && event.getClickCount() >= 2)

remove(current);

}

}private class MouseMotionHandler implements MouseMotionListener

{

public void mouseMoved(MouseEvent event)

{

// set the mouse cursor to cross hairs if it is inside

// a rectangle

if (find(event.getPoint()) == null)setCursor(Cursor.getDefaultCursor());

else

setCursor(Cursor.getPredefinedCursor(Cursor.CROSSHAIR_CURSOR));

}

public void mouseDragged(MouseEvent event)

{

if (current != null)

{

int x = event.getX();

int y = event.getY();

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// drag the current rectangle to center it at (x, y)

current.setFrame(x - SIDELENGTH / 2, y - SIDELENGTH / 2,

SIDELENGTH, SIDELENGTH);

repaint();

}

}

}

Actions

to separate the responsibilities of getting user input and executing

commands;

multiple ways to activate the same command: menu, keystroke,

button, etc.

one class object for each command;

one listener for multiple event sources

MulicastingOne event source, many listeners

Simply add multiple listeners to an event source.

There is no guarantee about the order in which the events are

delivered to the listeners.

Chapter 7 Graphics Programming

Java Graphics libraries

AWT (Abstract Window Toolkit) - for basic GUI (Graphical User Interfaceprogramming

Swing - additional set of user interface components provided by Java 1.

Frame - a top-level window (a window that is not contained inside

another window) with a title and a border is called a frame. Frame is a

subclass of class Container.

import javax.swing.*;

publicclass SimpleFrameTest

{publicstaticvoid main(String[]

args)

{

SimpleFrame frame = new

SimpleFrame();

frame.setDefaultCloseOperation(

JFrame.EXIT_ON_CLOSE);

frame.setVisible(true);

}

}

class SimpleFrame extends

JFrame

{

public SimpleFrame(){

setSize(DEFAULT_WIDTH,

DEFAULT_HEIGHT);

}

publicstaticfinalint

DEFAULT_WIDTH = 300;

publicstaticfinalint

DEFAULT_HEIGHT = 200;

}

// get screen dimensions

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Toolkit kit =

Toolkit.getDefaultToolkit();

Dimension screenSize =

kit.getScreenSize();

int screenHeight =

screenSize.height;

int screenWidth = screenSize.wi

2D Shapes

The Java 2D library organizes geometric shapes in an object-oriented

fashion. There are

abstract classes Line2D, Rectangle2D,

Ellipse2D, and etc. Each class contains two static inner classes Float

Double, e.g.,

Rectangle2D.Float floatRect = new Rectangle2D.Float( 10.0F, 25.0F,

22.5F, 20.0F );

// left x, top y, width, height; float values

Rectangle2D.Double doubleRect = new Rectangle2D.Double( 10.0, 2

22.5, 20.0 );// double values

//example

Rectangle2D rect = new Rectangle2D.Double(leftX, topY, width,

height);

g2.draw(rect);

// draw the enclosed ellipse

Ellipse2D ellipse = new Ellipse2D.Double();

ellipse.setFrame(rect);g2.draw(ellipse);

// draw a diagonal line

g2.draw(new Line2D.Double(leftX, topY, leftX + width, topY +

height));

// draw a circle with the same center

double centerX = rect.getCenterX();

double centerY = rect.getCenterY();double radius = 150;

Ellipse2D circle = new Ellipse2D.Double();

circle.setFrameFromCenter(centerX, centerY, centerX + radius,

centerY + radius);

g2.draw(circle);

Colors

g2.setPaint( Color.red ); // predefined red color

g2.drawString( Warning!, 100, 100 );

Predefined colors: red, blue, green, black, magenta, yellow, etc. (RED,

BLUE, etc. are also ok.)

Custom colors specification:

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cannot have any static method.

Any class can promise to implement the interface using the keyword

implements. Any class that implements an interface must

implement all its methods.

