1. B.Ramamurthy Copyright 1998 CSE Department
Jump Start : Java
B.Ramamurthy
Copyright 1998
CSE Department
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2. Topics for Discussion OO Design Principles Java Virtual Machine
Java Application Structure
Class and objects
Methods and Variables
Access/Visibility Modifiers
Debugging
Inheritance
Abstract classes, interfaces and implementation
JDK Event Model
Applets and Appletviewer
Summary
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3. Object-Oriented Principles
OOP
Inheritance
-- Hierarchy
-- Reusability
-- Extensibility
-- Expressive power
-- Reflects many
real-world problems
Polymorphism
-- Many forms of
same function
-- Abstract Methods
-- Abstract Classes
Encapsulation
(class)
-- Information Hiding
-- Interface and
Implementations
-- Standardization
-- Access Control mechanisms
(private /public etc.)
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4. Conventional Compiled Languages
C++source
Mac Compiler
Mac Hardware
PC Hardware
PC Compiler
Sun Compiler
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5. Java Virtual Machine Java compiler :javac JVM Mac Hardware C++source
Byte code
C++source
Mac Compiler
Mac interpreter
JVM
PC Hardware
Byte code
C++source
PC Interpreter
JVM
PC Hardware
Byte code
C++source
Sun Interpreter
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6. “Run-anywhere” Capability
On any machine when you compile Java source code using javac byte code equivalent to the source is generated.
Byte code is machine-independent. This enables the “run-anywhere” capability.
You invoke java command which will feed the byte code to the machine-dependent interpreter.
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7. Java Application Program Interface (Java API)
Package of related
classes : java.awt
Random
java.util
Date
Dictionary
Java.io, java.beans,..
Etc..
package
class
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8. Java API : A Simplistic View
packages
classes
methods and data declarations
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9. Java API Classes
Unlike many other languages, you will referring to the classes in the API.
Where is the API?
Packages are in:
/util/lang/jdk1.1.6/src/java
“cd” to any package of interest:
awt, lang, io, graphics…..
You will see the methods and data defined in the classes in the package.
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10. Types of Programs Java is fully object-oriented.
Every “function” has to be attached to a class.
You will mainly deal with three types of programs:
class: methods and data (variables + constants) describing a collection (type) of object
application: class that has a main method: represents a our regular program
applet: class that is meant for execution using a appletviewer/browser
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11. Problem Solving Using Java
OO Design and Progamming in Java
Write an
applet
class
Identify classes needed
Write an
application
class
Reuse API
classes
Reuse
your classes
Design new
classes
Create and use objects
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12. What is an Object?
Object-oriented programming supports the view that programs are composed of objects that interact with one another.
How would you describe an object?
Using its characteristics (has a ----?) and its behaviors (can do ----?)
Object must have unique identity (name) : Basketball, Blue ball
Consider a ball:
Color and diameter are characteristics (Data Declarations)
throw, bounce, roll are behaviors (Methods)
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13. Classes are Blueprints
A class defines the general nature of a collection of objects of the same type.
The process creating an object from a class is called instantiation.
Every object is an instance of a particular class.
There can be many instances of objects from the same class possible with different values for data.
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14. Example objects Object References redRose class Rose blueRose class
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15. Instantiation : Examples
class FordCar defines a class name FordCar
FordCar windstar; ---- defines a Object reference windStar
windstar = new FordCar(); instantiates a windstar Object
class HousePlan1 { color….
HousePlan1 blueHouse;
blueHouse = new HousePlan1(BLUE);
HousePlan1 greenHouse = new HousePlan1(GREEN);
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16. Operator new and “dot”
new operator creates a object and returns a reference to that object.
After an object has been instantiated, you can use dot operator to access its methods and data declarations (if you have access permissions).
EX: redRose.bloom(); greenHouse.color
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17. Elements of a Class class data declarations (variables, constants)
methods
header
body
header
statements
modifiers,
type, name
variables,
constants
parameters
repetition
others
selection
assignment
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18. Class Structure
class
variables
constants
methods
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19. Defining Classes Syntax: class class_name { data-declarations
constructors
methods }
Constructors are special methods used for instantiating (or creating) objects from a class.
Data declarations are implemented using variable and constant declarations.
