Java Programming

2
Java Programming
Simon Ritter
simon.ritter@uk.sun.com

3
Agenda
u Java Language Syntax
u Objects The Java Way
u Basics Of AWT
u Java Hints & Tips
u Demonstration
u Where Next?

5
Comments
u C Style
/* Place a comment here */
u C++ Style
// Place another comment here

6
Auto-Documenting Comments
u javadoc utility part of JDK
/** Documentation comment */
– @see [class]
– @see [class]#[method]
– @version [text]
– @author [name]
– @param [variable_name] [description]
– @return [description]
– @throws [class_name] [description]

7
Java Data Types
u Primitive Types
– byte 8 bit signed
– short 16 bit signed
– int 32 bit signed
– long 64 bit signed
– float 32 bit IEEE 754
– double 64 bit IEEE 754
– char 16 bit Unicode
– boolean true or false
u No Unsigned Numeric Types

8
Java Data Types
u Reference Types
– class
– array
– interface (cannot be instantiated)

9
Type Promotion/Coercion
u Types Automatically Promoted
– int + float Automatically Returns float
u No Automatic Coercion
float pi = 3.142;
int whole_pi = pi; /* Compile Time Error */
int whole_pi = (int)pi; /* Success! */

10
Variables
u Cannot Use Reserved Words For Variable Names
u Variable Names Must:
– Start With A Letter (‘A’-’Z’, ‘a’-’z’,’_’,’$’)
(Can Use Unicode)
– Be A Sequence Of Letters & Digits
u Can Have Multiple Declarations
– int I, j, k;
u Constants Not Well Supported (Class Only)

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Variable Assignment
u Assignments Can Be Made With Declaration
int I = 10;
u char uses single quote, strings use double
– Unicode can be specified with 4 byte hex number
and u escape
char CapitalA = `u0041`;

12
Variable Modifiers
u static
– Only One Occurrence For The Class
– Can Be accessed Without Instantiating Class
(Only If Class Variable)
u transient
– Do Not Need To Be Serialised
u volatile
– Do Not Need To Be Locked

13
Arrays
u Arrays Are References To Objects
u Subtely Different To ‘C’
int x[10];
x[1] = 42; /* Causes runtime nullPointer exception */
u Space Allocated With new
– int[] scores; /* Brackets either place */
– results = new int[10];
– short errors[] = new short[8];

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Arrays (Cont’d)
u Can Create Multi-Dimensional Arrays
u Can Create ‘Ragged’ Arrays
u Space Freed When Last Reference Goes
Out Of Scope

15
Operators
u ! ~ ++ --
u * / %
u + -
u << >> >>>
u < <= > >=
u == !=
u &
u ^
u |
u &&
u ||
u = += -= *= /= %= &= |=
^= <<= >>= >>>=

16
Conditional Statements
u Syntax
if ( [test] ) { [action] } else { [action] }
u Example:
if (i == 10)
{
x = 4;
y = 6;
} else
j = 1;

17
Determinate Loops
u Syntax
for ( [start]; [end]; [update] )
{
[statement_block];
}
u Example
for (i = 0; i < 10; i++)
x[i] = i;

18
Indeterminate Loops
u Syntax 1:
while ( [condition] )
{
[statement_block]
}
u Syntax 2:
do {
[statement_block]
}
while ( [condition] );

19
Multiple Selections: Switch
u Syntax:
switch ( [variable] )
{
case [value]:
[statement_block];
break <label>;
default:
[statement_block];
}

20
Labeled Statement
u Label Must Preceed Statement To Jump To
u Label Must Be Followed By Colon (:)
u Use break And continue With Label
int i;
i:
for (i = start; i < max; i++)
{
int n = str.count, j = i, k = str.offset;
while (n-- != 0)
if (v[j++] != v2[k++])
continue i;

21
Java Memory Handling
u Allocation
– Use new with declaration
u Deallocation
– Handled Automatically By Garbage Collector
» Background Thread
– finalize Method
» File Handles
» Graphics Contexts

22
Error Handling In Java
u Catching Exceptions
u Syntax:
try
{
[statement_block];
} catch ( [exception_type] )
{
[statement_block];
} finally
{
[statement_block];
}