Differences between using interfaces and abstract classes:

If multiple inheritance is important, use interfaces, otherwise use

abstract classes. This is because an abstract class can provide someor all of the methods that can be inherited easily.

Interfaces never have instance fields, and the methods are never

implemented in the

interfaces.

Arrays.sort(staff); //Library method calls compareTo().

class Employee implements Comparable

{

public Employee(String n, double s){ }

publicint compareTo(Employee other)

{

if(salary < other.salary) return -1; //to compare two string return

name.conmpareTo(other.name);

if(salary > other.salary) return 1; //to compare two hireday

return hireday.compareTo(other.hireday);

return 0;}

}

x = new Comparable( ); // error

Comparable y; // ok

y = new Employee( ); // ok if Employee implements Comparable

if ( y instanceof Comparable ) { } // ok

You can extend interfaces. You can also put constants in interfaces. The

fields are always

public static final.

Object Cloning

Employee original = new Employee(John Public, 50000);

Employee copy = original; // copying

copy.raiseSalary(10); // oops also changed original

Using the clone() method in the Object class copies only the first level of

fields. If there are immutable subobjects in the

data fields (such as a String object), this library clone() method works fiEmployee copy = original.clone();

copy.raiseSalary(10); // OK original unchanged.

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Quite frequently, subobjects are mutable, and one must override the clo

method to make

a deep copy that clones the subobjects as well. The relevant class mu

also

implement the Cloneable interface.

class Employee implementsCloneable {

public Employee(String n, double s) { ... }

public Employee clone() { ... }

}

public Employee clone() throws CloneNotSupportedException

{

// call Object.clone()

Employee cloned = (Employee) super.clone();

// clone mutable fields

cloned.hireDay = (Date) hireDay.clone();return cloned;

}

Inner classes

Inner classes are classes defined within another class.

Only inner classes can be private. Regular classes always have either package or

public visibility.

Advantages: ability to access instance fields of the outer classes; inner classes can beprivate to ensure encapsulation

publicvoid start()

{

// Creating an inner class object.

ActionListener listener = new TimePrinter();

Timer t = new Timer(interval, listener);

t.start();

}Local inner classes

Define a class within a method. The class is usable only in that method (always private to

method). No access modifier is needed.

For example, in Example 6-4, the inner class TimePrinter is used only once.

public void start()

{

class TimePrinter implements ActionListener

{

public void actionPerformed(ActionEvent event)

{

Date now = new Date();

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methods of the old class (superclass). However, objects of the superclass cannot use fields newly

defined in the subclass.

A subclass object cannot access the private fields of the superclass object.

Polymorphism

Another way of formulating the is a rule is the substitution principle. You can use a

subclass object whenever the program expects a superclass object.

Employee e;

e = new Employee( ); // Employee object expected

e = new Manager( ); //OK, a Manager object can be used as well.

Object variables in Java are polymorphic, i.e., an object variable of a class can refer to an

object of this particular class or any of the subclasses.

Another form of polymorphism is discussed in the section about overriding (see below),

where one method name is being overloaded or overridden.

We can assign a subclass object to a superclass variable.

Manager boss = new Manager ( John, 70000, 1990, 1, 2 );

Employee em1 = new Employee ( Lee, 50000, 1992, 3, 1 );

Employee em2 = new Employee ( Wong, 55000, 1994, 9, 4 );

Employee em3 = new Employee (Chan, 60000, 1996, 3, 8 );

Employee EM = boss; // ok!

/* In this case, EM and boss refer to the same area of memory. The actual type of

EM is Manager, but the variable type of EM is Employee. */Employee[] staff = new Employee[4];

staff [0] = boss;

/*Ok, although the type ofstaff[0] isEmployee and the type ofboss is Manager. */

staff [1] = em1;

staff [2] = em2;

staff [3] = em3;

However, the converse (assigning a superclass object to a subclass variable) is false in general.

superclass object cannot be used as a subclass object.Overloading and overriding

The scenario that methods in different classes of an inheritance hierarchy have the same

signature (method name and parameter list) and same return type but different contents is

called overriding that method name (polymorphism). Notice that the return type is not

part of the signature.

