Object-Oriented Design & Patterns

Cay S. Horstmann

Chapter 3

Guidelines for Class Design

Chapter Topics

• An overview of the Date classes in the Java library
• Designing a Day class
  
• Three implementations of the Day class
  
• The importance of encapsulation
• Analyzing the quality of an interface
• Programming by contract
• Unit testing
  

Date Classes in Standard Library

• Many programs manipulate dates such as
  "Saturday, February 3, 2001"
• Date class:
  

  Date now = new Date();
     // constructs current date/time
  System.out.println(now.toString());
     // prints date such as 
     // Sat Feb 03 16:34:10 PST 2001
  

• Date class encapsulates point in time

Methods of the Date class

Methods of the Date class

• Deprecated methods omitted
• Date class methods supply total ordering on Date objects
• Convert to scalar time measure
• Note that before/after not strictly necessary
  (Presumably introduced for convenience)

Points in Time

The GregorianCalendar Class

• The Date class doesn't measure months, weekdays, etc.
• That's the job of a calendar
• A calendar assigns a name to a point in time
• Many calendars in use:
  • Gregorian
  • Contemporary: Hebrew, Arabic, Chinese
    
  • Historical: French Revolutionary,  Mayan

Date Handling in the Java Library

Designing a Day Class

• Custom class, for teaching/learning purpose
• Use the standard library classes, not this class, in your own programs
• Day encapsulates a day in a fixed location
• No time, no time zone
• Use Gregorian calendar

Designing a Day Class

• Answer questions such as
  • How many days are there between now and the end of the year?
  • What day is 100 days from now?
    

Designing a Day Class

Designing a Day Class

• daysFrom computes number of days between two days:
  

  int n = today.daysFrom(birthday);
  

• addDays computes a day that is some days away from a given day:
  

  Day later = today.addDays(999);
  

• Mathematical relationship:
  

  d.addDays(n).daysFrom(d) == n
  d1.addDays(d2.daysFrom(d1)) == d2
  

• Clearer when written with "overloaded operators":
  

  (d + n) - d == n
  d1 + (d2 - d1) == d2
  

• Constructor Date(int year, int month, int date)
  
• getYear, getMonth, getDate acccesors
  

Implementing a Day Class

• Straightforward implementation:
  

  private int year
  private int month
  private int date
  

• addDays/daysBetween tedious to implement
  • April, June, September, November have 30 days
  • February has 28 days, except in leap years it has 29 days
  • All other months have 31 days
  • Leap years are divisible by 4, except after 1582, years divisible by 100 but not 400 are not leap years
  • There is no year 0; year 1 is preceded by year -1
  • In the switchover to the Gregorian calendar, ten days were dropped: October 15, 1582 is preceded by October 4

Implementing a Day Class

• Ch3/code/day1/Day.java
• Ch3/code/day1/DayTester.java
• Note private helper methods
• Computations are inefficient: a day at a time

Second Implementation

• For greater efficiency, use Julian day number
• Used in astronomy
• Number of days since Jan. 1, 4713 BCE
• May 23, 1968 = Julian Day 2,440,000
• Greatly simplifies date arithmetic
• Ch3/code/day2/Day.java

Third Implementation

• Now constructor, accessors are inefficient
• Best of both worlds: Cache known Julian, y/m/d values
• Ch3/code/day3/Day.java
• Which implementation is best?

The Importance of Encapsulation

• Even a simple class can benefit from different implementations
• Users are unaware of implementation
• Public instance variables would have blocked improvement
  • Can't just use text editor to replace all
    

    d.year
    

    with
    

    d.getYear()
    

  • How about
    

    d.year++?
    

  • d = new Day(d.getDay(), d.getMonth(), d.getYear() + 1)
    

  • Ugh--that gets really inefficient in Julian representation
• Don't use public fields, even for "simple" classes

Accessors and Mutators

• Mutator: Changes object state
• Accessor: Reads object state without changing it
• Day class has no mutators!
• Class without mutators is immutable
• String is immutable
• Date and GregorianCalendar are mutable

Don't Supply a Mutator for every Accessor

• Day has getYear, getMonth, getDate accessors
• Day does not have setYear, setMonth,setDate mutators
• These mutators would not work well
  • Example:
    

    Day deadline = new Day(2001, 1, 31);
    deadline.setMonth(2); // ERROR
    deadline.setDate(28);
    

  • Maybe we should call setDate first?

    Day deadline = new Day(2001, 2, 28);
    deadline.setDate(31); // ERROR
    deadline.setMonth(3);
    

• GregorianCalendar implements confusing rollover. 
  • Silently gets the wrong result instead of error.
• Immutability is useful
  

Sharing Mutable References

• References to immutable objects can be freely shared
• Don't share mutable references
• Example
  

  class Employee
  {
     . . .
     public String getName() { return name; }
     public double getSalary() { return salary; }
     public Date getHireDate() { return hireDate; }
     private String name;
     private double salary;
     private Date hireDate;
  }
  

Sharing Mutable References

• Pitfall:
  

  Employee harry = . . .;
  Date d = harry.getHireDate();
  d.setTime(t); // changes Harry's state!!!
  

