OO Concepts

1
OO Concepts
2
Chapter 3
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Object-Oriented Concepts (review)

• Encapsulation hiding unimportant details
• Black box something that magically does its
  thing
• Abstraction taking away inessential features
• Example car
• if engine control module fails, replace it
• mechanic can provide inputs, test outputs
• doesnt need to know how it works
• OO challenge finding the right abstractions
  (what black boxes do we need to solve the
  problem at hand?)

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Object-Oriented Concepts (review)

• How would you represent
• a bank account?
• a book?
• a library?
• a customer?
• an oil field?
• a mutual fund?
• an employee?

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BankAccount

• See Eclipse notes for example of creating class
  with comments
• As in C, constructors have name of class, can
  be overloaded
• Instance fields should be made private, accessors
  and modifiers (getters/setters) are used
• Additional methods are defined as needed

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More on Variables

• Instance variables
• each object has its own copy (as in C)
• objects are automatically garbage collected
  (unlike C!)
• fields are initialized with a default value
  (e.g., 0, null) if not explicitly set in
  constructor. Still good to do your own
  initialization.
• NullPointerException if you forget to call new
• Local variables
• must be explicitly initialized
• only exist inside method (as in C)

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javadoc comments

• Remember the API had a standard format.
  User-defined classes can easily create html
  documentation in that same format, by inserting
  comments that meet certain specifications
• Start with /
• First sentence describes purpose
• _at_ tags specify information (e.g., _at_param,
  _at_return, _at_throws, _at_author, _at_version)
• run javadoc from command line or Eclipse to
  generate html pages

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Chapter 8

• Class design, UML, cohesion coupling, static
  fields and methods

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Choosing Classes

• Common class names are nouns, methods names are
  verbs
• class BankAccount, methods deposit, withdraw etc.
• class CashRegister, methods enterPayment,
  recordPurchase, etc.
• Classes may also be actors that do some type of
  work for you
• Scanner scans stream for numbers and strings
• Random generates random numbers
• Utility class may have no objects, but exists to
  provide static methods and constants
• Math class
• Some classes just exist to start a program
  (degenerate classes)

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Choosing Classes (continued)

• Which of the following are good classes?
• ComputePaycheck
• PaycheckProgram
• Paycheck
• ChessBoard
• MoveChessPiece
• A class should represent a single concept

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Cohesion and Coupling

• Cohesion All public methods (i.e., the public
  interface) should be closely related to the
  single concept the class represents
• Coupling Number of dependencies between classes

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Coupling
low coupling
high coupling
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UML Dependency Relationship
Unified Modeling Language Booch, Jacobson,
Rumbaugh
In a class diagram, dependency is a dashed line
with a shaped open arrow that points to dependent
class. (i.e., CashRegister depends on Coin)
CashRegister
Coin
Note direction of arrow CashRegister depends on
Coin Coin is independent
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Static Methods

• Also known as class method
• Does not operate on an object, so it has only
  explicit parameters
• Example
• public class Financial
• public static double percentOf(double p, double
  a)
• return (p/100) a
• double tax Financial.percentOf(taxRate, total)

All data is passed in, no owned variables
Call with class name, not instance
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Static Fields

• Used to store values outside any particular
  object
• public class BankAccount
• private double balance
• private int lastAssignedNumber 1000
• // No
• should be
• private static int lastAssignedNumber 1000

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Instance vs Static vs Local Which to Use?

• Use instance variables for data that belong to
  one instance of the class and will be operated on
  by multiple methods
• Use local variables for temporary calculations
  within a method
• Use static ONLY for constants and when methods
  will be called with no object

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Intance vs Local vs Static Example

• public class Financial
• public static double percentOf(double p, double
  a)
• return (p/100) a
• public class BankAccount
• private double balance
• private double intRate
• // addInterest updates balance returns amt of
  interest
• public double addInterest()
• double interest balance intRate
• balance interest
• return interest

• Common errors
• Making all variables instance, even if only used
  in one function.
• Making all variables static like using globals!

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Chapter 9

• Interfaces

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Interfaces for Code Reuse

• Interface specifies common set of operations
• All methods are abstract (like prototypes), no
  implementation. Must be public.
• Interface may have constants, but no instance
  fields (never instantiate an interface directly)
• Other classes can then implement that interface

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Interface Example

• public interface Measureable
• double getMeasure()

public class Coin implements Measurable
public double getMeasure() return
value . . .
keyword public required, default is package

• public class BankAccount implements Measurable
• public double getMeasure()
• return balance
• . . .