Overloading a class provides more than one method with the same name, but with

different signatures.

Overriding a class hierarchy replaces the superclass's implementation of a method inthe subclass. Their signatures and return types must be identical. Note that only

non-static methods may be overridden. For example,

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// in class Employee

public void raiseSalary( double byPercent ) {

salary *= ( 1. + byPercent / 100. );

}

// in class Manager

public void raiseSalary( double byPercent ) {

Date today = new Date();

//addition 0.5% for each year of service

double bonus_percent =

0.5* ( today.getYear() getHireDay().getYear() );

super.raisesalary( byPercent + bonus_percent );

}

When you override a method, the subclass method must be at least as visible as the

superclass method. For example,

If the superclass method is public, the subclass method must also be public.

If the superclass method is private, the subclass method can be public. Also, in this case, if the method is named, say,func(), the subclass method cannot call

super.func().

When an object of a subclass calls a method name (this method may or may not have been

overridden in the inheritance hierarchy), which one will actually be invoked? The rules are:

(1) The subclass checks whether or not it has a method with that name and with exactly

the same parameters. If so, use it. If not,

(2) Java moves to the parent class and looks there for a method with that name and thoseparameters. If so, it calls that method. If not,

(3) Java continues to move up the inheritance chain. The parent classes are checked

until the chain of inheritance stops (an error) or until Java finds a matching method

This is one of the fundamental inheritance rules: a method redefined in a subclass hides orove

the method of the ancestor class.

Dynamic binding

Dynamic binding occurs when a superclass and a subclass both have a method of the same sig

and return type.

Ifan object of a subclass is assigned to a superclass variable, and the variable calls an overrid

method, which one of the overridden methods in the inheritance hierarchy will be invoked?

The answer is - according to the actual type of the object itself, not the type of the variable.

Consider the code from Example 5-1:

for (int i = 0; i < staff.length; i++) {

Employee e = staff[i];

// staff[0] is of type boss; staff[1] and staff[2] are of type EmployeeSystem.out.println( "name=" +

e.getName() + //static binding

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", salary = " + e.getSalary()); // dynamic binding

}

The Java compiler produces the method tables for the Employee and Manager classes (in Exa

5-1):

Employee:

getName() Employee.getName()

getSalary() Employee.getSalary()

getHireDay() Employee.getHireDay()

raiseSalary( double ) Employee.raiseSalary( double )

Manager:

getName() Employee.getName()

getSalary()Manager.getSalary()

getHireDay() Employee.getHireDay()

raiseSalary( double ) Employee.raiseSalary( double )

setBonus( double ) Manager.setBonus( double )

Constructors in inheritance hierarchy

Every constructor of a subclass must somehow call a constructor of the superclass (to set up

the data fields of the superclass).

(1) In a constructor of a subclass, we can explicitly usesuperwith the appropriate

(2) parameters to invoke one of the superclass's constructors. This must be done in the

first statement of the subclasss constructor.

(2) If the subclass constructor does not call a superclass constructor explicitly, the default

constructor of the superclass will be called automatically by the compiler.

By convention, if a class does not have any constructor, the Java compiler will

automatically generate a default constructor. However, if the class has one or more cons

that requires parameters (but no default constructor), the compiler will not

generate a default constructor. Thus

(i) if the superclass has no default constructor but does possess other constructors, w

explicitly use one of these constructors viasuper. Otherwise the compiler will rep

error;

(ii) if the superclass has no constructor at all and the subclass constructor does not usthe default constructor provided by the compiler will be called.