• Remedy: Use clone
  

  public Date getHireDate() 
  { 
     return (Date)hireDate.clone();
  }
  

Sharing Mutable References

Final Instance Fields

• Good idea to mark immutable instance fields as final
  private final int day;
• final object reference can still refer to mutating object
  private final ArrayList elements;
• elements can't refer to another array list
  
• The contents of the array list can change
  

Separating Accessors and Mutators

• If we call a method to access an object, we don't expect the object to mutate
  
• Rule of thumb:
  Mutators should return void
• Example of violation: 
  

  Scanner in = . . .;
  String s = in.next();
  

• Yields current token and advances iteration
• What if I want to read the current token again?

Separating Accessors and Mutators

• Better interface:
  

  String getCurrent();
  void next();
  

• Even more convenient:
  

  String getCurrent();
  String next(); // returns current
  

• Refine rule of thumb:
  Mutators can return a convenience value, provided there is also an accessor to get the same value

Side Effects

• Side effect of a method: any observable state change
  
• Mutator: changes implicit parameter
  
• Other side effects: change to
  • explicit parameter
    
  • static object
• Avoid these side effects--they confuse users
• Good example, no side effect beyond implicit parameter
  

  a.addAll(b)
  

  mutates a but not b
  

Side Effects

• Date formatting (basic):
  

  SimpleDateFormat formatter = . . .;
  String dateString = "January 11, 2012";
  Date d = formatter.parse(dateString);
  

• Advanced:

  FieldPosition position = . . .;
  Date d = formatter.parse(dateString, position);
  

• Side effect: updates position parameter
  
• Design could be better: add position to formatter state

Side Effects

• Avoid modifying static objects
• Example: System.out
• Don't print error messages to System.out:
  

  if (newMessages.isFull())
     System.out.println("Sorry--no space");
  

• Your classes may need to run in an environment without System.out
• Rule of thumb: Minimize side effects beyond implicit parameter
  

Law of Demeter

• Example: Mail system in chapter 2
  Mailbox currentMailbox = mailSystem.findMailbox(...);
• Breaks encapsulation
• Suppose future version of MailSystem uses a database
• Then it no longer has mailbox objects
  
• Common in larger systems
• Karl Lieberherr: Law of Demeter
• Demeter = Greek goddess of agriculture, sister of Zeus

Law of Demeter

• The law: A method should only use objects that are
  • instance fields of its class
  • parameters
  • objects that it constructs with new
• Shouldn't use an object that is returned from a method call
• Remedy in mail system: Delegate mailbox methods to mail system
  mailSystem.getCurrentMessage(int mailboxNumber);
  mailSystem.addMessage(int mailboxNumber, Message msg);
  . . .
• Rule of thumb, not a mathematical law

Quality of Class Interface

• Customers: Programmers using the class
• Criteria:
  • Cohesion
  • Completeness
  • Convenience
  • Clarity
  • Consistency
• Engineering activity: make tradeoffs

Cohesion

• Class describes a single abstraction
• Methods should be related to the single abstraction
• Bad example:
  

  public class Mailbox 
  { 
     public addMessage(Message aMessage) { ... } 
     public Message getCurrentMessage() { ... } 
     public Message removeCurrentMessage() { ... } 
     public void processCommand(String command) { ... } 
     ... 
  }
  

Completeness

• Support operations that are well-defined on abstraction
• Potentially bad example: Date
  

  Date start = new Date();
  // do some work
  Date end = new Date();
  

• How many milliseconds have elapsed?
• No such operation in Date class
• Does it fall outside the responsibility?
• After all, we have before, after, getTime

Convenience

• A good interface makes all tasks possible . . . and common tasks simple
• Bad example: Reading from System.in before Java 5.0
  

  BufferedReader in = new BufferedReader(new
     InputStreamReader(System.in));
  

• Why doesn't System.in have a readLine method?
• After all, System.out has println.
• Scanner class fixes inconvenience
  

Clarity

• Confused programmers write buggy code
• Bad example: Removing elements from LinkedList
  
• Reminder: Standard linked list class
  

  LinkedList<String> countries = new LinkedList<String>(); 
  countries.add("A"); 
  countries.add("B"); 
  countries.add("C"); 
  

• Iterate through list:
  

  ListIterator<String> iterator = countries.listIterator(); 
  while (iterator.hasNext()) 
     System.out.println(iterator.next()); 
  

Clarity

• Iterator between elements
• Like blinking caret in word processor
• add adds to the left of iterator (like word processor):
• Add X before B:
  

  ListIterator<String> iterator = countries.listIterator(); // |ABC 
  iterator.next(); // A|BC 
  iterator.add("France"); // AX|BC 
  

• To remove first two elements, you can't just "backspace"
• remove does not remove element to the left of iterator
• From API documentation:
  Removes from the list the last element that was returned by next or previous. This call can only be made once per call to next or previous. It can be made only if add has not been called after the last call to next or previous. 
  
