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Problem Solving, Object-Oriented Design and Java. B.Ramamurthy. Topics for Discussion. OO Design Principles Java Virtual Machine Java Application Structure Class and objects Methods and Variables Access/Visibility Modifiers Summary. Object-Oriented Principles. OOP. Polymorphism

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topics for discussion
Topics for Discussion
  • OO Design Principles
  • Java Virtual Machine
  • Java Application Structure
  • Class and objects
  • Methods and Variables
  • Access/Visibility Modifiers
  • Summary

B.Ramamurthy

object oriented principles
Object-Oriented Principles

OOP

Polymorphism

-- Many forms of

same function

-- Abstract Methods

-- Abstract Classes

Inheritance

-- Hierarchy

-- Reusability

-- Extensibility

-- Expressive power

-- Reflects many

real-world problems

Encapsulation

(class)

-- Information Hiding

-- Interface and

Implementations

-- Standardization

-- Access Control mechanisms

(private /public etc.)

B.Ramamurthy

conventional compiled languages

Mac Hardware

Mac Compiler

Mac Compiler

C++source

PC Hardware

C++source

PC Compiler

Sun Hardware

C++source

Sun Compiler

Conventional Compiled Languages

B.Ramamurthy

java virtual machine
Java Virtual Machine

JVM

Java compiler :javac

Mac Hardware

Byte code

Mac Compiler

Java source

Mac interpreter

JVM

PC Hardware

Byte code

Java source

PC Interpreter

JVM

Sun Hardware

Byte code

Java source

Sun Interpreter

B.Ramamurthy

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.

“Run-anywhere” Capability

B.Ramamurthy

java application program interface java api
Java Application Program Interface (Java API)

(JAVA API)

Package of related

classes : java.awt

Random

java.util

Date

Dictionary

java.io, java.beans,..

Etc..

package

class

B.Ramamurthy

java api a simplistic view
Java API : A Simplistic View

API

packages

classes

methods and data declarations

B.Ramamurthy

java api classes
Java API Classes
  • Unlike many other languages, you will referring to the classes in the API.
  • Where is the API?
  • Docuemntation for the packages are in:

http://java.sun.com/j2se/1.4/docs/api/

  • Open in it in your browser and traverse through the javadoc tree

B.Ramamurthy

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

Types of Programs

B.Ramamurthy

problem solving using java
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

B.Ramamurthy

what is an object
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)

B.Ramamurthy

classes are blueprints
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.

B.Ramamurthy

example
Example

objects

Object

References

redRose

class Rose

blueRose

class

B.Ramamurthy

instantiation examples
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);

B.Ramamurthy

operator new and dot
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

B.Ramamurthy

elements of a class
Elements of a Class

class

data declarations (variables,

constants)

methods

header

body

header

statements

modifiers,

type, name

variables,

constants

parameters

repetition

others

selection

assignment

B.Ramamurthy

class structure
Class Structure

class

variables

constants

methods

B.Ramamurthy

defining classes
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.

B.Ramamurthy

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()

Instance variable begin with an _

Classes: Begin with an uppercase letter. EX: Tree, Car, System , Math

Naming Convention

B.Ramamurthy

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 debugging)

Debugging and Testing

B.Ramamurthy

class components
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;

B.Ramamurthy

operations
Operations
  • Behaviors, methods or messages to which an object will respond to.
  • Methods:
    • Constructors
    • Set/get methods (mutators/accesors)
    • Predicate methods (boolean/status indicators; Ex: isEmpty())
    • Utility methods (for local use only: ex: internal sort after an insert)
    • Explicit methods or interface methods that define the interface an object offers to the world.

B.Ramamurthy

method invocation call
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);

B.Ramamurthy

defining methods
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.

B.Ramamurthy

method definition syntax
Method Definition : Syntax

visibilityreturn_type method_name

(parameter_list)

{

statements

}

B.Ramamurthy

return type
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.

B.Ramamurthy

return statement
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

B.Ramamurthy

parameter list
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.

B.Ramamurthy

parameter list syntax
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....)

B.Ramamurthy

method definition review
Method Definition : review

definition

header

body

Visibility

modifiers

parameter list

return type

Name

{ statements }

B.Ramamurthy

method definition example
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

B.Ramamurthy

example contd
...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

}

B.Ramamurthy

what happens when a method is called
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.

B.Ramamurthy

method invocation semantics
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

8

1

2

Main method

Rect.area(….)

3

7

4

8

area

method

5

6

B.Ramamurthy

constructors
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. (overloading)

B.Ramamurthy

constructors contd
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 = new Car(RED); // initializing constructor for Car class with color as parameter

B.Ramamurthy

visibility modifiers
Visibility Modifiers

type

Method/variable name

public

protected

“nothing”

DEFAULT

Package

Visibility

private

static

“nothing”

DEFAULT

for

class methods

and variables

for

object

methods and

variables

B.Ramamurthy

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.

..Modifiers (contd.)

B.Ramamurthy

classpath
Many times classes needed are available in a packages other than the Java API.

Set the CLASSPATH environment variable of your .cshrc file to point to such packages. One such is located at /projects/bina/CSE116/

setenv CLASSPATH .:/projects/bina/CSE116/other paths

You may now import a class from a package in this directory.

CLASSPATH

B.Ramamurthy

arrays
Arrays
  • Array is a numbered collection of variables all of the same type.
  • Length attribute gives the capacity of the array
  • Cells or the individual elements of the array distinguished by an index.
  • Lets look at an example.
  • Common error: ArrayIndexOutofBounds

B.Ramamurthy

summary
Summary
  • An overview of OOP, problem solving using OOP and Java language was presented.
  • Next class we will “review” interface, abstract class, concrete class, polymorphic dispatch and relationship among classes.

B.Ramamurthy