Classes Redux

1
Classes Redux

• 10-31-05

2
Choosing Classes

• Should be nouns
• Methods are verbs
• Entities
• Should represent a simple concept
• Point, ellipse, rectangle
• BankAccount, CashRegister

3
Choosing Classes

• Actors
• Do some kinds of work for you
• Scanner class
• Rarely does a class have no objects
• Utility class
• Math
• Classes with only a main method

4
Not a Good Class

• Cant tell from the class name what an object of
  the class is suppose to do
• Trying to turn an action into a class
• These are generally methods

5
Cohesion

• A class should represent a single concept
• The public methods and constants should be
  cohesive
• All interface features should be closely related
  to the single concept that the class represents

6
Coupling

• Many classes need other classes in order to do
  their jobs
• Dependence can be depicted in UML
• If many classes of a program depend on each
  other, coupling is high
• If there are few dependencies between classes,
  then coupling is low

7
Consistency

• When you have a set of methods, follow a
  consistent scheme for their names and parameters
• messageDialog inputDialog have inconsistent
  parmaters (null)
• Inconsistencies are generally not a fatal flaw,
  but they are an annoyance

8
InConsistency

• Public class CashRegister
• public void enterPayment(int dollars, int
  quarters, int dimes, int nickels, int pennies)
• public static final double NICKLE_VALUE 0.05
• public static final double DIME_VALUE 0.10
• public static final double QUARTER_VALUE 0.25

9
Better Consistency

• public class Coin
• public Coin(double aValue, String aName)
• .
• public double getValue()
• ..

10
Better Consistency

• Public class CashRegister
• public void enterPayment(int coinCount, Coin
  coinType)
• .

11
Accessors, Mutators, andImmutable Classes

• Mutator method modifies the object
• Accessor method retrieves information from the
  object
• Classes with just accessor methods are called
  immutable classes
• Safe to be used anywhere in a program

12
Side Effect

• A side effect of a method is any kind of
  modification of data that is observable outside
  of the method
• Updating an explicit parameter can be surprising
  to programmers, and it is best to avoid it
  whenever possible

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Implicit ExplicitMethod Parameters

• Implicit the object on which the method is
  invoked
• Explicit enclosed in the parenthesis

14
Minimize Side Effects

• In an ideal world, all methods would be accessors
• Mutator methods with no changes to any explicit
  parameter acceptable side effect
• Methods that change an explicit parameter
• Should be avoided
• Methods that change another object
• Not good

15
Precondition

• Is a requirement that the caller of a method must
  obey
• Requires that a method is only called when it is
  in the appropriate state
• If a method is called in violation of a
  precondition, the method is not responsible for
  computing the correct result
• Needs to be documented clearly

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Preconditions

• The method is free to do if a precondition is not
  fulfilled
• Two typical strategies
• Throw an exception
• Error handling
• Work under the assumption that the preconditions
  are fulfilled

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Assertion

• Is an condition that y0u believe to be true at
  all times in a particular program location
• An assertion check tests whether an assertion is
  true
• Typically tests a precondition
• If the assertion fails, and assertion testing is
  enabled then the program terminates with an
  AssertionError

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Assertion

• public double deposit(double amount)
• assert amount gt 0
• balance balance amount

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Bad Idea

• public void deposit(double amount)
• if (amount lt 0) return
• balance balance amount
• // legal but will lead to disaster

20
Postconditions

• The return value is computed correctly
• The object is in a certain state after the method
  call is completed

21
Static Methods

• A method that is not invoked on an object
• Also called a class method
• Math.sqrt(x)
• You want to encapsulate some computation that
  involves only numbers.
• Numbers are not objects
• x.sqrt() not legal in Java

22
Static Fields

• Sometimes you need to store values outside any
  particular object
• Class field
• Single variable(location) for the entire class
• Every method in the class can access its static
  field

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Initializing a Static Field

• Cant be done in a method
• Do nothing (use defaults)
• 0 for numbers
• false for Booleans
• null for objects

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Initializing a Static Field

• Use an explicit initializer
• public class BankAccout
• private stat int last AccountNumber 1000

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Initializing Static Fields

• Like instance fields, static fields should always
  be declared as private to ensure that the methods
  of other classes do not change their values
• Exception static constants