• Huh?
  

Consistency

• Related features of a class should have matching
• • names
  • parameters
  • return values
  • behavior
• Bad example:
  

  new GregorianCalendar(year, month - 1, day)
  

• Why is month 0-based?

Consistency

• Bad example: String class
  

  s.equals(t) / s.equalsIgnoreCase(t)
  

• But
  

  boolean regionMatches(int toffset, 
     String other, int ooffset, int len) 
  boolean regionMatches(boolean ignoreCase, int toffset, 
     String other, int ooffset, int len) 
  

• Why not regionMatchesIgnoreCase?
• Very common problem in student code

Programming by Contract

• Spell out responsibilities
  • of caller
  • of implementor
• Increase reliability
• Increase efficiency
  

Preconditions

• Caller attempts to remove message from empty MessageQueue
• What should happen?
• MessageQueue can declare this as an error
• MessageQueue can tolerate call and return dummy value
• What is better?

Preconditions

• Excessive error checking is costly
• Returning dummy values can complicate testing
• Contract metaphor
  • Service provider must specify preconditions
  • If precondition is fulfilled, service provider must work correctly
  • Otherwise, service provider can do anything
• When precondition fails, service provider may
  • throw exception
  • return false answer
  • corrupt data

Preconditions

/**
   Remove message at head
   @return the message at the head
   @precondition size() > 0
*/
Message remove()
{
   return elements.remove(0);
}

• What happens if precondition not fulfilled?
• IndexOutOfBoundsException
• Other implementation may have different behavior

Circular Array Implementation

• Efficient implementation of bounded queue
• Avoids inefficient shifting of elements
• Circular: head, tail indexes wrap around
• Ch3/queue/MessageQueue.java
  

Inefficient Shifting of Elements

A Circular Array

Wrapping around the End

Preconditions

• In circular array implementation, failure of remove precondition corrupts queue!
• Bounded queue needs precondition for add
• Naive approach:
  @precondition size() < elements.length
• Precondition should be checkable by caller
• Better:
  @precondition size() < getCapacity()

Assertions

• Mechanism for warning programmers
• Can be turned off after testing
• Useful for warning programmers about precondition failure
• Syntax:
  

  assert condition;
  assert condition : explanation;
  

• Throws AssertionError if condition false and checking enabled

Assertions

public Message remove() 
{ 
   assert count > 0 : "violated precondition size() > 0";
   Message r = elements[head]; 
   . . .
} 

• During testing, run with
  

  java -enableassertions MyProg
  

• Or shorter, java -ea
  

Exceptions in the Contract

/**
   . . .
   @throws NoSuchElementException if queue is empty
*/
public Message remove() 
{ 
   if (count == 0) 
      throw new NoSuchElementException();
   Message r = elements[head]; 
   . . .
} 

• Exception throw part of the contract
• Caller can rely on behavior
• Exception throw not result of precondition violation
• This method has no precondition
  

Postconditions

• Conditions that the service provider guarantees
• Every method promises description, @return
• Sometimes, can assert additional useful condition
• Example: add method
  

  @postcondition size() > 0
  

• Postcondition of one call can imply precondition of another:
  

  q.add(m1);
  m2 = q.remove(); 
  

Class Invariants

• Condition that is
  • true after every constructor
  • preserved by every method
    (if it's true before the call, it's again true afterwards)
• Useful for checking validity of operations

Class Invariants

• Example: Circular array queue
  

  0 <= head && head < elements.length
  

• First check it's true for constructor
  • Sets head = 0
  • Need precondition size > 0!
• Check mutators. Start with remove
  • Sets head_new = (head_old + 1) % elements.length
  • We know head_old >= 0 (Why?)
  • % operator property:

    0 <= headnew && headnew < elements.length
    

• What's the use? Array accesses are correct!
  

  return elements[head];
  

Unit Testing

• Unit test = test of a single class
• Design test cases during implementation
  
• Run tests after every implementation change
• When you find a bug, add a test case that catches it
  

JUnit

JUnit

• Convention: Test class name = tested class name + Test
• Test methods start with test

import junit.framework.*;
public class DayTest extends TestCase
{
   public void testAdd() { ... }
   public void testDaysBetween() { ... }
   . . .
}

JUnit

• Each test case ends with assertTrue method
  (or another JUnit assertion method such as assertEquals)
• Test framework catches assertion failures
  

public void testAdd()
{
   Day d1 = new Day(1970, 1, 1);
   int n = 1000;
   Day d2 = d1.addDays(n);
   assertTrue(d2.daysFrom(d1) == n);
}