CSE 501N Fall 06 01: OO Concepts Java Basics

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CSE 501NFall 0601 OO Concepts Java Basics

• 30 Aug 2006
• Rohan Sen

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Lecture Outline

• From idea to software
• Compiled Languages
• Interpreted Languages
• Evolution of programming languages
• Object-oriented programming concepts
• Java Basics

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From Ideas to SoftwareCompiled Languages (C,
C, VB, etc.)

• Code is translated directly to machine executable
  binary
• Each binary is specific to the type of platform
  (machine and OS)
• E.g., Mac, PC, Linux
• Fast execution
• No cross-platform compatibility

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From Ideas to SoftwareInterpreted Languages
(BASIC, Java, etc.)

• Code is translated to an interim format
• An interpreter translates between the interim
  format and machine code
• Slow execution due to translation
• Cross-platform compatibility
• Still require platform specific interpreter

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Evolution of Programming Languages

• Programming in binary
• 1011101010101
• 01000101010101
• 01010100101001
• Low level languages
• Assembly
• MOV AX 2
• MOV BX 3
• ADD AX BX
• MOV AX CX
• High level languages
• Functional languages, e.g., C
• add(int a, int b)
• a ab
• Object oriented languages, e.g., Java, C

compiles into
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Object-oriented ProgrammingEntities

• Class
• Blueprint for certain type of object, e.g., the
  class House
• Defines an internal state
• Defines methods to change its internal state
• Object
• A specific instantiation of a class definition,
  e.g., Bobs House
• The unit of programming in OO languages
• Multiple objects can be created from a single
  class definition Bobs House, Alices House,
  etc.
• Method
• A means of defining the behavior of the object
• May change the state of the object
• Accessors / Mutators / Subroutines
• Attributes
• Define the state of the objext

Bobs House
people_inside
enter
exit
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Object-oriented ProgrammingDifferentiating
between Classes and Objects

• It is extremely important to be able to
  distinguish between classes and objects
• A class defines the basic features of a certain
  type of entity
• Informally stated, a valid class definition for a
  house requires that anything claiming to be a
  house
• Must have a number of doors
• Must have a number of windows
• Must have some bedrooms
• Must have some bathrooms
• An object is a specific example of a certain type
  of entity
• Bobs House and Alices House are instances of
  the class House
• Notice that they obeys the definition of the
  general class House
• In addition, they also specify the attributes
  that are particular to each house

Bobs House
2 doors
10 windows
3 bedrm
2 baths
House Class Definition
Alices House
5 doors
7 windows
4 bedrm
1 baths
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Object-oriented ProgrammingMethods and Attributes
House Class Definition
Bobs House
2 doors
10 windows
3 bedrm
2 baths

• Attributes
• numDoors
• numWindows
• numBedrooms
• numBathrooms
• Methods
• addWindow()
• removeWindow()

• Attributes
• numDoors 2
• numWindows 10
• numBedrooms 3
• numBathrooms 2
• Methods
• addWindow()
• removeWindow()

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Object-oriented ProgrammingCore Concepts -
Abstraction

• The act of representing essential features
  without including the background details or
  explanations
• Classes use the concept of abstraction to hide
  implementation details
• For example, the method addWindow() of the class
  House adds a window to the instance of the house
• However, it does not specify exactly how it does
  this
• Abstraction is a very useful concept because it
  allows programmers to write code
• At a higher level
• Without the need to understand implementation
  details
• And promotes reuse of code

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Object-oriented ProgrammingCore Concepts -
Encapsulation

• Protect attributes by preventing open access to
  them
• Allow access via controlled methods
• Accessors and mutators

numWindows 30
Bobs House
addWindow
numWindows
removeWindow
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Object-oriented ProgrammingCore Concepts -
Inheritance

• Use a basic class definition to derive another
  more complex class definition
• Create class hierarchies
• The superclass is the most generic class
  definition
• The subclass is a more specific class definition
• Though called a subclass, capabilities of the
  subclass are greater than that of the superclass

Structure
Superclass
is-a
is-a
Garage
House
Subclass
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Object-oriented ProgrammingCore Concepts -
Polymorphism

• The ability to use the same code, yet process
  differently based on the class of an object
• Utilized in conjunction with inheritance to write
  general and abstract programming solutions
• Supported by dynamic binding

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Java Programming Basics

• In the Java programming language
• A program is made up of one or more classes
• A class contains one or more methods
• A method contains program statements

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Java Programming BasicsProgram Structure
// comments about the class
public class QuoteGenerator

class header
class body
Comments can be placed almost anywhere
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Java Programming BasicsProgram Structure
// comments about the class
public class QuoteGenerator

// comments about the method
public static void main (String args)

method header
method body
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Java Programming BasicsProgram Structure
// comments about the class
public class QuoteGenerator

// comments about the method
public static void main (String args)

System.out.println(Ask not)
statement
statement terminator
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Java Programming BasicsComments
// this comment runs to the end of the line
/ this comment runs to the terminating
symbol, even across line breaks /
/ this is a javadoc comment /
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Java Programming BasicsIdentifiers

• An identifier can be made up of letters, digits,
  the underscore character ( _ ), and the dollar
  sign
• Identifiers cannot begin with a digit
• Java is case sensitive

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Java Programming BasicsIdentifiers

• Identifiers may be chosen by the programmer
• Identifiers are used by the programmer
• Special identifiers called reserved words have a
  predefined meaning in the language

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Java Programming BasicsReserved Words
abstract boolean break byte case catch char class
const continue default do double
else enum extends false final finally float for go
to if implements import instanceof
int interface long native new null package private
protected public return short static
strictfp super switch synchronized this throw thro
ws transient true try void volatile while
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Java Programming BasicsWhite Space

• Spaces, blank lines, and tabs are called white
  space
• White space is used to separate words and symbols
  in a program
• Extra white space is ignored
• White space is FREE. Use it liberally to make
  your code legible

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Java Programming BasicsErrors

• A program can have three types of errors
• The compiler will find syntax errors and other
  basic problems (compile-time errors)
• A problem can occur during program execution,
  which causes a program to terminate abnormally
  (run-time errors)
• A program may run, but produce incorrect results
  (logical errors)

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The Java Style GuideGood Habits to Adopt Early

• It is very important that others can read and
  understand your code
• In this class
• In the real world
• The best way to ensure this is to write clean,
  well organized code
• The style guide provides guidelines that have
  been adopted as standard by the Java community
• You will be expected to adhere to the style guide
  for all your lab assignments

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The Java Style GuideGood Habits to Adopt Early

• import java.util.
• public class TreasureChest extends Chest
  implements RagsToRichesInterface
• //public variables
• public int numOfGoldCoins 0
• //protected and private variables
• public TreasureChest(int goldCoins, int
  silverCoins)
• // Statements
• public int lootChest()
• //Statements
• return numOfGoldCoins

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The Java Style GuideGood Habits to Adopt Early

• public void aDummyMethod()
• if(xtrue) Wrong!
• //Statements
• else
• //Statements
• boolean y true
• while(y)
• //Statements
• for(i 0 i lt 10 i)
• //Statements

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Conclusion

• Questions
• Lab 0 is assigned
• Get it online at http//www.cs.wustl.edu/ras6/CSE
  501/assignments.htm