Can implement more than one interface!
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Interface Example, continued

• public class DataSet
• public void add(Measurable x)
• sum sum x.getMeasure()
• if (count 0
• max.getMeasure() lt x.getMeasure())
• max x
• count
• public Measureable getMaximum()
• return max
• private double sum
• private Measurable max
• private int count

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Interface Example, continued

• BankAccount account new BankAccount(1000)
• Measurable x account // OK to convert
• Coin dime new Coin(0.1, dime)
• x dime // Also OK notice we dont know type
  of
• // x, just that it implements Measurable, so it
  is
• // OK to call getMeasure. May do cast if you are
• // sure of the type
• DataSet coinData new DataSet()
• coinData.add(new Coin(0.25, quarter)
• Measurable max coinData.getMaximum()
• String coinName max.getName() // Error
• Coin maxCoin (Coin) max // Error if youre
  wrong!
• String coinName max.getName()
• Measurable x new Rectangle(5, 10, 20, 20)
  //not OK

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Polymorphism

• How is correct method executed?
• Measurable x new BankAccount(1000)
• double m x.getMeasure()
• x new Coin(0.1, dime)
• m x.getMeasure()
• JVM locates correct method. What did we do in
  C?
• Overloading method early binding (e.g., default
  vs 1-parameter constructor static)
• Polymorphism late binding (dynamic)

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UML Diagram
stereotype indicator
ltltinterfacegtgt Measurable
DataSet
uses (open arrow tip)
is-a (triangular tip)
BankAccount
Coin
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Chapter 10

• Inheritance

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Inheritance Basics

• Models is-a relationship, same as C
• requires extends keyword
• All classes are children of class Object
• Object has several methods that are frequently
  overridden
• toString returns a string that describes the
  state of the object, automatically called by
  System.out.println(objectRef)
• clone creates a deep copy of an object
• equals does a field-by-field comparison, returns
  boolean result

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Simple Inheritance Example

• public class BankAccount
• public BankAccount()
• balance 0
• public BankAccount(double initialBalance)
• balance initialBalance
• public void deposit(double amount)
• double newBalance balance amount
• balance newBalance
• public void withdraw(double amount)
• double newBalance balance - amount
• balance newBalance
• public double getBalance()
• return balance

could be protected
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Simple Inheritance Example

• class SavingsAccount extends BankAccount
• public SavingsAccount(double balance, double
  rate)
• super(balance) // calls parent constructor
• interestRate rate
• void addInterest()
• // use getters/setters unless protected
• double interest getBalance() interestRate
  / 100
• deposit(interest)
• private double interestRate

cant directly update balance if private
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UML for Inheritance
Object
BankAccount -balance deposit(double) withdraw(doub
le) getBalance
should be hollow triangle tip
BankAccount
SavingsAccount
SavingsAccount -interestRate addInterest(double
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Can have Inheritance Hierarchy
JComponent
JPane
JTextComponent
JLabel
AbstractButton
JToggleButton
JButton
JTextField
JTextArea
JCheckBox
JRadioButton
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More on Inheritance

• SavingsAccount collegeFund new
  SavingsAccount(1000, 0.1)
• BankAccount anAccount collegeFund
• Object anObject collegeFund
• collegeFund.addInterest() // OK
• anAccount.addInterest() //not OK

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Typecasts

• If you know that a reference contains a specific
  type of object, you can perform a cast (will be
  an exception if youre wrong)
• BankAccount myAccount (BankAccount) anObject
• Can use instanceof to test class type
• if (anObject instanceof BankAccount)
• BankAccount myAccount (BankAccount) anObject

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Polymorphism

• In Java, method calls are always determined by
  the type of the actual object, not the type of
  the object reference (unlike C, where virtual
  keyword is needed)

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Abstract Classes

• Concept same as C, use in same types of
  situations (e.g., force subclasses to define
  behavior)
• Requires use of abstract keyword
• As in C, you cant initialize objects of an
  abstract type. But you can initialize a variable
  of an abstract class to a subclass.

public abstract class BankAccount public
abstract void deductFees() . .
. BankAccount anAccount anAccount new
BankAccount() // ERROR anAccount new
SavingsAccount() // OK (deductFees defined)
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Final Methods and Classes

• May occasionally want to prevent others from
  creating subclasses or overriding methods
• public final class String . . .
• public final boolean checkPassword(String
  password) . . .

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Access control

• public like C, good for constants
• private like C, good for instance fields
• protected like C but extends to package,
  convenient for inheritance
• package
• all methods of classes in the same package have
  access
• this is the default! (easy to forget)
• OK if several classes in package collaborate, but
  generally inner classes are better, packages are
  not secure (anyone can add a class to the
  package)
• public class BankAccount
• double balance // package access
• . . .

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Object Method Examples toString
class object describes class properties

• public class BankAccount
• public String toString()
• return getClass().getName() balance
• . . .
• public class SavingsAccount
• public String toString()
• return super.toString() Rate
• interestRate
• . . .

call to parent class
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Object Method Examples equals

• public boolean equals(Object otherObject)
• if (otherObject null) return false
• if (getClass() ! otherObject.getClass())
• return false
• BankAccount account (BankAccount) otherObject
• return balance account.balance
• // should call super.equals first if you write
  an
• // equals for a subclass
• // instanceof sometimes used instead of
• // getClass(), but it would return true for a
• // subclass

What happens if you dont override equals?
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Object Method Examples clones

• Read advanced topic 10.6 if you need to write a
  clone method