1. B.Ramamurthy Copyright 1998 CSE Department
Jump Start : Java
B.Ramamurthy
Copyright 1998
CSE Department
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2. CS1 (as in CSE115) CS1 Prog. Lang Algorithms BR Problem Solving
OO Design
Prog. Lang
Algorithms
-- Design
-- Tools
-- Techniques
-- Software Eng.
-- Class libraries
-- Class
-- Object
-- Methods
-- Attributes
-- Relationships
-- Encapsulation
-- Inheritance
-- Polymorphism
-- Elements
-- Control
Structures
-- Data
-- Sort
-- Search
Programming
Environment
Special Features
-- Design
using GUI
-- OS : Solaris 2.7
(unix-based)
-- JDK1.1.6
-- Application, Applets
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3. 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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4. 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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5. Conventional Compiled Languages
C++source
Mac Compiler
Mac Hardware
PC Hardware
PC Compiler
Sun Compiler
Sun Hardware
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6. Java Virtual Machine JVM Java compiler :javac Mac Hardware
Byte code
Java source
Mac Compiler
Mac interpreter
JVM
PC Hardware
Byte code
Java source
PC Interpreter
JVM
Sun Hardware
Byte code
Java source
Sun Interpreter
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7. “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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8. 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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9. Java API : A Simplistic View
packages
classes
methods and data declarations
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10. 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/jdk1.2/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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11. 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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12. 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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13. 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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14. 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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15. Example objects Object References redRose class Rose blueRose class
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16. 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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17. 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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18. 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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19. Class Structure
class
variables
constants
methods
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20. 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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21. 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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22. 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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23. 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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24. 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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25. 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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26. 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)
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27. 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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28. 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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29. 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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30. Method Definition : Syntax
visibility return_type method_name
(parameter_list) {
statements }
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31. 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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32. 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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33. 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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34. 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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35. Method Definition : review
header
body
Visibility
modifiers
parameter list
return type
Name
{ statements }
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36. 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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37. ...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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38. 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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39. 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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40. 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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41. 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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42. 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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43. ..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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44. 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/cs116/
setenv CLASSPATH .:/util/lang/jdk1.1.6/lib/classes.zip:/projects/bina/cs116/
You may import packages from this directory or use classes from
this directory.
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45. 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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46. 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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47. 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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48. Applet Runtime Structure
html
file
.html
Applet
file
.java
javac
includes
appletviewer
Java runtime
JVM
Applet
file
.class
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49. 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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50. 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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51. 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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52. 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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53. 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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54. On-line Information file:/util/lang/jdk1.2/docs/index.html
has user’s manual for Java API
It is extensive documentation with references to books, and other material of interest.
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