Recap (

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Recap (önemli noktalari yinelemek) from last week

• Paradigm
• Kays Description
• Intro to Objects
• Messages / Interconnections
• Information Hiding
• Classes

• Inheritance
• public class MammalTester
• // will work
• Mammal mam new Florist()
• // will not work
• Florist flo new Mammal()

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The JVM
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Key Aspects of OO Programming

• Abstraction
• just the essentials
• Composition
• building from parts
• Separation
• what vs. how
• Generalization
• finding the common elements

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Abstraction

• "Simplifying to its essentials the description of
  a real-world entity
• Encapsulation

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Properties of Good Abstraction

• Well named
• Coherent - attributes and behavior that are
  related and make sense
• Accurate - attributes and behavior match the
  entity being modeled
• Minimal - nothing extraneous

• Complete - all the attributes and behavior that
  are necessary
• Consistency - Operations should be consistent
  with each other and with respect to names,
  arguments, return values, and behavior.
• Decoupled - Different abstractions should not be
  inter-dependent

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Classes

• A class is a blueprint, or prototype, that
  defines the variables and the methods common to
  all objects of a certain kind." - Java Tutorial
• In Java, everything is defined in some class

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Classes

• Classes are either defined by the Application
  Program Interface (API) or are programmer defined
  data types.
• Standard Java API classes are organized in
  packages
• java.lang
• java.util
• ...

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Classes

• There will always be one file for each .class
  file
• At least to start, you will find it easiest if
  you maintain one .java source file for each class
  

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Anatomy of a Java class

• Package
• Imports
• Comments
• Declaration
• Fields
• Constructors
• Methods

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Methods

• A method is a named sequence of instructions - an
  action you can request an object to take
• Methods have a signature a name and zero or more
  arguments
• Methods declare a data type (primitive or object
  (or void) they return

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Methods

• Should generally be fairly short
• Constructors - a "method" to initialize objects
• Constructors and methods can be over-loaded
• Constructors and methods can have accessibility
  modifiers

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Constructors

• Constructors are the mechanism by which new
  instances of a class are created from the
  blueprint of the class definition.
• The purpose of a constructor is to initialize a
  new object.
• Constructors look something like method
  definitions except
• They always have the same name as the class
• They never return any type of value

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Constructors

• You "call" a constructor using the new operator
  and supplying any needed constructor arguments.
• Every class has, by default, a constructor
• public ClassName()
• that takes no arguments and does no special
  initialization.

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Constructors

• If you don't define one, Java will create a
  default, no-arg constructor.
• If you define any constructor, with or without
  arguments, Java assumes you know what you are
  doing and defines no default constructor.

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Constructors Rules of Thumb

• Remember the purpose of a constructor is to put
  a newly created object into a known, initial
  state.
• Constructors should not do a lot of processing.
• Try to separate object initialization from object
  behavior.

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Objects

• An object is a software bundle of variables and
  related methods." - Java Tutorial
• Everything is either a primitive data type or an
  object
• Objects are things
• Models of real world, physical things, like
  Students
• Abstract things, like courses, elections,
  financial transactions

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Objects

• In Java, declaring a variable to hold a primitive
  data type reserves space for that primitive data
  type.
• Declaring a variable to hold an object reserves
  space to hold a reference to an object of that
  type (or any derived type).

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Objects

• Declaring a variable to hold an object does not
  create the object
• The memory for an object instance is dynamically
  allocated using the new operator
• Create object types liberally

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Variable (Fields)

• Local Variables
• Variables you declare and use only within a
  method or a smaller block of code.
• Instance variables
• In a good object oriented Java class, just about
  all variables ought to be instance (or local)
  variables.
• Each object (instance) will have its own copy of
  these fields

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Variable (Fields)

• Class variables
• Qualifying a variable (field) with the keyword
  static makes that variable a class variable
• There is only one of those variables no matter
  how many instances of the class are instantiated.
  
• All instances share that class variable

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Using Class Variables

• There are only two good reasons to declare a
  class variable in Java
• As a static final constant
• As a private static internal item of information
  that is purposely shared among object instances

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Problem Specification CS Student

• Model enrollment in a CS class
• Permit student names to be given on the command
  line
• Assign lab partners as student pairs. An odd
  student will have no partner.
• List the students in the class (and their lab
  partner, if any) in reverse order of enrollment.

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Classes, Fields, Methods, Objects
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Accessibility

• The creator of a Java class controls what access
  objects outside the class have to the
  implementation (the inner details) of objects of
  her class by giving variables and methods
  accessibility qualifiers.
• Unlike in C, each method or field is given an
  accessibility modifier.

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Accessibility

• public
• All outside objects can call public methods.
• Any outside object can potentially change public
  variables.
• private
• methods are only callable within the instance
  methods of the class - not by subclasses.
• variables are only accessible within the methods
  of the class - not from subclasses.

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Accessibility

• protected
• methods are only callable from the methods of the
  class and any sub classes.
• variables are accessible within the instance
  methods of the class and any sub classes.
• "Package access
• the default if no other modifier is used
• Instances of any class in the same package may
  call methods with package access.
• Instances of any class in the same package can
  acess package variables

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Encapsulation

• Information Hiding
• objects have public methods that they expose for
  other objects to use. These methods permit other
  objects to send "messages" to it. Private
  variables and methods of the object are
  implementation details that can be changed at any
  time without affecting other objects.

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Encapsulation

• Expose Appropriate Methods
• Classes should expose to the outside (make
  public) just the methods needed to make the
  object do the things it is designed to do
• None of the internal details related to the
  implementation of the class should be visible to
  the outside.
• supports modularity - an object can be written
  and maintained independently of other objects.

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Composition

• "An organization of components interacting to
  achieve a coherent, common behavior"
• Composition extends the responsibilities of an
  object by delegating work to additional objects.
• Composition is the major mechanism for extending
  the responsibilities of an object. Nearly every
  object in an object model is composed of, knows
  of, or works with other objects.
• Java Design" Peter Coad and Mark Mayfield,
  Prentice Hall, 1996

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Composition

• Create object types liberally
• The "has-a" relationship

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Separation

• Distinguishing what an object can do versus how
  it does it
• Related to the use of Java interface definitions
  
• Related to Encapsulation

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Generalization

• Identifying, and possibly organizing, common
  elements among different entities
• Hierarchy - inheritance
• Polymorphism
• Patterns