Transcript Objects First With Java

Design: Coupling and Cohesion
How to write classes in a way that
they are easily understandable,
maintainable and reusable
Main concepts to be covered
•
•
•
•
Responsibility-driven design
Coupling
Cohesion
Refactoring
• Enumerations
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Software changes
• Software is not like a novel that is written
once and then remains unchanged.
• Software is extended, corrected,
maintained, ported, adapted …
• The work is done by different people over
time (often decades).
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Change or die
• There are only two options for software:
• Either it is continuously maintained ...
• Or it dies
• Software that cannot be maintained will
be thrown away.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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World of Zuul
Chapter 7: zuul-bad
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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An Example
• Add two new directions to the World of Zuul:
• up
• down
• What do you need to change to do this?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Code Quality
• Two important concepts for quality of
code:
• Coupling – as little as possible please
• Cohesion – as much as possible please
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Coupling
• Coupling refers to links between separate
units of a program.
• If two classes depend closely on many
details of each other, we say they are
tightly coupled.
• We aim for loose coupling.
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Loose Coupling
• Loose coupling makes it possible to:
• understand how to use a class without having
to understand how that class works;
• change the implementation of a class without
affecting those classes that use it.
This improves maintainability … makes change easier.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Cohesion
• Cohesion relates to: the the number and
diversity of tasks for which a single unit is
responsible.
• If a programming unit is responsible for
one logical task, we say it has high
cohesion.
• Cohesion applies both at the level of
classes and of methods.
• We aim for high cohesion.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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High Cohesion
• High cohesion makes it easier to:
• understand what a class or method does
(because it only does one thing);
• Choose and use descriptive names;
• reuse classes or methods.
This improves usability … and makes change easier.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Cohesion of Classes
Classes should represent just one,
and only one, well defined entity.
Cohesion of Methods
A method should be responsible for one,
and only one, well defined task.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Code Duplication
This is an indicator of bad design.
It makes maintenance harder.
It can lead to introduction of errors,
especially during maintenance.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Add up
and down
exits?
World of Zuul
Chapter 7: zuul-bad
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Chapter 7: zuul-bad
Problems
• Fields in Room are all public.
• The fact that a Room has four exits is
burnt into the interface of its setExits
method.
• The Game class references Room fields.
• So, the implementation of the Game and
Room classes are coupled. A change in
one (e.g. adding more exits to a Room)
means changing the other (e.g. the
createRooms, printWelcome and
goRoom methods of Game).
• This is BAD!
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Chapter 7: zuul-bad
Problems
• There are duplicated code fragments in
the Game class (in createRooms and
printWelcome methods).
• Changing the details of one of these
fragments means changing all.
• This is BAD!
• Factorise the duplicated code into a
single method (see Code 7.2, p209, of the
book: printLocationInfo) and
duplicate its invocation instead.
• Now, there is only one fragment of code
to maintain. This is Good!
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Chapter 7: zuul-bad

Chapter 7: zuul-better
• There are duplicated code fragments in
the Game class (in createRooms and
printWelcome methods).
• Changing the details of one of these
fragments means changing all.
• This is BAD!
• Factorise the duplicated code into a
single method (see Code 7.2 of the book:
printLocationInfo) and duplicate its
invocation instead.
• Now, there is only one fragment of code
to maintain. This is Good!
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Chapter 7: zuul-bad

Chapter 7: zuul-better
• Make fields in Room private and provide accessor
methods to get them (e.g. getExit in Code 7.4).
• In Game, can no longer reference directly the exit
fields (all Rooms) of Room.
• In Game.printLocationInfo (which does so
reference), we could just change it to use the
accessor method, rather than the field.
• However, it’s neater to invoke a (new) method of
Room to get the exit information it needs to print
(Code 7.6). Each Room object has that
information, so it is really its responsibility.
• For Game.goRoom (which also does so reference),
thirteen lines of code collapse to one (see p214 in
“Fourth Edition”, but p201 in “Third Edition”).
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Chapter 7: zuul-bad

Chapter 7: zuul-better
• Make fields in Room private and provide accessor
methods to get them (e.g. getExit in Code 7.4).
• Look carefully at this getExit accessor …
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Chapter 7: zuul-bad

Chapter 7: zuul-better
public Room getExit (String direction)
{
if (direction.equals ("north")) {
return northExit;
}
if (direction.equals ("east")) {
return eastExit;
}
if (direction.equals ("south")) {
return southExit;
}
if (direction.equals ("west")) {
return westExit;
}
return null;
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Chapter 7: zuul-bad

