Transcript Lecture 10: Encapsulation

Something to discuss
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Feedbacks on Midterm Exam
Final exam and term project
Final exam requires solid knowledge/skills in Java
 Be more serious for the class!
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Term project teams
Project topics
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Understand lecture AT CLASS
Practice at home (do homework) and at lab sessions.
Self study for term project
Game
Algorithm animation
Project proposal Next Week !
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Programming Fundamentals I
Java Programming
Spring 2007
Lecture No. 10
XuanTung Hoang
[email protected]
Encapsulation
1.
2.
3.
4.
5.
Overview
Encapsulation
Information Hiding
Composition
Singleton
What’s next after midterm … ?
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We have learned almost every basics of Java
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We can write java programs solve almost every
computing problem.
We will learn HOW TO ORGANIZE your
program in a clever way.
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For understanding
For reusability
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Specifically, what will we learn?
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Writing Java program …
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Creating class:
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If you follow (or use) those rules/tricks/patterns:
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Rules
Tricks
And patterns
You will write very good Java code!!!
You will be able to USE classes provided by others
effectively
Using class:
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Java Collection Framework
Java I/O
Java GUI
Java Multithreading ….
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Rule 1: Encapsulation
Group related information together
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Group related information into a class
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Use instance variables
Provide related actions to process those variables
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Rule 1.1: Information Hiding
1. Don’t allow instance variables to be changed arbitrarily
2. Only expose necessary methods
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Instance variable should be private
Standardizing:
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the way to change instance variables (with set methods)
the way to represent instance variables
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with get methods
with predicate methods
Expose necessary methods with public modifier
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Trick 1.2: Composition
You can combine classes to define another class
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Bottom up design !
Example:
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Define six classes
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Input, Output, Memory, CPU, ALU, Storage
Make Computer class that contains the above 6 classes
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Case Study: Time Class (1)
private instance
variables
Validate parameter
values before setting
instance variables
format strings, check
JavaAPI manual for
details
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Case Study: Time Class (2)
Create a Time1 object
Call toUniversalString
method
Call toString method
Call setTime method
Call setTime method
with invalid values
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Notes from the Case Study …
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hour, minute, second are private. They
cannot be accessed/modified from other classes
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Access/modify via set and get methods (e.g. setTime method)
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Making hour, minute, second public is allowed, but it isn’t a
good idea …
 Hour, minute, second should store valid values. If they can be
set freely, their values are very likely
invalid.
If hour, minute, second are private with appropriate set metho
ds we can make sure that their values are always valid
 appropriate set methods: Integrate validity checking in set
method
Other classes (clients) just call the set method without
worries about invalidity.
Complex detailed implementation
related to private fields are hidden
(information hiding)?
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set, get, and predicate methods
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Data/information hiding is a property of class instance
Typical classes has set, get, and predicate methods
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set methods (mutators): validate and set the new values to instance
variables
get methods (accessors): obtain values of instance variables and control
the format of the returned values
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E.g.: toUniversalString() can be considered as an
get method
Predicate methods: test whether a certain condition on the object is true
or false
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E.g.: we can add to class Time1 method is21Century()
that tests whether the date is in the 21st century or not.
Great feature of OOP
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Data abstraction and ADTs (1)
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Thanks to information hiding, clients care only
what functionalities a class offers and don’t
concern with how those functionalities are
implemented. This concept is called data
abstraction
Data is not just the information like in common
sense, data is the information and operations that
can be performed on the information  Abstract
Data Types (ADTs)
ADTs = Data representation + Operations
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In ADTs, data representation can be hidden
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Data abstraction and ADTs (2)
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ADT Is a term that can refer to any type in current
programming languages
Java:
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ADTs can be primitive types. E.g.: int (data representa-tion is int
eger number; operations are arithmetic operations)
ADTs can be classes. E.g. Time1 class, Employee class
Read section 8.15, text book
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Pattern 1.3: Singleton
Singleton = Universal public Instance
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In an application, if you have ONLY ONE instance
of a class, use Singleton pattern.
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Singleton pattern
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In a singleton class:
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1 private static instance variable of singleton class
(sInstance)
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Initialize it to null
1 public static method to:
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create object of Singleton class ONLY ONE
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Guarantee ONLY ONE object creation by checking sInstance
and return the singleton instance
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Singleton Pattern
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Examples of Singleton
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Universal Random number generator
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Universal Sequence number generator
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In CardGame project
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