Existence of superclasss constructors: When a constructor of asubclass does not invokethe superclasss constructorexplicitly, the Javacompiler

existence of

default (no-argument)constructor

existence of

constructorsrequiringparameters

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Case 1:No Nouses the defaultconstructor provided bythe compiler

Case 2:NoYes reports an error

Case 3: Yes No uses the defaultconstructor

Case 4: Yes Yes uses the defaultconstructor

Abstract Classes and Abstract Methods

When one moves up an inheritance hierarchy, the ancestor class may become sogeneralthat

we think of it more as a framework for other classes than as a class that we want to use. We

may use the keyword abstract to declare such general classes.Abstract classes cannot be instantiated, meaning that if a class is declared as abstract, no

instance of that class can be created.

Abstract Method - in an abstract class, you can define (deferring implementation) only the

signatures of some of its methods, leaving extended classes to provide specific implemen

tation of these methods.

Each method that has not been implemented in the class is specifically marked abstract.

For added clarity, a class with one or more abstract methods must itself be declared abstract. F

example,

abstract class Person {

public Person(String n){

name = n;

}

public abstract String getDescription();

public String getName() {return name;

}

private String name;

}

Abstract classes can have concrete data and methods. Also, a class can even be declared

as abstract even though it has no abstract methods.

All non-abstract subclasses of the abstract class must implement all the abstract methods of

the abstract class (overriding). So abstract methods act as placeholder methods that are

implemented in the subclasses.

final classes and methods

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upstatic fields or other necessary states.

public static double [] buffer = new double[BUFFER_SIZE];

static{

int i;

for ( i = 0 ; i < buffer.length ; i++ )

buffer[i] = java.lang.Math.random( );

}

Thus a static method cannot use the implicit parameterthis.

Finalizers

Every class in Java can have a finalizer method that releases resources back to the operating

system. This method is called before the garbage collector sweeps away the object. A

classs finalizer method always has the namefinalize, receiving no parameters and returning n

value. (I.e., its return type is void.)

Chapter 4 (Part 1)Fundamental Concepts ofObject-oriented Approach

Quality Factors:

Correctness --- Correctness is the ability of software products to exactly perform their tasks,

as defined by the requirements and specification.

Robustness --- Robustness is the ability of software systems to function even in abnormal

conditions.

Extendibility --- Extendibility is the ease with which software products may be adapted to

changes of specification.Reusability --- Reusability is the ability of software products to be reused, in whole or in part,

for new applications.

O-O Analysis

Tradition structured analysis focus upon the flow of data within a system; O-O analysis

emphasizes on the building of real-world models.

What is an object?

An object is an entity that contains both the attributes that describe the state of a real-world

object and the actions that are associated with the real-world object.With an O-O language, an object would capture both the data (attributes) and methods

( actions) associated with the object.

What is a class?

Object-Oriented programs usually operate with hundreds or thousands of objects. Many object

common properties ( same structure, same interface, same behavior ) --- they are

the same kind.

We define a class of objects to be one that has the same properties to describe objects in

this class, which messages they should accept, which methods they should execute in

response to these messages, and the attributes of them.

Polymorphism

Polymorphism represents a concept in type theory in which a single name

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(e.g., the name of a method) may denote things of many different classes that are

related by some common superclass.

The identically named method may have different implementations in the subclasses

Chapter 3: Basics of Java

Data Type

All data types are classes, except several "primitive" types built-in to support integer, floating-

point, boolean, and character values.

Classes

Classes are user-defined data types. (There are lots of libraries classes.) We will study how to

write user-defined classes in chapter 4.

Constants

Java uses the keywordfinalto denote a constant (In C, use the keyword const.) The keyword

finalindicates that you can assign to the variable only once.

Enumerated Types

enum Size {SMALL, MEDIUM, LARGE, EXTRA_LARGE}

Size pizza = Size.MEDIUM;

Further discussion in Chapter 5.

Strings (java.lang.String p.53)

The standard Java library contains a predefined class String to deal with sequences of character da

provide shorthand to declare and initialize them.

labeled break (breaking out of one or nested loops)

int n;read_data:

while ( ) { // This loop statement is tagged with the label.

for ( ) { // This inner loop is not labeled.