Chapter 7: zuul-better
• Make fields in Room private and provide accessor
methods to get them (e.g. getExit in Code 7.4).
• In
Gamea, String
can no longer
reference
exit
It maps
(direction)
intodirectly
an exit the
Room
.
(all Room
s) of
Room.of a:
• fields
But that’s
just the
function
• In Game.printLocationInfo
(which does so
HashMap<String, Room>
reference), we could just change it to use the
• So,
replace
the four
exit than
fieldsthe
with
one of the
accessor
method,
rather
field.
(private
of course)
– Code
7.5. Now,
we of
• above
However,
it’s neater
to invoke
a (new)
method
can
have
exitsinformation
as we like.it needs to print
Room
to as
getmany
the exit
• Finally,
replace
theEach
setExits
method
(Code 7.6,
p218).
Room object
has(which
that
information,
so it is“four
reallyexits”
its responsibility.
burns the message
into its definition)
a single
setExit
method
(which
we can
• with
For Game
.goRoom
(which
also does
so reference),
thirteen
of code
collapse
to one (see p214).
invoke forlines
as many
exits
as we like).
• Now, Room is independent of its number of exits.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
World of Zuul
MORAL: class
diagrams are
not enough to
tell whether the
design is good.
Chapter 7: zuul-bad
Chapter 7: zuul-better
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Responsibility-Driven Design
• Where should we add new fields and
new methods (and to which class)?
• The class that holds the data (fields)
processes (methods) the data.
• Responsibility-driven design leads to
low coupling.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Localising Change
• One aim of reducing coupling and
responsibility-driven design is to
localise change.
• When a change is needed, as few
classes as possible should be
affected.
Improve maintainability … make change easier.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Think Ahead
• When designing a class, think what
changes are likely to be made in the
future.
• Aim to make those changes easy.
Improve maintainability … make change easier.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Refactoring
• When classes are maintained, code
usually needs to be added.
• Lists of fields, lists of methods and
the code inside methods tend to
become longer.
• Every now and then, classes and
methods should be refactored to
maintain cohesion and low coupling.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
26
Refactoring and Testing
• When maintaining code, separate
the refactoring from making other
changes.
e.g. zuul-bad to zuul-better.
• Do the refactoring first, without
adding to the functionality.
• Test before and after refactoring to
ensure that nothing gets broken.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Design Questions
• Common questions:
• How long should a class be?
• How long should a method be?
Answered with regard to cohesion and coupling.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Design Guidelines
• A method is too long if it does more
then one logical task.
• A class is too complex if it represents
more than one logical entity.
Note: these are guidelines …
everything is still open to the designer.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Enumerated Types
• A new language feature.
• Uses enum instead of class to introduce a new
type name and a set of named constants of that
type.
• This is a better (safer) alternative to a set of
named static int constants.
• For the World of Zuul, we shall use names for
command words, rather than string literals
(which will appear only in one place).
java.sun.com/docs/books/tutorial/java/javaOO/enum.html
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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A Basic Enumerated Type
public enum CommandWord
{
// A value for each command word,
// plus one for unrecognised commands.
GO, QUIT, HELP, UNKNOWN
}
Enumerated objects are constructed implicitly.
Each name represents an object of the
enumerated type, e.g. CommandWord.HELP.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
31
Equivalent Class Type
public class CommandWord
{
// A object for each command word,
// plus one for unrecognised commands.
public final static Commandword GO = new CommandWord ();
public final static Commandword QUIT = new CommandWord ();
public final static Commandword HELP = new CommandWord ();
public final static Commandword UNKNOWN = new CommandWord ();
private CommandWord ()
{
// nothing to do
}
}
The constructor is declared private so that no CommandWord
objects, other than the ones declared here, can be constructed.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
32
Another Enumerated Type
public enum CommandWord
{
// A value for each command word,
// plus one for unrecognised commands.
GO("go"), QUIT("quit"), HELP("help"), UNKNOWN("unknown");
private final String commandString;
private CommandWord (String commandString)
{
this.commandString = commandString;
}
public String toString ()
{
return commandString;
}
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
33
Equivalent Class Type
public class CommandWord
{
// A object for each command word,
// plus one for unrecognised commands.
public final static Commandword GO =
new CommandWord ("go");
public final static Commandword QUIT =
new CommandWord ("quit");
public final static Commandword HELP =
new CommandWord ("help");
public final static Commandword UNKNOWN =
new CommandWord ("?");
private final String commandString;
...
...
...
}
private constructor {to construct the above constants}
public String toString {returns the commandString}
public static CommandWord[] values {returns an array
of above constants}
Automatically provided by the enum
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
34
Equivalent Class Type
public class CommandWord
{
// A object for each command word,
// plus one for unrecognised commands.
public final static Commandword GO =
new CommandWord ("go");
public final static Commandword QUIT =
new CommandWord ("quit");
public final static Commandword HELP =
new CommandWord ("help");
public final static Commandword UNKNOWN =
new CommandWord ("?");
private final String commandString;
private CommandWord (String commandString) {
this.commandString = commandString;
}
...
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
35
Equivalent Class Type
public class CommandWord
{
...
public final statics (GO, QUIT, HELP, UNKNOWN)
private final String commandString;
private CommandWord (String commandString) {
this.commandString = commandString;
}
public String toString () {
return commandString;
}
Automatically provided by the enum
public static CommandWord[] values () {
return new CommandWord[] {GO, QUIT, HELP, UNKNOWN};
}
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
36
See also …
projects\chapter07\
courses\co320_ca13\projects-phw
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
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Review
• Programs are continuously changed.
• It is important to anticipate change
and make it as easy as possible.
• Quality of code requires much more
than just performing correctly!
• Code must be understandable and
maintainable.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
38
Review
• Good quality code avoids duplication,
displays high cohesion, low coupling.
• Coding style (commenting, naming, layout,
etc.) is also very important.
• There is a big difference in the amount of
work required to maintain poorly structured
and well structured code.
• In fact, unless it is well structured, the code
is doomed … it will not be used for long.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
39