23
Returning Values
u Use return Expression
u If No Return Value Is Used Method Must Be
Declared void
u Don’t Get Confused By try/finally
Blocks
– finally Block Code Will Always Be executed Before
The Return Of Control To The Invoking Method

25
Basic Java Objects
u Syntax:
class [name] extends [super]
{
[variables]
name() {} /* Constructor */
[methods]
}

26
Object Variables
u Implicit Pointer To Object
u Must Be Initialised (Unless Declared static)
– Wombat fred;
c = fred.colour(); WRONG
– Wombat fred = new Wombat();
c = fred.colour(); RIGHT

27
Overloading
(Ad-Hoc Polymorphism)
u Methods Have The Same Name But Different
Arguments (different ‘signature’)
– setTime(short hour, short min, short sec)
– setTime(short hour, short min)
– setTime(short hour)
u Java Uses Static Dispatch For Methods In The
Same Class

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Constructor Methods
u Forces Initialisation Of The Object
u Can Be Overloaded
u Use super keyword for Superclass
public Wombat(String name, int age)
{
super(name); /* Must be first line */
wombat_age = age;
}

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Destructor Methods
u Use finalize() method
u Used To Tidy Up Resources Not Tracked
By Garbage Collector
u Not Guaranteed To Be Called Until
Garbage Collector Runs

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Accessor/Mutator Methods
u Mutator Methods
– setProperty()
u Accessor Methods
– getProperty()

31
Scope Modifiers
u public
– Available To All, Outside Object
u private
– Only Available Inside Object
u protected
– Only Available To Sub-Classes
u static
– Does Not Depend On Instatiation Of Object

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The this Object
u Refers To The Current Instatiation Of This
Object
u Can Be Used In Constructor With
Overloading
Wombat(String name, int age)
{
Wombat_age = age;
this(name);
}

33
Packages
u Provides Libraries Of Classes
u Use package Keyword At Start Of Source
u Use import Keyword To Use Classes
import java.util.*;
Date Today = new Date();
u Can Use Full Name (must be in CLASSPATH)
Date Today = new java.util.Date();
u Source Files Can Only Have One Public Class

34
Inheritance/Subclasses
u Allows Common Functionality To Be
Reused
u Subclasses Extend The Functionality Of
The Super Class

35
The super Method
u Refers To The Super-Class Of This Object
u On It’s Own Will Call The Constructor For The
Super Class
– Call Must Be First Line Of Sub-Class Constructor
u Can Be Used To Directly Call Methods Of The
Super Class
– super.setName(name);

36
The Cosmic Superclass
u Object Class Is Ultimate Superclass
u If No Super-Class Specified, Object Is
Default
u Has Useful Methods
– getClass()
– equals()
– clone()
– toString()

37
Polymorphism
u Method Signature:
– Name
– Parameters
u Used To Access Methods Defined In Super-Classes
u Signatures Must Match To Be executed
u A Method With The Same Signature Will Hide Those
in Super Classes
u Failure To Match Will Cause Compile Time Error

38
Final Classes
u Use final Keyword
u Used With class Prevents Inheritance
u Used With Method Prevents Overriding
u Two Reasons For Using final
– Efficiency
– Safety

39
Abstract Classes
u Creates Placeholders For Methods
u Classes With Abstract Methods Must Be
Declared Abstract
u Abstract Methods Must Be Defined in
Sub-Classes

40
Interfaces
u Java Only Allows One Superclass
u Provides A Cleaner Mechanism For
Multiple-Inheritance
– Less Complex Compiler
– More Efficient Compiler
u Interfaces Are Not Instantiated
u Allows Callback Functions To Be
Implemented

41
Object Wrappers
u Used To Make Basic Types Look Like
Objects
u Allows Basic Types To Be Used In Generic
Object Based Methods

42
The Class Class
u Used To Access Run Time Identification Information
u Use getClass() To Create Object Reference
u Useful Methods
– getName()
– getSuperclass()
– getInterfaces()
– isInterface()
– toString()
– forName()

44
Components
u Most Basic AWT class
u Contains Most AWT Methods
– event handling
– Physical Dimensions
– Properties (font, colour, etc)
u Useful Methods
– paint()/repaint()
– getGraphics()
– setFont()/getFont()