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20. Naming Convention
Constants: All characters in uppercase, words in the identifier separated by underscore: EX: MAX_NUM
Variables, objects, methods: First word all lowercase, subsequent words start with uppercase. EX: nextInt, myPen, readInt()
Classes: Start with an uppercase letter. EX: Tree, Car, System , Math
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21. A complete example
Problem Statement: You have been hired to assist in an secret encryption project. In this project each message (string) sent out is attached to a randomly generated secret code (integer) between 1 and 999. Design and develop an application program in Java to carry out this project.
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22. Identify Objects There are two central objects:
Message
Secret code
Is there any class predefined in JAVA API that can be associated with these objects? Yes ,
“string” of java.lang and “Random” of java.util
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23. The Random class Random class is defined in java.util package.
nextInt() method of Random class returns an integer between 0 and MAXINT of the system.
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24. Design Class Random Class String An instance of Random
An instance of string
Class Random
An instance of Random
number generator
Input and fill up message.
Generate Random integer
Attach (concatenate)
Output combined message.
For implementation see /projects/bina/java/secretMsg.java
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25. Debugging and Testing
Compile-time Errors : Usually typos or syntax errors
Run-time Errors : Occurs during execution. Example: divide by zero .
Logic Errors: Software will compile and execute with no problem, but will not produce expected results. (Solution: testing and correction)
See /projects/bina/java/Peets directory for an exercise.
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26. Class Components
Class name (starts with uppercase), constants, instance variables, constructors definitions and method definitions.
Constants:
public final static double PI = 3.14;
Variables:
private double bonus;
public string name;
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27. Method Invocation/Call
Syntax:
method_name (values);
object_name.method_name(values);
classname.method_name(values);
Examples:
computeSum(); // call to method from within the class where it is located
YourRose.paintIt(Red);
Math.abs(X);
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28. Defining Methods
A method is group of (related) statements that carry out a specified function.
A method is associated with a particular class and it specifies a behavior or functionality of the class.
A method definition specifies the code to be executed when the method is invoked/activated/called.
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29. Method Definition : Syntax
visibility return_type method_name
(parameter_list) {
statements }
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30. Return Type can be void, type or class identifier
void indicates that the method called to perform an action in a self-standing way: Example: println
type or class specify the value returned using a return statement inside the method.
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31. Return Statement Syntax of return statement:
return; // for void methods
return expression; // for type or class return value
// the expression type and return type should be same
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32. Parameter List
Parameter list specified in method header provides a mechanism for sending information to a method.
It is powerful mechanism for specializing an object.
The parameter list that appears in the header of a method
specifies the type and name of each parameter and
is called formal parameter list.
The corresponding parameter list in the method invocation is called an actual parameter list.
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33. Parameter list : Syntax
Formal parameter list: This is like molds or templates
(parm_type parm_name, parm_type parm_name, ....)
Actual parameter list: This is like material that fit into the mold or template specified in the formal list:
(expression, expression....)
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34. Method Definition : review
header
body
Visibility
modifiers
parameter list
return type
Name
{ statements }
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35. Method Definition : Example
Write a method that computes and returns the perimeter of a rectangle class.
Analysis:
Send to the method: Length and Width
Compute inside the method: Perimeter
Return from the method: Perimeter
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36. ...Example (contd.) public int Perimeter (int Length, int Width) {
int Temp; // local temporary variable
Temp = 2 * (Length + Width); // compute perimeter
return Temp; // return computed value }
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37. What happens when a method is called?
Control is transferred to the method called and execution continues inside the method.
Control is transferred back to the caller when a return statement is executed inside the method.
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38. Method Invocation : semantics
Operating
System
1. OS to main method
2. Main method execution
3. Invoke Area
4. Transfer control to Area
5. Execute Area method
6. Return control back to
main method
7. Resume executing main
8. Exit to OS 1 8 2
Main method
Rect.Area(….) 3 7 4 8
Area
method 5 6
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39. Constructors
A Constructor is used to create or instantiate an object from the class.
Constructor is a special method:
It has the same name as the class.
It has no return type or return statement.
Typically a class has more than one constructor: a default constructor which has no parameters, and other constructors with parameters.
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40. Constructors (contd.)
You don’t have to define a constructor if you need only a default constructor.
When you want initializing constructors :
1. you must include a default constructor in this case.
2. You will use initializing constructors when you want the object to start with a specific initial state rather than as default state.