System.out.print(Enter a number >= 0: );

n = in.nextInt( );

if ( n < 0 ) // should never happen cant go on

break read_data;// break out of read_data loop

}

}

// This statement is executed immediately after the break.

if ( n < 0 ) { // check for bad situation

// deal with bad situation

}

else {

// carry out normal processing

}

// Second example

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label:

{

if ( condition ) breaklabel; //exits block

}

// jumps here when the break statement executes

MP1

import java.io.*;

import java.util.*;

public class MP1 {

public static void main(String[] args)throws IOException{

int i = 0, j = 0;

int ii, jj;int maxpara = 1000;

int maxline = 1000;

String [][] contents = new String[maxpara]

[maxline];

if (args[0].substring(0,1).equals("-")){

if (args[0].substring(0,2).equals("-s")){

Scanner in = new Scanner(new File(args[1]));

while(in.hasNextLine()){String temp =

in.nextLine();

if(temp.equals("")){

i++;

j=0;

}else{

contents[i][j] =

temp;j++;

}

}

itemS(args[0], i, j, maxline,maxpara, contents);

in.close();

else{

Scanner in = new Scanner(new File(args[0]));

PrintWriter out = new PrintWriter(new

File(args[1]));

while(in.hasNextLine()){

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String temp = in.nextLine();

if(temp.equals("")){

i++;

j=0;

}else{

contents[i][j] = temp;

j++;

}

}

for(ii = 0;ii

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this.name = n;

this.acct_no = an;

this.customer_since = cs;

this.mileage_list = ml;

}

public Blue_Card(){}

public Blue_Card claim_ticket(int km){

Collections.sort(mileage_list);

return this;

}

class Trip implements Comparable{

public int compareTo(Trip other){

return

date_granted.compareTo(other.date_granted);

}

}public class MP2 {

public static void main(String[] args){

Scanner in = new Scanner(System.in);

ArrayList Ada_list = new ArrayList();

Ada_list.add(new Trip(20000, new Date(2007-

1900,1-1,5)));

Ada_list.add(new Trip(15000, new Date(2008-

1900,8-1,8)));System.out.print("Please enter mileage earned:");

int me = in.nextInt();

}

}

if(option == 6){

System.exit( 0 );

}

MP3import java.awt.*;

import java.awt.geom.*;


import java.awt.event.*;

import java.util.*;

import java.awt.event.*;


public class MP3 {

public static void main(String[] args){

KFrame frame = new KFrame();

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frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

frame.setVisible(true);

}

}

class KFrame extends JFrame{

public KFrame(){

setTitle("MP3");

setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);

KPanel panel = new KPanel();

add(panel);

}

public static final int DEFAULT_WIDTH = 315;

public static final int DEFAULT_HEIGHT = 635;

}

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

int x =e.getX();

int y = e.getY();

}

}

class MyKeyAdapter extends KeyAdapter

{

public void keyPressed(KeyEvent event){ int code = event.getKeyCode();

char code2 = event.getKeyChar();

}

}

MP4

public KFrame(){

setTitle("MP4");

main_panel = new JPanel();main_panel.setLayout( new BorderLayout() );

left_panel = new FillPanel();

left_panel.setPreferredSize(new Dimension(400,

700));

main_panel.add(left_panel,BorderLayout.WEST);

right_panel = new RPanel();

right_panel.setLayout( new BorderLayout() );

right_panel.setPreferredSize(new Dimension(300,

700));

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right_top = new RTPanel();

right_top.setPreferredSize(new Dimension(300,

250));

right_panel.add(right_top, BorderLayout.NORTH);

right_middle = new RMPanel();

right_middle.setLayout(new BorderLayout());

right_middle.setPreferredSize(new Dimension(300,

250));