45
Container
u Holds Other Components/Containers
u Subclassed From Component
u Subclasses:
– Panel
– Window
– Frame
– Dialog
u Uses a LayoutManager To Control Placement

46
Layout Managers
u FlowLayout
u BorderLayout
u CardLayout
u GridLayout
u GridBagLayout

47
FlowLayout
u Simplest Layout Manager (Default)
u Components Lined Up Horizontally
– End Of Line Starts New One
u Alignment Choices
– CENTER (Default)
– LEFT
– RIGHT

48
BorderLayout
u Allows Five Components To Be Placed In
Fixed Relative Positions
u Components May Themselves Be Containers
North
CenterWest East
South

49
CardLayout
u Allows For Flipping Between Components
u Look & Feel Is Not Ideal

50
GridLayout
u Lays Out A Grid Of Locations
u Grid Width & Height Set At Creation Time
u All Components Are The Same Size
u Useful For Organising Parts Of A Window

51
GridBagLayout
u Grid Style Layout For Different Sized Components
u Constraint Properties Used To Define Layout
– gridx, gridy Start Point Of Component
– gridwidth, gridheight Size Of Component
– weightx, weighty Distribution Of Resize
u Rows & Columns Calculated Automagically

52
Events
u Used To Detect When Things Happen
– Key press, Mouse Button/Move
– Component Get/Lose Focus
– Scrolling Up/Down
– Window Iconify/Move etc
u Processed By
– HandleEvent() and action() (JDK 1.0)
– Event Delegation (JDK1.1)

53
Delegation Event Model
u New For JDK 1.1
u HandleEvent() & action() Can Get Messy
– Too Many Subclasses
– Too Much in Specific Methods
u New Model Uses
– Event Source
– Event Listener
– Event Adapters

54
Delegation Event Model
Advantages
u Application Logic Separated From GUI
– Allows GUI To Be Replaced Easily
u No Subclassing Of AWT Components
– Reduced Code Complexity
u Only Action Events Delivered To Program
– Improved Performance

55
Menus
u Used With Separate Frame Object
u Supports Cascading Menus
u Supports Tear-Off Menus
u Supports Checkboxes (CheckboxMenuItem)
u Menu Objects Attached To Menu Bar
u MenuItem Objects Attached To Menus
u Menu Events Carry The MenuItem Label

56
Scrolling Lists
u Simple Scrolling List Of String Objects
– Vertical & Horizontal Scroll Bars
u Supports Multiple Selections
u Uses Specific Event Types
– LIST_SELECT
– LIST_DESELECT
u Use getSelectedItems() To Get Strings

57
Dialog Box
u Provides User Interaction in Separate
Window
u Can Be Non-Resizable
u Modal - User Must Respond To Dialog
Before Continuing
u Modeless - User May Continue Without
Interacting With Dialog Box
u Use Dialog Object

58
Images
u Applets use Applet.getImage()
u Applications Use Toolkit.getImage()
u Most Image Methods Use ImageObserver()
– Not Normally Used Directly
– Use MediaTracker
u Simple Methods
– getGraphics() Returns Graphics Object For
Drawing
– getHeight()/getWidth() Return Parameters

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The Media Tracker
u Allows Applet Thread To Wait For Images To
Load
u MediaTracker Object Can Optionally Timeout
u Can Detect Stages Of Image Loading
– Image Size Determined
– Pieces Of Image Downloaded
– Image Loading Complete

60
Animation
u Very Simple In Java
u Download Sequence Of Images Into Array
img[j] = getImage(“images/T1.gif”);
u Loop Through Array Calling Paint() With
Appropiate Image
public void paint(Graphics g)
{
g.drawImage(img[j], x, y, this);
}

61
Programming Hints & Tips
u CLASSPATH Variable
u One Public Class Per Source File
u Screen Flicker - Override update() Method
u Use Vector Class for Linked Lists
u Mouse Buttons
– ALT_MASK Middle Button
– META_MASK Right Hand Button

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Demonstration
Java Workshop

63
Where Next?
u Web Sites
– java.sun.com
– java.sun.com/products/jdk/1.1 (Free Download!)
– kona.lotus.com
u Tools
– www.sun.com/workshop/index.html
u Books
– Core Java 2nd Ed, Gary Cornell & Cay S. Horstmann
u Training Courses
– www.sun.com/sunservice/suned

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