3. Example: Car myCar(Red); // initializing constructor for Car class with color as parameter
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41. Visibility Modifiers type Method/variable name public protected
“nothing”
DEFAULT
private
static
“nothing”
DEFAULT
To indicate
class method/
variable
To indicate
object
method/
variable
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42. ..Modifiers (contd.) private : available only within class
“nothing” specified : DEFAULT: within class and within package
protected : within inherited hierarchy (only to sub classes)
public : available to any class.
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43. CLASSPATH
Many times classes needed are available in a repository other than the Java API.
Set the CLASSPATH environment variable to point to such repositories. One such is located in /projects/bina/cs114/structures
setenv CLASSPATH
.:/util/lang/jdk1.1.6/lib/classes.zip:/projects/bina/cs114/structures
(all in a continuous line)
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44. CLASSPATH
Now you may refer to any one of the classes in the structures directory.
The source code is available in
/projects/bina/cs114/sources
Checkout ReadStream and Keyboard classes for nice inputs.
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45. Inheritance
Inheritance is the act of deriving a new class from an existing one.
A primary purpose of inheritance is to reuse existing software.
Original class used to derive a new class is called “super” class and the derived class is called “sub” class.
Inheritance represents “is a” relationship between the superclass and the subclass.
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46. Syntax class subclass extends superclass { class definition } Example:
class Windstar extends FordCar // meaning it inherits from class Fordcar{ ....}
Windstar myCar();
In this example, FordCar is the super-class and Windstar is a sub-class and myCar is an object Windstar class.
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47. Representing the Relationship
BankClass
has a
has a
has a
Account [ ]
MortgageSVC
BrokerageSVC
is a
is a
Savings
Checking
is a : use inheritance
has a : use composition, or membership
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48. Modifers Visibility modifiers: private, public, protected
Protected modifier is used when the entity needs to be available to the subclass but not to the public.
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49. Example : Words Main class Book class Super class Uses is a
Dictionary Class
subclass
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50. Example : School
Student
Main class
uses
Grad Student
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51. Abstract Class (ABC) and methods
An abstract class or method is defined using “abstract” modifier.
An abstract method has no definition in the super class.
ABC cannot be instantiated.
Sole purpose of ABC is to provide an appropriate super class from which classed may inherit.
Classes from which objects can be instantiated are called concrete classes.
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52. Example Food Abstract super class Pizza Hamburger HotDog subclasses
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53. Interface
An abstract method is one that does not have a definition within the class. Only the prototype is present.
An interface is collection of constants and abstract methods.
Syntax
interface interface_name {
constant -declarations;
abstract methods; }
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54. Example interface EPA { bool emissionControl();
bool pollutionControl(); … }
class NYepa implements EPA {
bool emissionControl () {
details/semantics /statements how to implement it }
class CAepa implements EPA {
bool emissionControl () {….
// different details on implementation….}
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55. Inheritance and Interfaces
In Java class may inherit (extend) from only one class. (C++ allows multiple inheritance).
But a Java class may implement many interfaces.
For example,
public class Scribble extends Applet implements MouseListner, MouseMotionListner {
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56. Types of Programs Console programs:
Control-driven: You start the program and the program code determines the sequence of events.
Mostly the actions are predetermined.
Windows-based programs:
Event-driven: The operation of the program depends on what you do with the controls presented (usually by the GUI)
Selecting menu items, pressing buttons, dialog interaction cause actions within the program.
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57. Events
1. Actions such as clicking a button, moving a mouse, are recognized and identified by the operating systems(OS).
2. For each action, OS determines which of the many currently running programs should receive the signal (of the action)
3. The signals that the application receives from the OS as result of the actions are called events.
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58. Event Generation User Interface Other event sources
Mouse actions, Keystrokes
Operating Systems
JAVA API
Events
Methods and handlers
Methods and handlers
Application 1
Application 2
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59. Event Handlers
An application responds to the events by executing particular code meant for each type of event.
Not all events need to be handled by an application. For example, a drawing application may be interested in handling only mouse movements.
As a designer of an event-driven application you will write methods to handle the relevant events.
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60. Event Handling Process
Source of an event is modeled as an object. Ex: button click’s object is a button
Type of the event: ActionEvent, WindowEvent, MouseEvent etc. Ex: An ActionEvent object is passed to the application that contains information about the action.