JPanel right_middle_bottom = new JPanel();

right_middle_bottom.setPreferredSize(new

Dimension(300, 150));

right_middle_bottom.setLayout(new

GridLayout(8,1));

top_scores = new JLabel(" Top Scores");

name = new JLabel(" Name Scores");

first = new JLabel("1.");second = new JLabel("2.");

third = new JLabel("3.");

fourth = new JLabel("4.");

fifth = new JLabel("5.");

JTextField tf = new JTextField (" ", 10);

right_middle_bottom.add(top_scores);

right_middle_bottom.add(name);

right_middle_bottom.add(first);right_middle_bottom.add(second);

right_middle_bottom.add(third);

right_middle_bottom.add(fourth);

right_middle_bottom.add(fifth);

right_middle_bottom.add(tf);

right_middle.add(right_middle_bottom,

BorderLayout.SOUTH);

right_panel.add(right_middle,BorderLayout.CENTER);

right_bottom = new RBPanel();

right_bottom.setPreferredSize(new Dimension(300,

200));

slider_panel = new SPanel();

slider_panel.setLayout( new BorderLayout() );

slider = new JSlider(1,100,50);


slider.setMajorTickSpacing(20);


slider.addChangeListener(new ChangeListener()

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}

}

);

top_left_down.add(square);

top_left_down.add(triangle);

top_left_down.add(circle);

top_left.setLayout(new GridLayout(2,1));

top_left.add(label);

top_left.add(top_left_down);

ButtonGroup group = new

ButtonGroup();

JRadioButton fill = new

JRadioButton("Filled figue", true);

JRadioButton outline = newJRadioButton("Outline only",

false);

fill.addActionListener(new

ActionListener()

{

public void

actionPerformed(ActionEvent

event){fill_out = true;

repaint();

}

}

);

top_right.setLayout(new

GridLayout(2,1));

group.add(fill);group.add(outline);

top_right.add(fill);

top_right.add(outline);

top_panel.add(top_left,BorderLay

out.CENTER);

top_panel.add(top_right,

BorderLayout.EAST);

bottom_panel.setLayout(new

BorderLayout());

bottom_left = new JPanel();

bottom_right = new JPanel();

JLabel label5 = new

JLabel("Status");

bottom_left.setLayout(new

GridLayout(2,1));

JLabel label1 = new JLabel("Sizeof square, triangle, or circles:");

JSlider slider = new

JSlider(5,300,50);


slider.setMajorTickSpacing(20);


bottom_left.add(label1);

bottom_left.add(slider);bottom_right.setLayout(new

GridLayout(2,3));

JLabel label2 = new JLabel("Red

intensity:");

JLabel label3 = new JLabel("Green

intensity:");

JLabel label4 = new JLabel("Blue

intensity:");JTextField tf = new JTextField("",

5);

JTextField tf1 = new JTextField("",

5);

JTextField tf2 = new JTextField("",

5);

bottom_right.add(label2);



bottom_right.add(tf);

bottom_right.add(tf1);

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bottom_right.add(tf2);

bottom_panel.add(bottom_left,Bo

rderLayout.WEST);

bottom_panel.add(bottom_right,B

orderLayout.CENTER);

bottom_panel.add(label5,BorderL

ayout.SOUTH);

add(top_panel,BorderLayout.NOR

TH);

add(middle_panel,BorderLayout.C

ENTER);

add(bottom_panel,BorderLayout.S

OUTH);

public void

paintComponent(Graphics g){

super.paintComponent(g);Graphics2D g2 = (Graphics2D) g;

int w = getWidth(), h =

getHeight();

int margin = h/2;

int w2 = w / 2;

if(figure == 2){

Polygon poly;

g2.setPaint( Color.green );

poly = new Polygon();

poly.addPoint( w2 - s, h -

margin );

poly.addPoint( w2 + s, h -

margin );

poly.addPoint( w2, h - margin -

s );

active.add(poly);

if(fill_out){

g2.fill( poly );

}else{

g2.draw( poly );}

//poly.reset();

}

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