Target of an event: Listener object of the event. Passing the event to a listener means calling the particular method of the listener object.
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61. Java Event Model
Java event model has changed quite significantly between JDK1.0 and JDK 1.1.
Many Web Browsers still use JDK1.0.
JDK1.0 event model is good for understanding event-driven programming and for simple applications but does not scale well for large applications.
It is impossible to take a in-depth look at any one. But we will learn enough to use some basic features.
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62. Java Class Hierarchy java.lang Object java.awt Component “others”
Label
Button
Container
TextComponent
java.applet package
Window
Panel
Applet
Dialog
Frame
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63. JDK1.1 Event Model : System support
Many Event classes
Low-level classes
Semantic classes
FocusEvent
MouseEvent
Etc.
Etc.
ActionEvent
TextEvent
Listener Interfaces
: methods
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64. JDK1.1 Event Model : Application Support
Instantiate objects from component classes such as buttons and checkboxes which will serve as source of the events.
Implement listener interfaces.
Define methods to handle events of interest.
Example: /projects/bina/java/Doodle.java
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65. Implementing Listeners
An application (class) becomes a listener when it implements the listener interface.
Java 1.1 event model provides a variety of listeners.
EX:
If an application is interested in button click, it will have to implement ActionListener interface.
actionPerformed() method of the ActionListener will have be implemented by the application.
When the button click event occurs, actionPerformed() method will be called with event object as argument.
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66. Example: Doodle.java
1. Include “implements” at the header of the class : This application uses MouseListener and MouseMotionListener interfaces
2. Invoke “ methods” to add/activate required listeners. Here we have three:
a) mouse click
b) mouse motion
c) window close
3. Set up board : use paint() and fillRect() methods.
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67. Example (contd.)
4. Implement all the methods (even if it is empty) of the interface (why?)
5. This application needs only MousePressed and MouseReleased for line color selection and MouseDragged for drawing lines.
6. Finally an adapter class that handles window closing.
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68. Applet
An applet is a Java program that operates within a browser and hence can appear in a web page.
It can be fetched from a remote computer and run on the local computer.
Applet’s enabling technologies:
Java Virtual Machine (JVM)
Interpretation rather than compilation
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69. Requirements
Besides the classes, an applet requires a .html file and a tool for viewing the applet (Ex: appletviewer of JDK)
An applet extends Applet class of java.applet package.
import java.applet.Applet;
An applet should have at least an init() or a paint() method (just as an application should have main() method)
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70. Applet Class Applet class has four important methods: init() start()
stop()
destroy()
Besides these paint() method of component class is commonly used for drawing graphics in an applet window.
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71. Applet Runtime Structure
html
file
.html
Applet
file
.java
javac
includes
appletviewer
Java runtime
JVM
Applet
file
.class
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72. Example 1: Using only paint()
//file : MyApplet.java
import java.applet.Applet; // step 1
import java.awt.Graphics;
public class MyApplet extends Applet //step 2 {
public void paint( Graphics g) // step 3
{ g.drawString(“To climb a ladder”, 20, 90); } }
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73. Example 1: interpretation
step 1: import Applet class
step 2: inherit from Applet class
step 3: Overload /redefine/customize paint() method
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74. html file 1. Create a html file:
<APPLET code=“MyApplet.class” width=300 height=200>
<\APPLET>
2. Save as MyApplet.html
3. To create class: javac MyApplet.java
4. To execute: appletviewer MyApplet.html
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75. Example 1: execution semantics
Java run-time system will look for init(), then start() methods.
Default definitions of init() and start() are executed.
Then user-applet-defined paint() method is executed.
Rest of the control follows default methods.
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76. More on Applets 1. main() is replaced by init() or paint() methods
2. An applet cannot access local programs or files
3. Very useful in creation of active/dynamic web pages
4. Implements “behavior streaming”
5. Popular component of Java language.
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77. On-line Information file:/util/lang/jdk1.1.6/api/API_users_guide.html
has user’s manual for Java API
/util/lang/jdk1.1.6/demo
has many demo programs
and of course, Sun’s home page has the JDK distribution.
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78. Summary
We studied Java classes and objects, object-oriented programming, applications and applets.
A good understanding of the event-model is necessary to write event-driven programs.
Graphics and awt features (buttons, panels etc.) will tremendously enhance the capabilities and presentation of the event driven programs.
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